UNIVERSITY OF CALIFORNIA PUBLICATIONS . COLLEGE OF AGRICULTURE. AGRICULTURAL EXPERIMENT STATION. THE BROWN ROT OF THE LEMON. By RALPH E. SMITH. Assisted by H. J. Ramsey, E. H. Smith, E. B. Babcock and C. O. Smith. ■ 5^2 BULLETIN No. 190 (Berkeley, Cal., July, 1907.) W. W. SHANNON, SACRAMENTO: : : : : : superintendent state printing, 1907. BENJAMIN IDE WHEELER. Ph.D., LL.D.. President of the University. EXPERIMENT STATION STAFF. E. J. WICKSON, M.A.. Acting Director and Horticulturist. E. W. HILGARD. Ph.D.. LL.D.. Chemist. W. A. SETCHELiIi, Ph.D.. Botanist. ELWOOD MEAD. M.S., C.E.. Irrigation Engineer. C W. WOODWORTH, M.S., Entomologist. R. H. LOUGHRIDGE, Ph.D.. Agricultural Geologist and Soil Physicist. (Soils, Alkali.) ' . M. E. JAFFA, M.S., Nutrition Expert, in charge of the Poultry Station. G. W. SHAW. M.A.. Ph.D., Agricultural Technologist, in charge of Cereal Stations. GEORGE E. COLBY, M.S., Chemist. (Emits, Waters, Insecticides.) RALPH E. SMITH, B.S., Plant Pathologist and Superintendent of Southern California Pathological Laboratory and Experiment Stations. A. R. WARD, B.S.A., D.V.M., Veterinarian and Bacteriologist. E. W. MAJOR, B.Agr., Animal Industry and Manager of University Farm. F. T. BIOLETTI, M.S., Viticulturist. (Grapes, Wine and Zymology.) H. M. HALL. M.S., Assistant Botanist. H. J. QL'AYLE, A.B., Assistant Entomologist. W. T. CLARKE, B.S., Assistant Horticulturist and Superintendent of University Extension in Agriculture. JOHN S. BURD, B.S., Chemist, in charge of Fertiliser Control. C. M. HARING, D.V.M., Assistant Veterinarian and Bacteriologist. E. B. BABCOCK, B.S., Assistant Plant Pathologist. E. H. SMITH, M.S., Assistant Plant Pathologist. J. W. MILLS, Assistant Horticulturist, j CUrus Experiment 8 tatio7i, Riverside. T. F. HUNT, B.S., " " \ H. J. RAMSEY, M.S., Assistant Plant Pathologist, { Southern California Pathologi- C. O. SMITH, M.S., " " " \ cal Laboratory. Whittier. R. E. MANSELL, Assistant in Horticulture, in charge of Central Station Grounds. GEO. W. LYONS, B.S., Assistant in Soil Laboratory. RALPH BENTON, B.S., Assistant in Entomology. A. J. GAUMNITZ, M.S., Assistant in Cereal Investigations. HANS C. HOLM, B.S., Assistant in Zymology. P. L. McCREART?, B.S., Laboratory Assistant in Fertilizer Control. C. WESTERGARD, B.S., Assistant in Farm Mechanics. M. E. STOVER, B.S., Assistant in Agricultural Chemical Laboratory. D. L. BUNNELL, Clerk to the Director. JOHN TUOHY, Patron, I Tulare Substation, Tulare. J. T. BEARSS, Foreman, S J. W. ROPER, Patron, ) University Forestry Station, Chico. E. C. MILLER, In charge, ) ROY JONES, Patron, 1 University Forestry Station, Santa Monica. N. D. INGHAM, Foreman, \ VINCENT J. HUNTLEY Foreman of California Poultry Experiment Station, Petaluma. The Station publications I Reports and Bulletins), so long as avail- able, "ill },, sent to any citizen of the State on application. Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://www.archive.org/details/brownrotoflemon190smit THE BROWN ROT Of THE LEMON. By RALPH E. SMITH. Assisted by H. J. Ramsey, E. H. Smith, E. B. Babcock, and C. O. Smith. The subject of this bulletin is a peculiarly virulent, rapid-spreading form of decay of lemons occurring in southern California. It has been named "Brown Rot" by the growers in distinction from the ordinary "Blue Mould" rot caused by the Penicillium fungus, the usual form of decay in citrus fruit. Great losses have been experienced from this source during the past few years, and it has remained for this investi- gation to determine the cause and means of control of the trouble. The history of the lemon industry in California has been one of much vicissitude and uncertainty, and has been marked particularly by a decidedly sudden and apparently permanent change from a condition of deep depression and discouragement to one of unusual profit and prosperity. This change came about most largely during the season of 1904-05, and was so complete as to transform the whole aspect of the business. The California lemon as a commercial product has had a poor repu- tation in the markets of the country, which it has only recently begun to live down. The defect has been in keeping quality, in which respect the native product, as compared with imported fruit, has been notably deficient. The reasons for this may have been several, but for present purposes it need simply be said that by improvements in their methods of handling the fruit the lemon growers and shippers of the State, taking advantage of the improved market conditions brought about by the short foreign crop of 1904-05, have largely overcome the prejudice against the keeping quality of the California lemon, and are doing so more and more with every car that leaves the State. Supported by the abnormally high prices of the last three seasons, our lemon industry has improved and strengthened itself all along the line to a really remark- able extent. Under the influence of the leaders of the industry methods of both production and handling have been improved, the cooperative selling associations have strengthened and improved their condition and enlarged the scope of their influence, and the whole lemon industry has entered upon a period in which there is nothing but encouragement for the future. L UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. Excessive decay, in curing, transit, and on the market has been the great trouble with the California lemon which has kept it from enjoying the appreciation of its otherwise excellent qualities to which it is properly entitled. The tree is peculiar in that it produces fruit at all - -nns of the year, which fruit is not allowed to mature naturally on the tree to normal ripeness and size, but is picked in a green condition on reaching a certain size and then stored for some time (several weeks or months to cure or ripen. A well-cured lemon does not rot. It can - rcely be made to decay. When the skin has been dried out to the ideal firm texture, thinness and yellow color, under the proper con- ditions of atmospheric moisture for good curing, the lemon can be kept for months without decaying, until it has completely dried up and mummified. It has been demonstrated that most of the decay which often occurs so extensively during the period from the tree to the consumer, and which formerly was nearer the rule than the exception in many houses. is directly traceable to bruising of the fruit caused by rough handling in the field and packing house; likewise that by careful handling this rot can be almost entirely eliminated. One of the greatest advances of the California lemon industry has come about through the recognition of this simple fact. Decay of this sort is commonly caused by the so-called ''Blue Mould" fungus, species of Penicillium, a fungus which is not particularly active as a parasite, but which is ever ready to attack and quickly causes the decay of citrus fruit which has been bruised or weakened in any way. Penicillium is the ever present, and, until recently, the only recognized cause of citrus decay in this State.* The recent improvement in the lemon industry just referred to has come about largely by the simple exercise of more care in handling the fruit to prevent bruising, and by improved methods of curing. Going back for a period of five or six years it has gradually come about that the lemon people have begun to recognize that some further (factor than the ordinary blue mould which they knew how to handle causing decay of their fruit. As early as the season of 1901 the more observant of them had come to the conclusion that something entirely distinct from the ordinary Penicillium rot, or at any rate, from their standpoint, a different proposition for handling, was confronting them. This fact was recognized earliest by the leaders in the business. since they were handling ordinary decay successfully and could quickly recognize a new condition, while in the less carefully conducted houses the usual amount of decay up to a very recent period was so great that a new cause coming in had little effect upon the general average Wood worth, Orange and Lemon Rot. Bull. 139, Cal. Agr'l Expt. Sta. Powell, The Handling of Fruit for Transportation. Year Book U. S. Dept. of Agr., 1905. Bulletin 190. brown ROT OF THE LEMON. 3 or upon the minds of those in charge. When, however, in the best houses, the shrinkage between picking and packing rose from 10 to 20 or 30 per cent, when an amount of fruit equal to 10 per cent of the entire output rotted in the house and was thrown away from a cause different from anything encountered in previous experience, when a large amount of extra help was constantly required in handling the fruit in order to keep down losses to even these abnormally high amounts, when fruit in transit decayed in an inexplicable manner and to an unheard of extent, and when the losses by decay in single houses rose to from ten to thirty thousand dollars per year above that which could be accounted for by any known cause, then it became evident that something of an unusual nature was going on, and that something must be done to meet this situation or the lemon business would again go back into a condition of depression and uncertainty. This was what actually occurred in many houses in southern Cali- fornia during the period from 1901 to 1905. Excessive decay occurred, whole pickings or entire lots of fruit were lost in some cases, and, except that something new and peculiar was at work, practically nothing Avas known as to the cause or remedy of the trouble. Some main- tained that the decay was nothing more than a severe attack of the ordinary blue-mould fungus, but the most observant of those who handled the fruit soon became convinced that this was not the case. In the spring of 1905 the "Lemon Men's Club," an organization of the leaders in the California lemon business, brought this matter to the attention of the State Experiment Station. Arrangements were made between Mr. C. C. Teague of the Limoneira Lemon Company and the writer for commencing an investigation of the trouble, and a financial contribution was made by the members of the club to assist in the work. The investigation was inaugurated by placing two assistants in the field, one at the Limoneira Ranch at Santa Paula, and one at Whittier, working at the latter place in conjunction with Mr. C. W. Leffingwell, Jr., and the Whittier Citrus Association. A large share of the credit for whatever good may have come from this work is due to the above companies, for furnishing fruit, labor and many facilities and conveni- ences for carrying on our experiments. The work has continued con- stantly since the beginning in the field, packing house and laboratory, most of the lemon packing houses of southern California contributing, in greater or less degree as called upon, to the progress of the work. It is with a feeling of more than ordinary appreciation that the writer acknowledges the support given to this investigation by the lemon industry of southern California. In the fall of 1905 a temporary laboratory was established at Whittier, through the courtesy of the Chamber of Commerce of that city, since which time most of the experi- 4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. mental work has been done at that place, combined with extensive orchard and packing-house demonstrations in other sections. Most of the experimental and field work of the investigation has been carried out by Messrs. 