UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE CONTROL OF DOTHIORELLA ROT ON AVOCADO FRUITS W. T. HORNE AND D. F. PALMER BULLETIN 594 JULY, 1935 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA CONTENTS PAGE Cause 3 Local nature of the trouble 4 Time of development of rot 4 The character of dothiorella rot 4 Sources of infection 6 Method of penetration by the fungus 6 The Fuerte avocado and dothiorella rot 7 Attempts to prevent dothiorella rot by means other than by spraying . 8 Experimental methods 8 Spray experiments for the control of dothiorella rot, 1931 to 1934 .... 9 Suggested spray program for dothiorella rot 14 Spray treatment without fumigation 14 Spray treatment with fumigation 14 Combination treatment for dothiorella rot and mottle-leaf 14 Summary 15 Acknowledgments 16 THE CONTROL OF DOTHIORELLA ROT ON AVOCADO FRUITS 12 W. T. HORNE 8 and D. F. PALMER 4 A decay of avocado fruit caused by the fungus dothiorella 5 was early observed by H. S. Fawcett and others, but its importance became evi- dent only with the growth of the California avocado industry. It is not the most widespread, nor is it the most rapidly destructive or most offen- sive form of spoilage. However, certain features, as pointed out further on, have made it peculiarly troublesome. It has at times occasioned se- rious economic losses in the affected districts, both by losses of fruit and through discrimination against fruit from certain districts regardless of whether it was affected or not. Early efforts at control based on experi- ence with citrus fruits were not successful. CAUSE Dothiorella is a comparatively common fungus of wide distribution. It was studied by H. S. Fawcett in connection with a large canker on trunks and limbs of English walnut in California, and the disease which it produces was called melaxuma (black sap). In this case, cankers in the wild willows growing nearby were found to be an important source of infection. The fungus is also frequently found in dead twigs of the walnut. In New York it was studied as the cause of a serious disease of cultivated red currants, which produced cane blight. It is sometimes important in moist seasons or situations, causing a stem-end rot of citrus fruits, with two other fungi which produce simi- lar effects, Diplodia natalensis Evans, and Phomopsis citri Fawcett. In the avocado the dothiorella fungus is found on decaying fruits, in the dead areas of tipburned leaves, and in the bark of dead twigs. i Received for publication May 8, 1935. 2 Paper No. 320, University of California Citrus Experiment Station and Grad- uate School of Tropical Agriculture, Riverside, California. 3 Associate Professor of Plant Pathology and Associate Plant Pathologist in the Citrus Experiment Station. 4 Agricultural Inspector, San Diego County. s Botryosphaeria ribis chromogena Grossenbacher and Duggar, which before the discovery of the perfect stage was called Dothiorella gregaria Sacc, and which has been referred to, probably incorrectly, as D. ribis. [3] 4 University of California — Experiment Station LOCAL NATURE OF THE TROUBLE Dothiorella rot is only one of a number of decays which attack the avo- cado, and except in certain places where it is severe, it is of no greater importance than various other types o,f rot. It probably occurs through- out California wherever avocados are grown, but merchants have expe- rienced serious losses only from fruit grown in certain coastal areas. A Fig. 1. — Lower end of a Fuerte avocado showing the light dots or flecks and speckles. This fruit does not have dothiorella rot or other aggressive form of decay. moist climate and tipburn of the leaves appear to be highly important predisposing conditions. TIME OF DEVELOPMENT OF ROT Dothiorella rot does not usually develop on sound fruits hanging on the tree or on the fruit after it is picked until softening begins. In typical cases with Fuerte fruits at the stage known as "breaking" (the first de- tectable softening) , small dark spots with vague boundaries appear. Not all the dothiorella spots develop at the breaking stage, but new areas continue to make their appearance as softening progresses. The fact that the firm fruit shows no sign of the trouble and that no method is available to distinguish those fruits which are to develop the decay from those which are not destined to develop it, gives opportunity for serious trouble between grower and merchant. THE CHARACTER OF DOTHIORELLA ROT Dothiorella rot spots may be seen when less than % inch in diameter. At this stage they are light umber, rather vaguely bounded, and not Bul. 594] DOTHIORELLA ROT ON AVOCADO sunken nor distinctly marked. They often spread out from a speckle or other blemish as a somewhat paler margin. Figure 1 shows speckles, none of which are developing dothiorella spots, while figure 2 shows dothiorella spots, some of which are spreading out from speckles. As the spot becomes more distinct, it darkens slightly but is not black in the early stages. 