11. J. Ramsey, E. B. Babcock and C. O. Smith, especially the first named, while the study of the fungus in the laboratory has Dived largely upon Miss E. H. Smith. METHODS OF GROWING AND HANDLING LEMONS. It will be necessary to give a brief consideration to this subject for the better understanding of what is to follow. The lemon, like other citrus fruits, is an evergreen tree of semi-tropic habit, somewhat more easily affected by frost than the orange. As already stated the tree is peculiar in producing its fruit all the year round; that is to say, at almost any time in the year mature fruit, blossoms, and fruit in all stages of development may be found on the same tree. (See cover illustration.) Inasmuch as winter is the usual season of maturity of citrus fruits, a time when lemons are least in demand, this is a very valuable feature. There is a tendency to produce the bulk of the crop in winter and spring, especially in some varieties like the Lisbon, which feature is not at all desirable from the standpoint of the grower. PICKING. Lemons allowed to mature and color on the tree become overgrown, rough, misshapen, thick-skinned, of poor keeping quality and deficient in acid. One of the principal faults of the California lemon on the market has been due to irregular picking and allowing the fruit to get too old on the tree. In well-conducted orchards pickings are made once a month or at the longest every six weeks. The selection is made by means of a ring of 2*4 inches diameter (or at times a little more or less . all lemons which will not pass through the ring being picked. Smaller ones which have matured, colored and ceased growing are also taken. The most desirable fruit is that which is of moderate size, unetrical shape and dark green color. The lemons are cut from the Tr^e by means of clippers, placed in a sack carried by the picker, emptied into a field box and then carried to the packing house. In all these operations the greatest care is necessary not to cut or bruise the fruit in any manner. WASHING. It is at present the almost universal practice in California to wash all Lemons as soon as they come from the orchard. The practice originated Bulletin 190. BROWN ROT OF THE LEMON. from the necessity of washing decidedly dirty fruit, especially that which was affected by scale insects or the black, sooty fungus which follows their attacks. But the washing of even the cleanest fruit when green is so beneficial to its appearance after curing, giving it a gloss and finish not seen in the cleanest unwashed fruit, and the process is so inexpensive, that washing is now a routine process in lemon handling without regard to the original condition of the fruit. This washing is done in most of the houses with the machine shown in figure 1. This consists of a sheet iron tank of the shape shown, containing a circular arrangement of fixed brushes in the round portion, with an inner circle PIG 1. Usual form of lemon washer. (Photo by L. B. Williams.) of brushes which revolves rapidly when the machine is running. The tank is filled with water up to the brushes, the fruit is dumped carefully into the portion shown in the foreground, and crowded along up into the circular portion where it enters the brushes, is carried around between them and then lifted by revolving arms onto the grading table, a moving belt on which the lemons are carried past the graders and distributed by them according to color— dark green, light green or yellow— into different compartments. This machine, in various modi- fications, is in almost universal use. A new washer gotten up by the Limoneira Company has a square wooden tank, brushes in the form of square troughs, and the grading table, all in a straight line. (Fig. 2.) The fruit is dumped into the tank, carried up by revolving arms and belts into the brushways, rolled and carried along between the brushes 6 UNIVERSITY OV CALIFORNIA — EXPERIMENT STATION by moving- brushes which form the bottom, and so on out onto the moving belts to the graders. (Fig. 3.) The severe washing and scrubbing to which much of the fruit 1 *■**" m HP w f- 1 r? i it iBr 1 1 Ik n /J • II FIG 2. Limoneira washer. Tank end. be subjected in certain districts is a very undesirable and largely unnecessary operation. The scale and smut, to remove which the wash- ing is done, may be very completely controlled by systematic and Bulletin 190. BROWN ROT OF THE LEMON. thorough fumigation, and the necessity of severe scrubbing thereby eliminated. The latter operation is expensive and can not fail to injure the fruit. The ordinary washing, however, even where nothing FIG. 3. Grading table of lemon washer. more than an almost invisible dust is removed, is decidedly beneficial to the appearance of the cured fruit. As it comes from the washer the fruit is placed at once into shipping boxes while still wet, and is then put away for curing. UNIVERSITY OF CALIFORNIA— EXPERIMENT STATION. CURING AND STORING. The methods of holding lemons for curing and storing have been greatly improved in recent years. This is largely due to the develop- ment of the Limoneira system of tent-curing in open houses. The method consists in storing the fruit as soon as washed, usually in shipping boxes, in open-sided sheds exposed to the freest circulation of air and outdoor temperature. The boxes of fruit are stacked in square piles of about a carload each, each lot being covered with a canvas tent, as shown in figure 4. These tents consist of canvas tops and sides, the latter lacing together at the corners and rolling up on the sides. Under FIG 4. Packing lemons for shipment. Shows also curing tents. (Photo by L. B. Williams.) this system each lot of fruit can be given its proper amount of ventila- tion, according to its condition and that of the atmosphere. The gen- eral object aimed at is to gradually dry out or cure the fruit, developing a juicy inside, thin, tough rind, yellow color, and smooth velvety surface. Too much moisture causes softening and decay; too little results in wilting and withering of the fruit and poor curing generally. The "button" or stem end which is left on the lemon in picking is the best indicator in the curing process. If proper conditions are maintained for gradual, uniform curing the button holds tightly to the fruit and does not drop off. If, on the other hand, the fruit is weakened by roper treatment the buttons begin to drop off and separate very easily and poor keeping quality results. Under certain conditions Bulletin 190. BROWN ROT OF THE LEMON. 9 shallow trays, like those seen in figure 4, are used for storage instead of boxes. In the drier interior regions of southern California the tent system is not as satisfactory as nearer the coast, on account of too rapid drying of the fruit in summer, even with the tents closed continuously. For this reason some of the houses in that section, notably the Arlington Heights Fruit Company of Riverside and the Corona Lemon Company of Corona, are now using large basement rooms to a considerable extent for curing the fruit. These are arranged for ventilation and have various features adapted to their peculiar needs. In dry summer weather, even near the coast, it is usually necessary to cover the top boxes in the tent with papers to prevent excessive drying. The condition of the fruit is carefully watched during curing, and if necessary it is taken down and sorted during the process to remove decay. This, however, should not ordinarily be necessary except on account of brown rot, a phase which will be considered later. It is desirable to handle the fruit as little as possible, as a certain amount of bruising and consequent decay is bound to result, even under the best conditions. When fully cured the fruit is either shipped at once, or, if in good condition, may be held in storage in the tent for weeks or even months, according to market conditions. PACKING. When ready to ship the fruit is sorted from the storage boxes into trays according to quality. The packer, with his bench for the box and bunch of wrapping paper, then wraps each lemon in tissue paper and packs them uniformly in the boxes, picking out the particular size which he is packing as he goes along. (Fig. 4.) By thus selecting the sizes each box is made to contain a certain standard number of lemons, and is thereafter designated by the number and quality which it con- tains. The pack is uniform and systematic, the packer knowing just how many lemons he will put into the box and arranging them accord- ingly, selecting his size and putting a certain number in each tier. The packed boxes are taken to the nailer (see background, Fig. 4), who presses the cover down with a foot power press, nails it at the ends, and fastens an iron strap band about the center. SHIPPING. In forwarding the fruit to the Eastern markets, lemons are shipped in refrigerator cars in order to control the ventilation and maintain uniform temperature and moisture conditions. It has been thoroughly demonstrated, however, that with fruit properly cured and handled 10 UNIVERSITY OF CALIFORNIA—EXPERIMENT STATION. the use of ice is not necessary in shipping lemons, and a number of houses are shipping the year round without it. Not only is the expense of icing saved (a comparatively small matter in proportion to the value of the shipment were icing desirable), but the fact that fruit will carry across the continent without ice is the strongest recommenda- tion of its future keeping quality on the market. On the other hand, lemons which will not carry without ice will show decay sooner or later in any event. Icing, in other words, does not prevent, but simply post- pones or temporarily checks decay. The individual shipper may be able to sell more of his poorly handled fruit by this means, but the whole industry is bound to suffer in reputation and market standing by the subsequent decay of this fruit after it reaches the retailer and consumer. Shipping successfully without ice is the standard which every progressive lemon-house manager will set for himself where this is not already accomplished. A word should perhaps be added in favor of shipping under ice on long distance shipments in hot summer weather, in order to preserve the fresh, unwilted appearance of the fruit. This is not a matter of decay, but simply one of drying and shriveling, where the fruit may be three weeks or more in the car at a high temperature. Under such con- ditions the use of ice may often be advisable for the reasons described, but the shipper will do well to first demonstrate to his customers his ability to ship sound fruit under any conditions without the use of ice, and establish what is by far the most valuable asset in the lemon trade, a firm reputation for keeping quality. THE BROWN ROT. Were actual conditions which have obtained in the past few years portrayed in the above account of lemon handling, it would be necessary to insert in almost every sentence the clause, "Except for brown rot." This condition, as already intimated, has come to affect every phase and operation of the business, and has presented a very serious problem indeed to an industry already with its full share of complications. WHAT BROWN ROT IS. Brown rot is a certain form of decay of the lemon of an appearance and nature very characteristic to one familiar with it. The orange, pomelo and other citrus fruits are also affected, but not to the extent of the lemon, on account of the methods of handling the latter fruit. Brown rot in the packing house is distinguished most clearly by two features: its rapid spread in the fruit by contact, and its characteristic odor. In the ordinary blue mould, or Penicillium rot, the spread of the Bulletin 190. BROWN ROT OF THE LEMON. 11 decay from fruit to fruit in the box by contact is not common or abundant. Rather the condition shown in figure 5 occurs. Here is -***BJ 'i , .."' ; Mi ;.; i i.- ■* '# S ■■' i # | * 1 ) IP 1 s 1 i . ' J< i^ *. ' :/: ' - 5** _ . ■ 7^gg^ FIG. 5. Blue mould, or Penicillium rot, in box of lemons, without infecting others. One lemon affected seen a box of lemons in which one has the blue mould, but this has com- pletely rotted and melted down without infecting any of its neighbors, 12 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION. though the latter are smeared with the spores of the fungus and the decayed remains of the lemon. This is the usual condition in well- handled fruit in respect to this fungus. A lemon here and there which presumably has been bruised takes the Penicillium rot, but no spread by eontact occurs. Figure 6 shows a typical condition in a box of lemons affected by brown rot. Starting with one lemon the decay rapidly spreads to those next to it. from these to the next and so on, until the whole box is gone. This is the most destructive form of brown rot, because of the fact that a single affected lemon in a box is almost certain to spread the rot to the whole half-box in a short time. The decay resulting from brown rot is a very filthy, putrid one. It is characterized further by the pres- ence of innumerable small flies which invariably collect in swarms about such lemons, and breed in the decaying fruit. The maggots of these flies contribute very largely to the complete decomposition which takes place in affected lemons. The odor of brown rot in citrus fruit is most characteristic, and to one familiar with it serves to detect even a very small amount of rot in a large amount of fruit. A few affected lemons will betray the rot in a tent, while at a time of considerable prevalence the whole house smells of it. This odor is not a particularly repugnant one, but has a characteristic rancidity and penetration, originating presumably in a fatty acid produced in the decomposition of the oil in the rind of the fruit. The little flies mentioned above seem quick to detect this odor, and the first cases of brown rot in a tent of fruit are sufficient to attract these insects in a manner which is not the case with other forms of decay. Brown rot, then, is distinguished from blue-mould rot in a practical way by its rapid spread, peculiar odor and the presence of numerous small flies on the boxes containing affected fruit, even in closed tents. The experienced man seeking to detect the rot in his house goes through the tents, jarring the boxes for the flies and sniffing here and there for the odor. By these means he will detect even a surprisingly small amount of the trouble. OCCURRENCE OF BROWN ROT IN THE ORCHARD. Little has been said as yet as to the occurrence of this trouble in the orchard. It is common there in affected regions, however, on oranges, pomelos, tangerines, etc., as well as on lemons. It is indeed on orchard-affected fruit that we see the trouble more commonly in its pure condition, unmixed with other forms of decay. Our colored frontispiece shows typically brown rot-affected lemons from the orchard. Such fruit is found lying on the ground beneath Bulletin 190. BROWN ROT OF THE LEMON. 13 the trees, and also still hanging on the tree, usually within two feet of the ground, but occasionally higher. It occurs only in wet weather PIG. 6. Brown rot in box of lemons. Abundant spread by contact. or on low ground after irrigation, always more abundantly on the lower, heavier soils with more moisture, and has become extremely 14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. abundant in most of our lemon orchards .during the winter and spring season. On oranges the rot is commonly seen in the orchard only on fruit lying on or close to the ground in very wet places. The rot is par- ticularly liable to occur on trees standing in a "wash." in the orchard. where drainage water runs over the surface and keeps the ground beneath the trees bare, moist, and covered with sediment. It is easily seen that a considerable amount of fruit is affected while still on the tree and then falls to the ground. In this way many lemons are lost in bad cases, as in figure 7, where the fallen brown rot-affected fruit ran be seen on the ground. Fully a box of fruit to the tree is lost FIG. Brown rot in the orchard. Fallen lemons affected over bare, uncultivated ground. in some orchards every winter in this way. The affected fruit is not badly decayed, that is, it is not softened or disintegrated to any extent. The rind is still firm and the tissue sound. No appearance of mould or fungus is seen upon the surface. There is simply a slight discoloration, gradually extending from a center, of a light brownish and slightly purple color, not extremely prominent, but showing light on dark green fruit and dark on that which is yellow. Lemons of any size may be found affected on the tree, from the very smallest to those which are mat ure. The decay is not in any sense one of old or weakened fruit, but affects the youngest and most thrifty to an equal extent. The char- acteristic odor is distinguishable from the very first, and is practically identical in the different citrus fruits. Bulletin 190. BROWN ROT OF THE LEMON. 15 OCCURRENCE OF BROWN ROT IN THE PACKING HOUSE. At times of brown-rot prevalence there is always more or less orchard- affected fruit which escapes the pickers and is brought into the house. P^ ,,■;.,.,'.. - % ' MM ■I ? * I ■ B m W ^nn FIG. 8. Lemons with brown rot from boxes showing white growth of the Pythiucystis fungus. This has the same appearance as that seen in the orchard and is mostly thrown out at the washer by those through whose hands the fruit goes. FIG. 9. Contact infection by brown-rot fungus. If fruit in which brown rot is developing be examined after about a week's storage, the condition shown in figure 8 will be found. On the 2— bul. 190 16 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. surface of affected lemons a white, delicate, quite abundant mould develops, and it is natural to conclude that it is by the growth of this mould that the contact infection and spread take place. Where an affected lemon touches another the growth of the mould from fruit to fruit, as in figure 9, is evident, and in advanced cases, like figure 6. the same white mould can be seen advancing onto new fruit. Affected fruit iu the parking house soon becomes covered with the blue-mould fungus, which obscures any other characteristics, but when new the brown rot has a very distinct appearance from any other form of decay. The worst feature of the rot in the house is the frequent development of the trouble in great abundance in fruit which showed no si^n of the f f: r ^^^%^tK 1 EiR # ' ' FIG. 10. Lemons infected with brown rot by soaking in infected water. rot when washed and stored. Granting that an occasional lemon with orchard infection may escape the men at the washer and go into the tent, it is still a fact that by far the largest amount of brown rot in the tent develops in fruit which was perfectly green and sound when put up. Early in this work the writer has carefully watched, for hours at a time, the fruit coming from the washer, finding nothing but the finest, dark green sound lemons, and on examining the same fruit later found perhaps several affected lemons in each of a majority of the boxes in the tent. This, with the rapid spread by contact which begins jooii as the rot develops, has been the most serious and discouraging feature of the whole situation. The occurrence of* brown rol in this manner begins to show, under Bulletin 190. BROWN ROT OF THE LEMON. 17 average conditions, in about five days to a week from the time of washing and storing. The first sign of the rot is a dark, sunken spot on the surface of the lemon, like one of the spots shown on the fruit in figure 10. Not more than one spot on a lemon is commonly seen in ordinary cases. This spot rapidly spreads, a discolored area develops on the lemon, and, if there is moisture enough, the white mould appears on the surface. This produces the stage shown in figure 8, or that of the colored frontispiece if there is not moisture enough to develop the surface mould. Contact infection of more fruit begins at once, and the conditions shown in figures 8 and 6 result. Lemons infected by contact also show at first a dark colored spot at the point of infection. At the very first of infection of this sort the delicate white mould can be seen attaching itself to the lemon, growing from the one previously affected, and in many cases this can be wiped off, leaving the fruit apparently intact. In the majority of cases, however, and practically always after the dark spot has appeared, lemons on which the fungus has begun to grow develop the rot and can not be saved. THE CAUSE OP THE BROWN ROT. The brown rot is caused by a fungus, that which produces the white mould seen on the surface of affected fruit, as in figures 8 and 9. This fungus has proved to be a hitherto unknown form, and has been named Pythiacystis citrophthora* It is an active parasite of citrus fruit, readily causing infection of green growing fruit by means of its spores or by growth of the fungus itself from one fruit to another when in contact. DISCOVERY AND IDENTIFICATION OF THE PYTHIACYSTIS FUNGUS AS THE CAUSE OP BROWN ROT. In a case of this sort it is not sufficient to simply find a fungus of some kind growing in connection Avith affected fruit, in order to posi- tively identify the cause of the trouble. For the strictest proof in this respect the organism must first be found to occur regularly in affected tissues; it must, if possible, be isolated and grown in pure cultures, and above all it is necessary to produce the typical disease artificially by inoculating healthy material with the suspected organism in pure condition. When this has been accomplished we are safe in assuming that we have found the cause of the trouble. In the case of the brown rot, where the cause was entirely unknown at the beginning of the investigation, the probability was quite strong that the white-mould fungus which appeared on the fruit was the cause of the rot. On ♦Botanical Gazette 42:215-221, Sept. 1900. 