6 Speckles might be confused with dothiorella rot; they are Fig. 2. — A Dorothea fruit with typical dothiorella rot; notice eye-rot at the stem attachment. dark and sunken and rarely more than y 1Q inch in diameter, and, unless some complication appears, involve only the skin of the fruit. The dothi- orella spot, by contrast, may become % inch in diameter within 3 or 4 days. The latter is circular, not sunken, and is rather uniformly col- ored. If the surface is scratched, it is found to be softer and more easily broken than the sound surface. If cut, most of the rot will be found to be in the skin, and if the flesh is at all affected, only the outermost part shows a slightly watery condition. From this stage the spot spreads more rapidly, becomes soft and somewhat sunken and uneven, while a watery rot spreads rather slowly into the flesh. A rank odor develops. Sometimes the rotting fruit fades out to a greenish gray. The surface settles and becomes uneven. Later, the whole fruit shrivels up and becomes dark. If a bit of the decaying flesh is put in a drop of water, it is easily crushed, and under the com- 6 Color of the rather well-established spots on Fuerte fruits by Ridgway's color chart is approximately mummy-brown to bone-brown. The later, softer phase may be raw umber to light brownish olive or lighter. 6 University of California — Experiment Station pound microscope shows strong branching fungus threads of varying breadth. The seed is not ordinarily affected. At a wound or at the stem attachment, dothiorella rot may appear earlier than on other parts of the fruit and penetrate more deeply and rapidly. When it attacks the stem attachment, it produces a form of eye-rot 7 (fig. 2). "While the rot shown in figure 2 is mainly superficial, some odor will have penetrated the flesh, and utilization after this stage will be disappointing. SOURCES OF INFECTION When the decayed fruits become old and dark, the surface develops nu- merous small pimples at the tops of which minute spore masses may ap- pear as drops or coils. These spore masses emerge from fungus pockets (pycnidia), and similar structures are formed in the dead areas of tip- burned leaves and in the bark of dead twigs. If the surface of an af- fected twig is lightly shaved so as to cut off the tops of the prominences, white dots may be seen, each dot surrounded by a black ring. In rainy or wet times, spores emerge from these pycnidia and are freely washed and spattered about. Spores of another sort are also formed which may be shot to a distance of about % inch. While these spores are compara- tively large and heavy, they still may be carried considerable distances in a driving rainstorm. It is believed that the spores are mainly of rather local distribution, and that they drift very little in dry air, in this way being unlike the excessively light and dusty spores of citrus blue mold. Spores on fallen leaves and twigs probably do not readily get up into the tree so that in our opinion it is unnecessary to remove the mulch. An additional light cover of straw or other material should make it entirely safe. Dead twigs should be removed periodically, which can best be done at the time of the principal leaf drop in the spring and again in the autumn. METHOD OF PENETRATION BY THE FUNGUS Once a spore is deposited on the moist fruit, it germinates by sending out a tube-like thread; this grows down through one of the openings in the skin by which the fruit is supplied with air, and into one of the air spaces below. These air spaces are marked by the yellow flecks of the avocado skin, and in them the fungus makes considerable growth, but is unable to attack and destroy the living skin until the time of softening of the fruit. Preparations for microscopic study 8 clearly show the fungus in the 7 Eye-rot is any rot which spreads from the point of attachment of the fruit stem ; it may be caused by various organisms, but most of these are less capable of becom- ing established on the sound skin than is dothiorella. s Preparations used were made by Dr. Ira Ayer, of Carlsbad, California. Bul. 594] DOTHIORELLA ROT ON AVOCADO open spaces below the surface of the skin of sound avocado fruits (fig. 3). Protection of the fungus in these spaces will explain the difficulty of preventing the rot by treating the surface of harvested fruit with fun- gicides. THE FUERTE AVOCADO AND DOTHIORELLA ROT Fuertes in the coastal area bloom through a long period; in fact, some flowers can be found at almost any time. The principal time for blossom- Fig. 3. — A vertical section of the surface of a Fuerte fruit showing a stoma or breathing pore with the