18 IMVI'KSITY OF CALIFORNIA EXPERIMENT STATION. lemons free from other fungi it could plainly be seen that this mould was distinct from Penicillium, and the occurrence of contact infection E.H.S- FIG. 11. Section of lemon rind, showing brown-rot fungus. like that shown in figure 9 left little doubt that this fungus was the eause of the spread of the trouble. Examined with the microscope it Bulletin 190. BROWN ROT OF THE LEMON. 19 was found that the tissue of affected lemons, even when no mould was visible upon the surface, was full of the filaments of a fungus, and when kept in a moist chamber the typical mould at once developed upon the surface of such fruit. (Fig. 11.) Two significant facts were discovered early in the investigation. First, that if affected lemons were soaked in a pail of water for a few hours, then taken out and good fruit put in for a similar time, the latter would invariably become badly infected with the typical rot. (See experiments 1-19, 1-24, 1-31, 1-51, and checks in many others.) It was also found that if the water thus infected was boiled before the good lemons were put in, no infection resulted. (Experiment 1-24.) This was satisfactory evidence that some living organism was causing the brown rot. Second, it was found that if affected lemons were laid on moist soil or buried just beneath the surface, the same white mould soon appeared upon the soil. Good lemons laid on the same ground soon took the rot. (Experiments 1-30, 1-33, 1-66, 1-83.) All then that remained was to grow this fungus in pure cultures, identify it and produce the rot from cultures. This has been accomplished. After securing a pure growth of the fungus in culture media in flasks, it was found that by emptying such a culture into the pail of water instead of soaking affected lemons in it, typical brown rot resulted in lemons soaked in the water. (II-10a.) This completed the chain of evidence as to the cause of the trouble. The fungus, after much technical study, proved to be a form not previously known and was given the above scientific name and described in the journal cited. NATURE OF THE BROWN-ROT FUNGUS. Pythiacystis citrophthora is apparently in its original nature a harm- less and inconspicuous fungus inhabiting moist soils and water. But for its acquirement of this remarkable parasitism of the lemon it might very likely have never been discovered or attracted any more attention than many other microscopic, unimportant moulds which live ,m the soil. The Fungus on the Fruit. — As it occurs in the form of the white mould seen on affected lemons and causing contact infection, the most important feature of the fungus is the fact that it is entirely sterile, developing no spores of any sort, The fungus in the tissues of the lemon and on the surface consists simply of a mass of filaments of rapid, vigorous growth when supplied with moisture, but no spores. This is a most important fact. In a case like that of the blue-mould fungus the surface of affected fruit is covered with a fine dust which is composed of inconceivable numbers of the spores which propagate 20 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. the organism. The air, the dust, every box, lemon and part of the packing house carries an abundance of these spores to start the growth of the fungus when favorable conditions occur. Were this so with the brown-rot organism the case would indeed be serious, since the ability of the latter to attack sound fruit is so much greater than that of PenicUlium. But this is not the fact. The fungus on the fruit develops no spores, and there is absolutely no more infection of lemons after they leave the washer except by contact. The fungus simply grows from one lemon to another, starting from those which were already infected when put away, and no new lemons become infected except in this way. Many affected lemons may go into the tent in which the rot has not developed enough to show, but we have absolutely proven that the fungus produces no spores in the packing house and does not have the slightest means of spreading there except by vegeta- tive growth from lemon to lemon. It should not, however, be under- stood that, even by the closest examination, all affected lemons can be detected as they come from the orchard or washer. One of the worst features of the whole situation has been the abundant rot which often starts in the tent, in fruit which was apparently in the soundest, finest possible condition when put away. Such occurrences result from infection before the fruit is stored, but the rot does not develop enough to show to the eye for several days. Some further observations are to be noted in regard to this fungus, in addition to the description already published.* While the characteristics there given have since been repeatedly confirmed, we may modify one clause in the description to the following extent. The original reads, "Aquatic mycelium typically sterile, with occasional conidia or sporangia." Sporangia have since been found to develop in water or liquid cultures rather profusely in many instances when a week or two old. In these cases, however, the zoospores seldom develop normally, the proto- plasmic contents usually emptying through the pore without division, or remaining within the sporangium. The fungus grown on moist soil never fails to produce normal sporangia, which, when placed upon a microscopic slide, rapidly eject their swarmspores. That spores are ejected in the soil is evident from the empty sporangia on removal. In soil too dry to produce swarming the sporangia retain their vitality for a certain period after maturity, but after remaining in this condition for about a week they fail to function, even after prolonged soaking. As stated in the former paper, dilute prune juice appears to be the best medium for procuring the vegetative stage of the fungus, in which it develops as a vigorous. profusely branched, continuous, globular mass of mycelium, suspended in the liquid, i See Fig. 1 in paper above quoted.) A quick and sure way to obtain sporangia is to take a bit of mycelium from a prune culture and place in water in a shallow dish. Within 48 hours the fungus fringes out at the edges of the mass, changing from the thick vegetative type to slender threads and sporangiophores. Almost limitless variations from the typical shape of sporangia are to be found in soil and water cultures. (Fig. 12.) All the types illustrated have been found in soil, though most with less frequency than in water. Sporangia with two or three pores I Pig, L2, B, C, D) are common, though frequently only one of these opens for 'See Botanical Gazette 42:215-221, Sept. 1006. E.tt.S. FIG. 12. Sporangiophores and sporangia of Pythiacystis, showing range of form. -- UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. the passage of the spores. Intercalary sporangia are rarely found (Fig. 12, E and I) with all gradations between this and the long- water sporangiophores as figured in A. figure V2. The length of some of the latter is extreme, especially when developed from affected lemons in deep water, one measured being over 1.6 mm. In all media a slight swelling at the base of the sporangiophore is quite character- istic, which may be so developed as to present the appearance of a series of nerve ganglia. i See K. Fig. 12 and others.) The manner in which the sporangium joins the sporangiophore varies widely. In many cases the sporangiophore widens toward the junction, which we at first took to be characteristic. (Fig. 15.) In the majority of later cultures, however, the sporangiophore remains of uniform width at the tip. In rare instances the sporangiophore has been seen to grow on into the empty sporangium from the point of insertion, as De Bary records for some species of Pyrhium (Fig. 13. K), but never to develop further than as figured. There is the most gradual gradation possible here between De Bary's "conidia' - and the sporangia with zoospores. (Fig. 13, D- J. ) The two organs are apparently identical in development up to a point where ceriain conditions cause them in the ease of conidia to germinate from the pore (which is always formed), while other conditions previously described bring about a rounding off of the protoplasm into zoospores. A conidium on germinating will sometimes produce a true sporangium within a few microns of its tip. (Fig. 12, F and L.) Most of the figures of germinating conidia were drawn from those which had developed on an affected lemon floating in water. It is under these conditions that the conidia develop most freely, the sporangia being seldom found. In pure cultures in tap or distilled water, if at all fertile, sporangia are more commonly developed than conidia. most of the latter, however, being imperfect in their formation of swarmspores as before stated. In the search for the reproductive organs of Pythiacystis the numerous sprouts which are at first more or less globular have been rather misleading. (Fig. 13, A-C. ) Innumerable variations as to shape are found among these, especially in old water cultures. From old seemingly dead mycelium new branches will push out in this way after a period of inaction, without a change of medium, though the growth in this case is spindling and not long maintained. As yet no evidence has been found of a sexual stage in the life history of the fungus. Various means have been tried in the attempt to develop this stage, and a careful watch kept on the fungus in its natural environment. We have satisfied ourselves that it does not occur as an economic or otherwise important factor, or that if present at all it does not assist the fungus to live through periods of drought. It is interesting to note in this connection that on several of the older prune cultures a crusty white layer developed, one-eighth inch or less in thickness, with very much the appearance and brittleness of lard which has cooled on the surface of a liquid. At first this was thought to be an impurity, but a layer of the same kind has since been formed on cultures known to be pure. The formation has been noted to some extent. After the flask becomes filled with the fungus, which then comes to the surface of the liquid, a thin surface scum is formed which grows up slightly on the sides of the flask. This is a network of slender mycelial threads, on which tight balls of mycelium develop, with a beginning much like the so-called bud formation. < Fig. 13, B.) The completed layer is a close, sclerotium-like mass of mycelium. We shall investigate further along this line. As previously stated, this fungus grows only on very moist media, most of our work being done with liquid or wet soil cultures. If placed on a solid medium, such agar-agar or potato agar, the fungus may spread through the medium to some at, potato agar seeming to be the most favorable of the solid media. In this the organism will occasionally produce sporangia of a degenerate type. The Modi of Sport Formation and Infection. — It was particularly important in this investigation to determine accurately the manner and place in which the primary infection of the fruit takes place. Such E.H.S. FIG. 13. Vegetative bud formation and conidia of Pythiacystis. 24 I-NIYKKSITY OF CALIFORNIA— EXPERIMENT STATION. knowledge must underlie any logical attempt to devise means for checking the occurrence of brown rot. Fortunately, this has been accomplished, and the results of the investigation in this direction have proven of very great prac- tical value. It has been found, as will be explained, that there are two modes of infection, in the orchard and in the washer, and methods have been devised by which each of these may be pre- vented, or controlled to a very large extent. ORCHARD INFECTION. The fact has already been mentioned that an affected lemon allowed to lie on the ground soon spreads the fungus to the soil and makes it infectious to sound fruit laid upon the surface. This demonstrates that the fungus is able to live on the soil and cause infection by contact, just as it grows from one lemon to another. It leads further to the supposition that the soil of the orchard must be a breeding place of brown rot, on account of the large number of affected lemons which lie and decay on the ground under the trees. Further than this, however, we have found that it is in the soil that the spores of the fungus are produced, and that this is the original source of all the infection of lemons except that wdiich takes place by contact. The Pythiacystis fungus is by nature a soil dweller. Its growth on the lemon is, in a sense, an abnormal condition. Its spores, as in other fungi of the same class, are produced only in abundant moisture, a FIG. 14. Brown-rot fungus, with sporangia, in wet soil. After Smith & Smith, Bot Gaz., loc. cit. Bulletin 190. BROWN ROT OF THE LEMON. 25 condition which is not found on the surface of a lemon in air. Laying an affected lemon on wet soil, or on moist paper, or placing a mass of the fungus from a culture in a similar situation, we soon find the spore condition developed in great abundance. This is of the rather com- plicated type known as sporangia with zoospores. The fungus grows out into the soil producing the visible mould on the surface, and amongst this are found numerous rounded bodies of microscopic size, which are the spores or sporangia. (Fig. 14.) Ordinary fungus FIG. 15. Showing development of svvarmspores from sporangia in the brown- rot fungus. In the sporangium on the right the swimming swarmspores are escaping. After Smith & Smith, Bot. Gaz., loc. sit. spores are simply little round bodies which germinate by sending out a sprout wherever they happen to be lying, if moisture and other favor- able conditions are present. These sporangia of fungi of this type, however, have a different process. They develop only when in water. When proper conditions come, the inside of each one divides into about thirty smaller spores or zoospores. These minute bodies are of a nature more animal than vegetable in their appearance and action. Each is provided with two fine, hair-like appendages which lash vigorously about in water and enable the spore to swim about with all the appear- ance and activity of animal life. (Fig. 15.) They soon come to rest, lose their power of movement, and, if conditions are right, each 26 UNIVERSITY OF CALIFORNIA— EXPERIMENT STATION. one germinates, sending out a sprout which grows into a filament and produces the brown-rot fungus again. (Fig. 16.) This. then, is what occurs in the orchard. Lemons affected with brown rot fall to the ground. The fungus grows out into the soil and produces its mycelium or mould growth, which, when there is abundant moisture, as in we1 weather, develops great numbers of the primary FIG. 16. Germinating swarmspores about six hours after coming- to rest spores or sporangia among the particles of the soil. These lie until they become decidedly wet, then break up internally into a large number of swimming zoospores, which move about freely in Avhatever moisture is present, finally coming to rest and germinating to produce the fungus again. It is then not difficult to conceive of what happens in regard to infection of lemons on the tree with brown rot. With each succeeding occ urrence of the disease the soil becomes more and more infested with the fungus. In wet weather and particularly in low. wet ground, the Bulletin 190. BROWN ROT OF THE LEMON. 27 organism is active in the soil and develops a great abundance of sporangia. These lie mixed with the soil, forming a part of the finer surface portion. In heavy rain they spatter up with the dirt onto the FIG. 17. Experiment in preventing brown rot by covering ground under tree with burlap. lower limbs and fruit of the tree. Zoospores are formed in the moisture, occasionally on the surface of lemons, and these germinate and produce infection. Thus a certain amount of rot occurs on the tree during the wet season, and on account of the nature of the process it is confined mostly to the lower portion of the tree. It is very easv to demonstrate that the soil under the trees in infected 28 UNIVERSITY OV CALIFORNIA — EXPERIMENT STATION. orchards is full of infection. This was determined in our investigation bv soaking good lemons in water containing soil from such places. In the brown-rot season a small amount of soil taken from under a tree where the fruit is affected, and placed in a bucket of water, will cause abundant infection of almost every lemon which is thereafter soaked in the water. Such soil, indeed, has been found to be the most favorable material for infection and disinfection experiments, producing 100% of brown rot almost invariably, and a large number of separate infec- tions on each lemon, as in figure 10. (Experiments 1-83, 1-96, and all later disinfection experiments.) For a practical demonstration of the fact that infection by brown rot does come from the ground under the tree in the manner described, the experiment illustrated in figure 17 is quite convincing. In this case four representative trees in an orchard badly affected with the rot were chosen, and a covering of burlap stretched under each tree on a wooden frame about six inches from the ground. This was in February, when the rot was very prevalent on all the trees chosen for the experiment and much of the bottom fruit had already been lost. A number of similar trees were chosen as checks. All lemons affected with brown rot were picked from all the trees in the experiment and from the ground beneath them. Ten days later all the affected fruit was again removed, in order to get all that had been infected before the experiment started. Had the experiment been begun before the large amount of fruit removed in these two pickings had become infected, the results would have been even more striking. During the period from February 8 to April 15 two brown-rot lemons developed on each of two of the burlaped trees, none on the other two. In each case these two lemons were some which hung low, out over the edge of the burlap, and were not protected from the ground. No lemons became affected over the burlap, and none at all on the other two trees. Thirty- nine check trees surrounding the others (being all of which records were kept) showed brown-rot lemons, respectively, as follows: 8, 11, 14. 10. 14, 15. 12, 28, 19, 16, 19, 21, 21, 19, 9, 33, 10, 15, 12, 14, 24, 23, 17. 11, 20, 26; 38, 17, .4, 31, 26, 33, 31, 20, 27, 27, 33, 46, 53, or an average of over 20 per tree. It should also be noted that the amount of brown rot on these check trees was decidedly reduced by the greater or less covering of the soil under them with cover crop and weeds. In every case those showing a comparatively small amount of rot had a considerable amount of soil covering, which had an effect of the same nature as that of the burlap in preventing infection. Trees under which the ground was nearly bare fill showed from 30 to 50 brown rots, and also had lost very .ilv l^-fore the experiment began. This experiment substantiates the statement that orchard infection with brown rot comes entirely from the ground. Bulletin 190. BROWN ROT OF THE LEMON. 29 INFECTION IN THE WASHER. It has already been stated that one of the most puzzling features of the brown-rot situation has been the great prevalence of the rot in the curing tent, in fruit which when put up seemed perfectly sound and free from the slightest sign of decay. It is admitted that some orchard- infected lemons must get into the tents when the trouble is abundant. This could hardly be otherwise, when lemons in all stages of infection come in, with the affected spots not particularly conspicuous and some of them very small. A great deal is thrown out by the men at the washer, but naturally some escape them. Without question, too, there are times at which a large amount of fruit is brought from badly infested orchards which has been so recently infected that it does not show the presence of the rot to the eye at all. This fruit looks perfectly sound, and only after several days in the tent does the fungus develop enough to show itself. This often happens. But a common, and indeed the worst, occurrence of brown rot can not be accounted for in this way. At times, infection and spread by contact develop in the tents when no rot whatever is showing in the orchard, usually in early summer after the ordinary brown-rot season is over, and in amounts out of all proportion to the apparent orchard occurrence, whatever the season. This has proved to be infection from the washer, and we have demonstrated beyond a doubt that thousands of boxes of sound lemons have been infected with brown rot in the washing machine in the packing houses, that the rot has been enormously spread in this way, and that in this lies the principal, most general, and most easily prevented source of infection by brown rot. In dumping the fruit into the washer tank, box and all are submerged, if possible, in order to float the lemons . into the water gently. With them go leaves, twigs, blossoms, the dust and dirt of the orchard and considerable soil which clings to the bottom of the boxes- where they have rested on the ground. At the end of a day 's run there is a large amount of sediment of this sort in the bottom of the tank. Knowing the nature and habitat of the fungus, as explained above, it is. easy to see how the wash tank may become a prolific source of infection of sound fruit with brown rot. The orchard soil and dust are full of the sporangia of the fungus. When dumped into the washer zoospores are produced in great abundance, become attached to the lemons, and start an infection which does not become visible until several days later, after the fruit has been stored. One of the chief problems early in the investigation was to discover how the wash water became infected, since the fact that it was infectious w T as soon discovered. It had been demonstrated that affected lemons soaked in water for some little time made such water infectious, but the small number