surrounding cells more or less broken down (a) and the air spaces in the tissue below (h, b). Mycelium of dothiorella fungus has grown into the open passage of the breathing pore and into the air space below. (Figure drawn somewhat diagram- matically from a photomicrograph kindly made by Dr. Ira Ayer, of Carlsbad.) ing and setting fruit is from March to May, but many fruits are set at other times; these are "off-blooms." They tend to be small and ill-shaped and do not mature with the principal crop. The most skillful growers can now judge fairly well the state of maturity of a Fuerte fruit, if it can be seen in a good light, but selective picking is troublesome and diffi- cult. Many off-blooms in the past have become excessively senile, so that they failed to soften normally when picked. In these cases, dothiorella rot often developed early, and the whole surface was rotted before the inner flesh was fully soft. The foregoing statement is based on general observations, but careful study of the softening process and its failure with senility of fruit is lacking. There is a growing opinion that it would be better to cut down and destroy the off-blooms at about the time the main crop sets. There is reason to believe, from experience, that fruit at 8 University of California — Experiment Station its earliest satisfactory stage for harvesting softens more perfectly, and that dothiorella rot develops later and less actively on it than on fruit which has become gra}^ with age and has lost its resilience. Specific evi- dence is furnished by the difference in behavior of the two lots in the spray experiment of 1934 (page 12) . ATTEMPTS TO PREVENT DOTHIORELLA ROT BY MEANS OTHER THAN BY SPRAYING Early in these studies, avocado fruits were submitted to various treat- ments which should destroy spores on the surface of the fruit. None of these was effective. Fruit from dothiorella areas may be wet with strong alcohol and flamed without preventing the rot, and pieces of the skin planted in culture media shortly after treatment will give cultures of the fungus. Treatment with nitrogen trichloride gas (Agene) conducted with the aid of L. J. Klotz, and with and without the vacuum process, failed to prevent the rot, although some applications were severe enough to speckle the fruit. Prevention of this rot is evidently a problem unlike that involved with blue and green-mold rots of citrus fruits. Control by preventing tipburn and by removing dead twigs presents difficulties concerning the evidence secured, so that we do not have fig- ures to present, although measures of this kind are doubtless of the greatest importance. EXPERIMENTAL METHODS We ourselves sorted the fruit at the Calavo Growers packing-house, usually assisted by an experienced member of the Calavo staff. Injuries and blemishes due to physiological breakdown were disregarded. These include cuts, bruises, wounds, scald, speckles, carapace spots, and others. Many doubtful cases were studied further at the laboratory. Decays other than dothiorella rot were comparatively few so that error from misjudging the kinds of decay spots was not important. Many spots of true dothiorella rot appear late and develop so slowly that some of them have done no serious harm until the fruit is spoiled by oversoftness (natural overripening). Most of the fruits classed as "clean" which were studied further, later developed some dothiorella, but usually this was not of much practical importance. Fruits from the check plots classed as "clean" suffered more on longer keeping than fruits from the sprayed plots. In general, the advantage of the better lots of fruit increased on keeping. This is probably a true result and related with the manner of dothiorella infec- tion. Experiments with Fuertes from the Citrus Experiment Station at Riverside, where dothiorella is so rare as to be negligible, showed that BUL. 594] DOTHIORELLA ROT ON AVOCADO 9 some infection might be caused by spores getting on the fruit at harvest, but that the number of infections is so small as to be unimportant in ordinary marketing. In such experiments as those in which young fruits were disinfected and bagged at different stages to find the earliest size at which infection occurs, the results were believed to be approxi- mately correct. However, if all possible error is to be avoided, special methods of handling would need to be employed. The infections pro- duced by inoculating after picking appeared late and developed rather slowly. The good quality of fruits in the better lots, especially in 1934, and other results difficult to express in figures, indicate that the evidence given by the tables is not unduly favorable to the treatments. SPRAY EXPERIMENTS FOR THE CONTROL OF DOTHIOREKLA ROT, 1931 TO 1934 With the completion of four years of experimental work on