of affected lemons which get into the washer and the short time that they stay there seemed to preclude any general UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. infection from thai source. The discovery of the spore-producing habit of the fungus in the orchard soil first revealed the real manner by which the spores of brown rot get into the washer. Definite experiments have proven that fruit can and does become infected in the tank in this way. By putting a shovelful of orchard soil into the 1,000-gaIlon tank of a washer, and then running lemons through in the ordinary way for washing, it was found that from 10% to 40% of the fruit took the rot. (As to the infectiousness of the ordinary wash water, see experiments 1-12, 1-36, II-3 and 11-16. On the infection of the washer with soil, see IV-1, IV-11, IV-14 and several following. Also Fig. 25.) Xot only does the fungus become introduced from the orchard into the washer in the manner described, keeping the water continually infected, but experiments have shown that Pythiacystis may also come to inhabit the machine permanently if not destroyed by thorough clean- ing or disinfection. It has seemed in many cases as though the fungus was being bred in the washer itself, rather than being continually introduced anew with each day's run. This condition has been sug- gested especially by the fact that the rot would suddenly and completely stop appearing in the fruit after a machine had been very thoroughly cleaned, dried out for some time, and painted inside. Again, fruit of the same lot would rot much more badly when washed in one washer than in another. To test this we have experimented by soaking various parts of the machine in water with sound lemons. Besides the metal tank, solid frame work and brushes, these washing machines contain various pieces of cloth, sheepskin and other padding to protect the fruit from bruising. Soaking this padding in water with good lemons. we have been able to produce brown rot in considerably large amounts m the fruit. This therefore indicates that the fungus is able to grow and propagate in the machine more or less, attaching itself to materials like those mentioned above which are favorable for its development. The brushes also become quite foul, and accumulate large amounts of material which would permit the growth of Pythiacystis. The whole operation of washing lemons under ordinary conditions is peculiarly favorable to infection with a fungus of this sort. (See experiment 1-41 on infection from washer padding.) THE CONTROL OF BROWN ROT. To the practical lemon man the all-important question is. How shall prevent or control this trouble in an economical and practical manner? This problem we have attempted to solve as fully as that of the nature and habits of the organism causing the brown rot, but the former depends upon the latter. Bulletin 190. BROWN ROT OF THE LEMON. 31 With our present complete knowledge of the modes of infection by brown rot, it is not difficult to see the lines along which we must proceed to look for methods of completely checking the trouble. We have seen in the above account of the nature of the rot that there are three very distinct sources or places of infection of lemons, and no more. These are : in the orchard, in the washer, and by contact. Upon the extent to which the fruit can be protected from infection at these three points depends absolutely the extent to which the rot may be eliminated from our lemons. CONTROL IN THE ORCHARD. Since the infection with brown rot, or the spores of the fungus which produce it, originates primarily in the soil of the orchard, it is self- evident that if we could exterminate the fungus at that point we would succeed in checking the rot absolutely by that means alone. If this be not practical in its entirety, it is at least possible to aid very materially in eliminating brown rot by orchard methods. This is to be accom- plished, according to our results, by two general methods: first, by checking the development of the fungus in the soil ; second, by prevent- ing the infection of the fruit on the tree by spores from the ground, by chemical, cultural, or other means. In • connection with this we have made considerable study of the life and distribution of the fungus in the soil. The organism is doubtless a native of our soils in wet ground, but might not be expected to be common or easily found under ordinary circumstances. We have never succeeded in finding it in any soil out- side of citrus orchards. Our method of soil testing for this fungus has been to place in a bucket of water about a pint of the soil to be tested. Sound lemons were then soaked in the water for various lengths of time, then put away in a box in a curing tent. With soil where the fungus is abundant every lemon will in a week's time show very numerous infections, as in figure 10. According as the fungus is less abundant less infection will result. A very small amount of badly infested soil is sufficient to produce very abundant infection. Pythiacystis is most abundant in the soil directly under the trees, where the fungus is maintained by the affected fruit which falls each year upon the soil. The distribution or extent of the occurrence of the fungus in the soil away from the trees is governed entirely by moisture conditions, and in fact its whole developmeift depends upon a large amount of moisture, and too much dryness is fatal to it. In the wet winter season soil taken from anywhere in the orchard in affected regions is highly infectious and full of the fungus. The same is true of soil which washes from the orchard, even where it is carried for considerable distances. In depth the soil becomes infested for at least 4% feet, as soil down to that distance has been found to infect lemons. (For experiments on infec- 3— bul. 190 32 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. tiousness of soil from different depths, locations, etc., see 1-83, 1-96, IY-17. IV-28, IV-30, IV-31, IV-32, V-6, V-7.) As the dry summer comes on the soil away from or between the trees gradually loses its infectiousness, and after it has thoroughly dried out lemons may be soaked in water with any amount of it without producing brown rot. Samples of the most infectious soil taken in winter and allowed to dry show the same change. A soil which produces 100% of infection and very numerous infections on every lemon, becomes entirely innocuous after several weeks' drying. (Experiments 1-80, 1-87.) In the orchard the activity and distribution of the fungus keep on narrowing and lessening with conditions of increasing dryness, until, in soils which are not kept too wet by irrigation, the organism may die out altogether on the surface. Similarly in depth the fungus recedes with the moisture, leaving the upper soil, and remaining alive only farther down where there is more dampness. Near ditches and flumes, where the ground keeps wet, or in low ground where irrigation floods the surface, the fungus often remains active all summer and may produce infection of fruit on the trees in such situations. In the fall, when rain occurs again, Pythiacystis again begins to spread. The soil becomes infectious first on' the northwest side of the trees, where there are most shade and moisture. Soil from this location always produces infection before that from the other side of the tree or that between the trees has any effect. The rot also begins to show first on fruit on the tree at this point. As more rains occur and the soil becomes saturated with moisture, more and more rot occurs on the trees on all sides, 'the affected fruit falls to the ground and is scattered about, rains wash the surface soil through the orchard, the fungus grows vigorously in the wet ground, and soon the whole body of soil becomes infested and infectious again. After w T orking out quite fully these features in the life history of our fungus, with the results described, it seemed very clear that in orchard practice anything which would tend to stir up and dry out the surface soil in summer would be of practical value in checking brown rot. We therefore have urged the practice of thorough summer cultivation, especially under the tree, and have made extensive observations as to the results of this practice. In most lemon orchards no attempt whatever is made to cultivate the ground beneath the trees. The plowing and cultivation go up and down and across the rows, working the soil between the rows in both directions, but leaving about each tree a square of soil which receives no cultivation whatever. This shows quite plainly in figure 7. Upon this soil the brown rot-affected lemons drop and decay. With them are the leaves and rubbish, accumulation of sediment washed in by the winter rains, lemons decayed by other causes than brown rot, Bulletin 190. BROWN ROT OF THE LEMON. 33 and all the various refuse materials which accumulate in such a place. Moisture remains here near the surface late in the season and more or less all summer. The soil is never broken up or aerated, and, in short, presents ideal conditions for the development and propagation of the brown-rot fungus. Theoretically, to thoroughly break up the soil under the tree and keep it cultivated during the dry season, to mix up the old surface layer with fresh soil and let it dry out, and to clean out the accumulated debris and decayed matter of years should be a valuable means of keeping down the brown rot. Practically, the growers who have done this, thoroughly and persistently, have obtained excellent results. This does not mean that, in orchards where brown rot is abundant and the soil under the trees has been undisturbed for years, a single stirring of the ground will stop all the rot. Neither will cultivation under the trees during the wet season have much effect on the develop- ment of the trouble that season, as some have seemed to expect. The practice, to be successful, must consist in a thorough and rather frequent breaking up and stirring of the soil well under the trees, commencing in spring after the rains cease and continuing all through the summer. If this is accomplished the surface soil under the trees is pulverized and dried out to a considerable depth, access of air is permitted, and the fungus is killed. Instead of remaining active near the surface all summer, ready to start into more active growth and infect all the lower fruit on the tree with the first rains or even a heavy irrigation, the fungus in the infested soil is more and more dried out with each culti- vation and subjected to the most unfavorable conditions for its develop- ment. By continuing the operation all summer and keeping the ground well stirred, brown rot the next winter can be checked, as shown by actual experience, to a very decided extent. We know of no case where the practice has been made of cultivating under the trees all through the summer, where the amount of brown rot as compared with neighbor- ing orchards not so treated has not been very markedly reduced. In certain districts some of the very best and very worst orchards in respect to the rot are situated practically adjoining each other, and the only difference to which the absence of rot in the former can be ascribed is the fact that they are cultivated under the trees in summer while the others are not. (See experiment 11-17, A and B.) Methods of Cultivating Under the Trees. — These recommendations, we are aware, are somewhat ill-adapted to the style of pruning most commonly seen in many of our best lemon orchards, where the lower branches are allowed to spread down upon the ground with little attempt at cutting out. A large amount of fruit is produced on these low branches, where it can be easily and cheaply picked, and we would not be understood as unreservedly condemning this style of tree. The 34 UNIVERSITY OF CALIFORNIA — FXI'FRI.MFNT STATION. grower must use some judgment in this respect. Our strongest point would be, however, that even with the tree very close to the ground it is possible to stir up the soil under the branches much more thoroughly and closer to the trunk than is commonly done. Even if the lower limbs are dragged and the lowest fruit scratched a little, it is better to carry out this most useful method of checking brown rot, rather than to lose all the lower fruit wuth the rot when winter comes, and allow the fungus to flourish unchecked in the soil to be carried into the house. With trees like those shown in figure 18, where the limbs just touch the ground without lying prostrate upon it, it is an easy matter to %£ \ «J0 ^whU 4 k%M l fr ^_; *^m '."..-••■■ : ' .* •■'''• ''■'-. ■.-> *• "■■■•'•■ -V "-."■.■■.■'' - "' , \ A A tfSS - £$£2* J ySM £MBgBj$^ 8§T, |iHfl|BS||a }: *&Sk V* *" %$iy *"H'jf| ' *> -. . >' v v'->; t-'jy* $&t s : ^SfrSSSfiP* 1 ?-^^--?£aW'£^ % *3£^PMui^J,«8'W *fttl jit* >► dTL W jml ~MJ«jM5 ■ -\ .^MJt^l ^Br ^vVffSv' Tfl\flfcfiBPr^C>C-''^Si 'wul ^Afri'ti lMmt%tinhk :i '' * "'-'>,,■ -**-••■ «^* <^flHH FIG. 18. Low-pruned lemon tree, which can still be cultivated under. cultivate sufficiently under the tree. This condition may be maintained by slight pruning at the bottom, with heavier cutting at first if the limbs are low. One of the most efficient means of cultivating under the tree known to the writer is a spring-tooth harrow drawn by a considerable length of chain. After the ordinary cultivation of the orchard between the rows in both directions, a team is attached to this contrivance and the trees are "zig-zagged," the team going to the right of the first tree in the row, as close to it as possible, then swinging to the left and .going on that side of the next tree, then to the right again, and so on down the row, coming back on the opposite sides. As the team passes around each tree the harrow, on its long chain, is pulled through under it. stirring and breaking up the ground in the corners left untouched in Bulletin 190. BROWN ROT OF THE LEMON. 35 the ordinary cultivation. This can be clone with no serious damage with trees which look very low on the ground. There are also on the market several cultivators of more or less excellence, made for the special purpose of getting under the trees. In many cases growers have profitably had the ground under the trees broken up by hand, using mattocks or hoes, especially in cultivating for the first time ground which had long laid undisturbed, or for getting farther under than could be done with a team. As a general cultural proposition, without regard to brown rot, the breaking up and aeration of these dead, decay breeding, uncultivated areas is highly desirable. For destroying the rot fungus, the more the ground under the trees- can be stirred during the summer the better. The work should begin in spring, before the ground becomes hardened, and an effort be made all through the summer in connection with the usual cultivation to stir and work the ground under the trees and get it thoroughly dried out and stirred as deeply and as often as possible. The theory is very simple: Brown-rot fungus lives over summer entirely in the orchard soil. It is killed by dryness. Cultivation produces dryness. Chemical Treatment of the Soil.— After discovering the soil habit of the fungus, and the fact that it is carried over the summer entirely in the ground under the trees, the idea presented itself that it might be feasible to kill the rot-producing organism in the soil by applying some fungicidal substance or liquid. The possibilities in this direction have been tried out quite thoroughly, but with no very promising results. While it is entirely feasible to kill the fungus in this manner, the amount of liquid required to saturate the soil to a depth sufficient to be of any value, or the amount of any dry substance necessary to cover the surface heavily enough to keep down the fungus, is so great that the expense and labor involved prohibit the use of methods of this sort. Figuring the depth and area of soil necessary to be treated, the cost of material which would be required in an orchard of any size, with the labor of applying it, would amount to more than the value of the fruit. The experiments along this line were made principally with copper sulphate, formalin, lime and sulphur. Experiments with Copper Sulphate.— U sing small boxes of soil in which brown rot-affected lemons had been mixed, it was found that good lemons laid on the surface soon became affected with the typical decay. When boxes of soil thus prepared were saturated with a strong solution of copper sulphate, the fungus and the power of infection appeared to be completely destroyed. Good lemons laid on such treated soil remained sound indefinitely, if not destroyed by other forms of decay. The solution used in these experiments contained 3% copper UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. sulphate and was applied in sufficient quantity to thoroughly saturate the soil. This strength is far above what could be used in a practical way in the orchard, both on account of the expense and the injurious effect on the soil. Experiments with the use of 1-10, 1-12, 1-100 and 1-200% copper sulphate showed little or no retardation of the rot. See experiments 1-30, 1-65, 1-66.) In experiments on orchard soil it was found that very large amourits of water were required to saturate even an inch or two of the surface. On rather dry ground 500 gallons per tree was not sufficient to give results of any promise in regard to thorough saturation. Furthermore, it was found that the neutralizing •effect of the soil upon the sulphate solution was such as to make the minimum strength required to kill the fungus much greater than that found effective when the organism or its spores were in water. It is in fact practically impossible to sterilize the soil to any depth by this means, except by the use of enormous quantities of material. We have therefore given up all thought of the use of this method. (See experi- ment 1-90.) Experiments with Formalin.— The strength of this disinfectant is not neutralized by the soil to the extent that copper sulphate is thus affected. With small amounts of soil, as in boxes, it is possible to completely exterminate the brown-rot fungus, but in orchard practice this is out of the question. Experiments with Lime. — It was found that a heavy covering of air- slaked lime has a decided influence in checking the rot fungus in affected soil. To have the desired effect, however, it is necessary to put on a solid covering an inch or more in depth all over the surface, and keep this undisturbed. No way has appeared of doing this economically and practically. The effect on the soil of the amounts of lime which would soon accumulate from the continued applications necessary is also an objection to this treatment. Experiments with Sulphur. — A coating of sulphur on the soil has shown some effect in keeping down the fungus in box experiments, but nothing of value for actual practice. From the results of these various experiments in soil treatment it will be seen that the only apparently practical method of checking the rot by such means lies in thorough summer cultivation of the ground under the trees. We lay particular stress on this matter because it represents a rational method of ordinary cultural practice and a desir- able improvement in the present practice in many orchards, as well as a means of overcoming the brown rot to a very marked extent. In our mind it is the highest achievement in plant pathology to demonstrate Bulletin 190. brown ROT OF THE LEMON. 37 methods which in themselves represent improved practice from the cultural standpoint, whereby definite pathological difficulties may at the same time be overcome. The results attained where this method has been properly carried out have been so satisfactory that we have not thought it necessary or advisable to go further with investigations having in view the saturation of the soil with substances injurious to plant life, even though superficially the latter may show results of value. THE PREVENTION OF INFECTION IN THE ORCHARD. We have previously stated that brown rot may be controlled in the orchard either by eliminating the fungus from the soil or by protecting the fruit from infection, and have discussed the chemical and cultural means of attaining the former end. While it is true that the practice of cultivation under the trees in summer has succeeded in entirely eliminating the rot in a number of cases, and always shown most satis- factory results, yet in connection with this method, or where it has not been, or for any reason can not be carried out, it is very desirable to find every other possible way of preventing infection of the fruit. Even though the fungus be present in the soil, it is possible to largely reduce the infection which ordinarily takes place. Pruning Up the Trees. An effective, though not entirely practical means of controlling brown rot, is found in pruning up the trees so that none of the fruit hangs within two or three feet of the ground. (See Fig. 19.) We have experimented somewhat along this line and found that in the worst infected orchards trees pruned up to the latter height showed no more rot. This simply amounts, of course, to cutting off the lower fruit instead of leaving it to become infected. There is a practical benefit, however, in greatly lessening the amount of infected fruit which is carried into the washer and tents. It therefore will pay, to a moderate extent at least, to prune up the trees and get the fruit up off the ground. In low, heavy ground, where it is difficult to prevent surface flooding around the trees in irrigation and where the water from the rains settles and stands, we believe it will pay the grower to adopt a taller, higher style of tree and keep the fruit up at least two feet from the ground. This will save an immense amount of trouble from such places in wet weather. Considerations of pruning and cultivation under the tree naturally go together. In cases of bad rot in the orchard we recommend radical treatment in both directions, trimming the trees up quite severely and stirring the ground under them in summer very thoroughly and fre- 38 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. FIG. 19. Lemon trees pruned high. quently. The limbs may then be allowed to gradually come down again until the condition is reached which seems to be the most desirable under the circumstances. As said above, a large amount of cheaply handled fruit is produced near the ground ; the grower must decide, according to the extent of his trouble with rot or his success in handling it by other means, what policy he shall adopt as to pruning. The Use of Cover Crops as a Means of Preventing Orchard Infection. One of the earliest orchard observations made in connection with this work was the fact that on trees under which the ground was covered with a green growth in winter there was much less rot than over bare ground. Carried further, and observed more carefully, it was found that on the same tree the fruit on a limb hanging over bare ground would rot badly, while that over a clump of weeds or other growth remained unaffected. When fully appreciated and looked for particularly, in lemon orchards in all the different sections, the impor- tance of this discovery became evident. It was thought at first, theoretically, that weeds or cover crops under the trees in winter would favor the rot by holding moisture. Practically this is not the case, but just the opposite is true. Invariably where there is much growth on the ground the rot occurs on limbs hanging over bare ground, and only to a very limited extent over the covered ground. With a knowledge of the nature of the fungus, the fact thus established is readily Bulletin 190. BROWN ROT OF THE LEMON. 