the control of dothiorella rot, enough data have now been collected so that certain conclusions of a practical character may be drawn. The experiments were carried on in a commercial manner, in that the orchards used were of an average grade in regard to their condition of thrift and care. The spraying was done by commercial operators. The fruit was picked in a commercial manner and shipped by the usual common carrier to the Calavo house at Los Angeles. Here it was handled in the same way as other shipments except that it was allowed to soften in the packing- house instead of being distributed while firm to the various markets. Determinations were made on the fruit in an early-soft condition. These experiments were conducted upon trees of the Fuerte variety ranging in age from four to eight years. They were started in 1931, at a time when very little was known about the disease. The first problem was to determine whether or not it could be controlled by the use of a fungicide. For this reason, a maximum number of applications on plots S-l and D-l were made the first year (table 1) . The first application was made before the bloom had opened and at two-week intervals, or after each rain or sprinkle until the bloom was over. Thereafter, applications were made monthly until the fruit was matured and picked. These plots received ten applications in all, the first one having been made April 30, 1931. Plots S-2 and D-2 were treated before the first bloom had opened and at two-week intervals, or after each rain or sprinkle until the bloom was over. Plots S-3 and D-3 were treated once only, that treatment being given after the bloom. Each of these plots was composed of three trees. 10 University of California — Experiment Station All fruit was picked on November 30, 1931, the earliest date at which the fruit had attained a satisfactory oil content. By the time the 1932 experiment (table 2) was laid out, it was ap- parent that the insect problem would be of considerable importance TABLE 1 1931 Experiment tor Control of Dothiorella Rot Material Plot No. Number of treat- ments Total number of fruits Fruits with slight or no dothiorella Fruits with pro- nounced or severe dothiorella Per cent of fruits with dothiorella fs-i* 10 98 98 i S-2 [ S-3 4 1 101 146 93 104 8 42 8 29 [D-l 10 118 111 7 6 Monohydrated copper sulfate \ D-2 4 103 77 26 25 [ D-3 1 104 38 66 63 No treatment (basin irrigation) check 154 80 74 50 No treatment (overhead irriga- f check 54 11 43 80 tion) \ check 40 11 29 70 In plots lettered "S," the treatment was applied as a spray; in plots lettered "D," as a dust. TABLE 2 1932 Experiment for Control of Dothiorella Rot Material Bordeaux 4-4-50. Zinc sulfate, 8 lbs Hydrated lime, 8 lbs- Flotation sulfur, 6 lbs. Casein spreader, 1 lb... Water, 100 gals Copper sulfate, 8 lbs... Hydrated lime, 8 lbs... Flotation sulfur, 6 lbs. Casein spreader, 1 lb... Water, 100 gals No treatment Plot No. f S-l* I S-2 I S-3 j S-4 S-5 ( S-6 ' S-7 S-8 S-9 S-10 [ S-ll S-12 < S-13 [ S-14 check Number Total of treat- number ments of fruits 97t 36 6 21 110 152 62 23 J: 07 Fruits with slight or no dothiorella Fruits with pro- nounced or severe dothiorella Per cent of fruits with dothiorella * "S"=Treatment applied as a spray. t The fruits from plots S-l and S-2 were combined. t The fruits from plots S-12, S-13, and S-14 were combined. BUL. 594] DOTHIORELLA ROT ON AVOCADO 11 in avocado-growing districts. Where latania scale 9 was present, fumiga- tion with hydrocyanic acid gas was found necessary. As this material is not compatible with bordeaux or the copper compounds, the necessity for a copper substitute was apparent. For this reason attempts were made to substitute zinc sulfate for copper sulfate, since some investiga- tors reported good results from the use of this material as a fungicide. In districts where the avocado red mite is a pest, it is necessary to use sulfur in some form for its control. For this reason sulfur was added to several of the formulas. During this season there was a demand on the part of the growers for a simple method for determining in the field the approximate amount of dothiorella that could be expected to show up in the fruit at the time of softening. It was found that by making an inspection of an orchard and paying careful attention to the following points, a very close estimate could be made : (1) amount of tipburn, (2) number of dead twigs in the trees, (3) amount of dothiorella infection in the tipburned leaves and dead twigs, (4) amount of off-bloom, (5) method of irrigation, (6) insect and other injuries, (7) fungicidal material used, and (8) maturity of the fruit. From a commercial standpoint, we decided that it would not be prac- tical to make more than three applications of a fungicide in a single sea- son, and for that reason three applications were selected as a maximum number to be used. All fruit was picked as soon as a satisfactory oil con- tent had been attained, which was on November 16, 1932. Owing to a short crop of Fuertes in 1932, the number of fruits har- vested in this experiment was not sufficiently large to be satisfactory for drawing very definite conclusions. However, the 1932 experiment strongly indicated that the addition of sulfur increased the fungicidal value of bordeaux and that zinc sulfate had some promise as a fungi- cide in the control of dothiorella rot. For this reason, these materials were included in the formulas used in the 1933 experiment (table 3) . The fruit in this experiment was not picked until February 12, 1934, since it had not attained a satisfactory oil content until that date. The 1933 Fuerte crop was again light, and the quantity of fruit examined was rather small. The lime-sulfur which was used as an insecticide for the control of avocado red spider showed marked fungicidal value. For this reason flotation sulfur applied as a fungicide was tried in the 1934 experiment (table 4). 9 Latania scale, Aspidiotus lataniae Sign. 12 University of California — Experiment Station Since the addition of sulfur to bordeaux had given 100 per cent con- trol of dothiorella rot in 1932 and 1933, it was again used in the 1934 experiment. In an attempt to find out when infection in the sense of penetration of fruit actually takes place, 100 fruits which had attained a diameter of TABLE 3 1933 f Experiment for Control of Dothiorella Eot Material Plot No. Number of treat- ments Total number of fruits Fruits with slight or no dothiorella Fruits with pro- nounced or severe dothiorella Per cent of fruits with dothiorella Zinc sulfate crystals, 8 lbs... . Fresh hydrated lime, 8 lbs Wettable sulfur, 6 lbs Casein spreader, 1 lb Water, 100 gals Zinc sulfate crystals, 16 lbs.. Fresh hydrated lime, 8 lbs.. .. Wettable sulfur, 6 lbs Casein spreader, 1 lb Water, 100 gals Copper sulfate crystals, 8 lbs ] Commercial bordeaux, 16 lbs \ Casein spreader, 1 lb ( Water, 100 gals J Commercial bordeaux, 16 lbs 1 Wettable sulfur, 6 lbs [ Casein spreader, 1 lb I Water, 100 gals j Copper sulfate, 8 lbs ) Commercial bordeaux, 16 lbs Wettable sulfur, 6 lbs Casein spreader, 1 lb | Water, 100 gals J Dry lime-sulfur, 6 lbs 1 Casein spreader, 1 lb Water, 100 gals S-l' S-2 S-3 S-4 S-5 110 118 32 !>4 115 check 4S 292 47 244 25 6 14.5 2.5 0.0 2.0 18 * "S" = Treatment applied as a spray, t Fruit not picked until Feb. 12, 1934. iy 2 inches were dipped in 70 per cent alcohol and each covered with a paper bag to prevent further infection. These fruits were bagged on August 2. The fruits in this experiment were picked in two separate lots, the first lot on November 17, and the second lot 41 days later. The object of this was to determine differences in the amount of infection in early- harvested fruit as compared with that picked at a later date. Fruit from Bul. 594] DOTHIORELLA ROT ON AVOCADO 13 the check plot of the first picking developed 40 per cent of rot, while that of the second picking gave 70 per cent of rot. Fumigation experiments were conducted by II. J. Quayle to deter- TABLE 4 1934 Experiment for Control of Dothiorella Rot Material Plot No. Number of treat- ments Total number of fruits Fruits with slight or no dothiorella Fruits with pro- nounced or severe dothiorella Per cent of fruits with dothiorella Zinc sulfate crystals, 16 lbs... Fresh hydrated lime, 8 lbs Flotation sulfur, 6 lbs Blood albumin spreader, 6 oz. No. 3 tank-mix oil, y% gal Water, 100 gals s-r Zinc sulfate crystals, 16 lbs Copper sulfate crystals, 1 lb... Fresh hydrated lime, 8 lbs Flotation sulfur, 6 lbs Blood albumin spreader, 6 oz. No. 3 tank-mix oil, \i gal Water, 100 gals S-2 Zinc sulfate crystals, 32 lbs Fresh hydrated lime, 16 lbs Flotation sulfur, 6 lbs Blood albumin spreader, 6 oz. No. 3 tank-mix oil, J^ gal Water, 100 gals S-3 Commercial bordeaux, 16 lbs Flotation sulfur, 6 lbs Blood albumin spreader, 6 oz... No. 3 tank-mix oil, }/$ gal Water, 100 gals Flotation sulfur, 10 lbs Blood albumin spreader, 6 oz... No. 3 tank-mix oil, Yz gal Water, 100 gals J No treatment S-4 S-5 Fruit dipped in alcohol and bagged when fruit had reached 13^ inches in diameter Check Bagged fruit 200 211 200 176 164 410 72 153 47 179 :V2 190 176 157 221 24 15 8.0 0.5 4.0 54.0 1.0 'S" = Treatment applied as a spray. mine the compatibility of cyanide with the various materials used in this experiment. The results were as follows : 1. Zinc sulfate sprays were found to be compatible with cyanide. 2. The addition of 1 pound of copper sulfate to each 100 gallons of zinc sulfate spray did not increase its incompatibility markedly. 