39 ft ' Jfy . V"- ' '■■"■■' •v>;\ ' .4 ,. -^ , : '] ■ '""■' '■'"» ' ■ : ; — ; — — — sati. FIG. 20. Sound lemons hanging over cover-crop of vetch. The previous winter this same tree with the ground bare was badly affected. explained. It is simply that the growth covers the ground and prevents the dust and dirt, with the spores of the fungus, from getting on the fruit. The experiment of tenting under the trees with burlap, shown in figure 17, was devised with the demonstration of this particular point in mind. By this means, as already described, infection was entirely checked, and the fact demonstrated that orchard infection takes place by spores coming from the ground under the tree. It is not probable that any grower will see his way clear to cover the ground under his lemon trees with burlap or other similar material, though, if he could do this, it would be with the absolute assurance that all orchard infection with brown rot would be prevented, no matter how badly the soil was infested. The fact is a positive one, that any covering between the soil and the lower limbs of the tree prevents brown-rot infection to the extent that it actually covers the ground. The whole area of the orchard does not need to be covered, as infection apparently takes place only from the soil directly under a lemon by dirt spattering up with the rain, except in the case of large, thick trees, which retain a large amount of moisture. After observing the effect of weeds on the ground in keeping down the rot, the most natural thought was of cover crops, another oppor- 40 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. tunity of making good cultural practice do double duty. If a chance growth of weeds would show such decided effects, it seemed reasonable that much more would be gained from a regular, sowed crop. With this in mind growers have been urged to sow cover crops in their lemon orchards, especially under the trees, and the results have been entirely in accord with the earlier observations. These crops have naturally been more or less successful, both as cover crops and as rot preventives, and the fact has been very plainly established that such a covering will prevent infection in proportion to the amount of growth which the crop makes under the trees during the season when the rot is active. A heavy, solid growth early in winter, under trees which had formerly lost all the bottom fruit, has almost entirely eliminated the trouble, lemons close to the ground remaining unaffected where over bare ground they would never reach half size. (See Fig. 20.) As to the kind of crop to be used, the time of sowing, method of handling, etc., we would simply say that the only consideration so far as brown rot is concerned is to get a good heavy growth all over the ground, under the trees, early in winter. To accomplish this, the crop must be planted early, in September or October, and irrigated, if necessary, to produce a rapid growth. Under the trees the seed must be sown by hand, and may have to be raked in to some extent. As to the crop to be sown, there is some latitude of choice, and no one plant can be named as preeminently the best under all conditions. Of the leguminous plants in common use as cover crops the vetch, Canada pea, fenugreek and native bur-clover have been used in our experiments and observed in many orchards. The pea, used alone, is rather too coarse and spreading a plant to cover the ground as completely and quickly as is desirable, and does not grow as w^ell in the shade as some of the other plants. It may be used mixed with other seed to thicken and support the growth. Fenugreek grows poorly in the shade and is easily set back by dryness, so that it does not seem desirable for this purpose. Bur-clover is somewhat capricious in its nature, requiring an early start and plenty of water to cover the ground as early in the season as is necessary to keep down brown rot. It grows well in the shade under the trees. Vetch has the same requirements, but does very well under the tree and when well grown makes an ideal ground cover- ing. If started not later than October and given water enough (which must be done with any crop to get a thick growth); this may be regarded as the best plant for our purpose. In most of our lemon sections there is already sufficient information on hand to indicate what crop will best answer the requirements for that particular region. To cover the ground completely, well under the trees, and early in winter before the rainy, brown -rot season is too far advanced, is the only object which is of any value to this method. FIG. 21. Ideal summer condition of lemon orchard for brown-rot control. Lower branches just touching the ground, and soil beneath well cultivated. FIG. 22. Ideal winter condition of lemon orchard for brown-rot control. Heavy crop of vetch under the trees. Trees need not be pruned so high. 42 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. Ill brief, then, we recommend as one of our major conclusions in brown-rot control, the sowing of a cover crop in lemon orchards in the fall, planting the crop for this purpose especially under the trees. Naturally the crop will also be planted all over the ground for the ordinary purposes of a cover crop. The plant to be used must be one which makes a good winter growth and a thick, dense covering on the ground in as short a time as possible, particularly in the shade under the trees. Vetch and bur-clover seem to be among the best plants for this purpose, especially the former. For good results the crop should be planted in September or early October, sowing by hand under the trees, and the ground then furrowed out so that it can subsequently be irrigated. If fall rains are lacking, the ground should be irrigated and the cover crop pushed along as fast as possible. If this is done, a good ground covering can be obtained by the time that the rot begins to be abundant in January. If possible, the crop should be left on the ground until the rainy season is nearly over. After plowing-in the crop summer cultivation should begin as already described. Figures 21 and 22 show the ideal summer and winter condition of a lemon orchard for controlling brown rot. We have considered only the leguminous crops, owing to their value as soil enrichers, as well as the benefit to be derived from them in suppressing brown rot. There are cases, however, where a sowing of barley or any quick-growing, vigorous crop is of great value in control- ling brown rot in the orchard. The worst cases of rot are usually in places where there is a wash through the orchard, the ground being kept bare and moist by surface water running down from farther up the slope. There are also cases where, for one reason or another, it is impossible to get a leguminous cover crop started and properly grown in time to keep down the rot. In such cases a sowing of barley will often be most useful. By this means a quick growth is obtained, which prevents the water washing over the surface to quite an extent, keeps the ground covered under the trees, and aids very materially in pre- venting infection by brown rot. In badly affected orchards the first aim of the grower should be to prevent any washing of water over the surface, and to get a green covering of some kind on the ground, espe- cially under the trees, before the heavy rains come on. The bare slopes, where the water of every storm washes over the surface unrestricted, and the low, wet ground where water accumulates and stands, are the principal breeding places of brown rot in the orchard. Tin: Use of Artificial Coverings o.\ the Ground for Preventing Brown Rot Infection. The decided success obtained by covering the ground with green crops, and by the burlap method, suggested the possibility of spread- Bulletin 190. BROWN ROT OP THE LEMON. 43 ing some other substance under the trees to protect the fruit from infection. It is undoubtedly true that a layer of any substance which would cover the ground thoroughly would answer the purpose and keep down rot effectively, the only question being that of economy. The only substance which has suggested itself as being at all cheap and abundant enough for this purpose is straw. This in some cases can be obtained cheaply enough to make feasible a heavy covering under each tree in the orchard. Figure 23 shows an experiment of this nature. Four trees were chosen where rot had been abundant, and all affected fruit removed, as with the burlap trees. The ground under FIG. 23. Control of brown rot by covering ground under tree with straw. each tree was thoroughly covered with straw, using about a bale to the tree. During the remainder of the winter only two lemons became affected with brown rot on the straw-protected trees, and these were on limbs which hung out over bare ground. The record of the 30 unprotected check trees is given on page 28, in connection with the burlap experiment, showing from 4 to 53 brown-rot lemons per tree, with an average of over 20. There is no question that this is a most effective measure, and in bad cases of orchard infection, where the straw can be obtained at any practical figure, we strongly recommend its use in the winter season, both in connection with summer cultivation, and, if possible, with the cover-crop method. The amount of straw 44 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. available is the only limitation. If the ground can be solidly covered, brown-rot infection can be absolutely prevented. There