14 University of California — Experiment Station 3. The flotation sulfur was incompatible with cyanide and consider- able damage resulted in fumigation over trees sprayed with this mate- rial. It is thought that the incompatibility was due to substances other than the sulfur present in this particular spray preparation. It is also felt that the incompatibility would not be of a residual nature as is some- times the case with the copper compounds. 4. Fumigation over trees sprayed with bordeaux did not result in as serious injury as was expected. However, the experiments were not ex- tensive enough to be conclusive, and serious injury might result from further attempts under different conditions. SUGGESTED SPRAY PROGRAM FOR DOTHIORELLA ROT Spray Treatment Without Fumigation. — Where no fumigation is con- templated, there should be two applications with the following formula, the first application to be made when the fruit is about 1% inches in diameter, the second application to be made about two months later : Commercial bordeaux (or 4^4-50 homemade bordeaux) 16 pounds Wettable sulfur 6 pounds Blood albumin spreader 6 ounces Water 100 gallons Spray Treatment with Fumigation. — Where fumigation is contem- plated, it should take place as soon as the fruit attains the size of l 1 /^ inches in diameter. Fumigation should be followed at once with an application of wettable sulfur used without the bordeaux; at least one subsequent application of this spray material should be made about 6 to 8 weeks later. The formula for use follows : Wettable sulfur 6 pounds Blood albumin spreader 6 ounces Water 100 gallons Combination Treatment for Bothiorella Rot and Mottle-Leaf. — Treat- ment to prevent dothiorella rot may be combined with treatment for mottle-leaf. The first spray should be applied when the fruit has at- tained a diameter of about 1% inches, and the second about two months later. If fumigation is practiced, it should precede the first spraying. The formula recommended for this dual purpose follows : Zinc sulfate crystals 16 pounds Copper sulfate 1 pound Fresh hydrated lime 8 pounds Flotation sulfur 6 pounds Blood albumin spreader 6 ounces Water 100 gallons BUL. 594] DOTHIORELLA ROT ON AVOCADO 15 SUMMARY Dothiorella rot can be completely controlled by the use of fungicides. Liquid fungicides are more effective than those applied as dusts. Dothiorella rot is increased by overhead irrigation. Copper sulfate is more effective than zinc sulfate in controlling dothi- orella rot. The fungicidal value of copper sulfate when used as bordeaux is greatly increased by the addition of sulfur. Sulfur alone when applied as a liquid with spreader is a very effective fungicide against dothiorella rot. The fungicidal value of zinc sulfate is increased by the addition of a small amount of copper sulfate. Copper sulfate when added to zinc sul- fate in small amounts apparently does not interfere with fumigation. For the control of dothiorella rot, zinc sulfate alone, that is, without copper sulfate, must be applied at such strong concentrations as to make it impractical from a commercial standpoint. It has been indicated that the fruit is not infected until some time after a diameter of l x /2 inches has been reached. Therefore, early spray- ings are not as important as applications made after the fruit is fairly well developed. The most effective spray for the control of dothiorella rot is the bor- deaux and sulfur mixture prepared according to the formula given on page 14. The next most effective fungicide is wettable sulfur used alone. Either of the first two formulas recommended on page 14 also give satisfactory control of the avocado red spider. If the fruit is picked as soon as possible after it has attained a satis- factory oil content, infection will be greatly reduced, particularly on unsprayed trees. Spray recommendations for the control of dothiorella rot must be based upon the combination of insects and diseases for which control measures are necessary. 16 University of California — Experiment Station ACKNOWLEDGMENTS The writers desire to acknowledge the helpful interest of Dr. L. D. Batchelor, Director of the Citrus Experiment Station, and of Dr. H. S. Fawcett, head of the Division of Plant Pathology in the Citrus Experi- ment Station, without whose support the work could not have been car- ried out; and of Mr. R. R. McLean, Agricultural Commissioner of San Diego County, who provided the materials and equipment used in the experiments. Mr. James D. Hoffman, especially in the earlier years of these studies, assisted generously in making trees and various facilities available. The Calavo Growers of California cheerfully furnished fruit, facilities, and assistance to the full extent of our requests. The Univer- sity Agricultural Extension agents and the agricultural officials, and many growers and friends of the avocado industry, have helped in va- rious ways. llm-8,'35