CALIFORNIA AGRICULTURAL EXTENSION SERVICE CIRCULAR 67 October, 1932 The Diamond Canker Disease of the French Prune in California RALPH E. SMITH Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. B. H. Crocheron, Director, California Agricultural Extension Service. THE COLLEGE OF AGRICULTURE UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA Digitized by the Internet Archive in 2011 with funding from University of California, Davis Libraries http://www.archive.org/details/diamondcankerdis67smit THE DIAMOND CANKER DISEASE OF THE FRENCH PRUNE IN CALIFORNIA EALPH E. SMITHi A disease of the French ( Agen) prune (Prunus domestica) which has apparently never been described in scientific literature forms the subject of this paper. The name diamond canl?:er is derived from the fact that Fig. 1. — Typical case of diamond canker on a twenty-year-old French- prune tree. the cankers or outbreakings in the bark of affected trees show a tendency toward a diamond shape (fig. 1) . This is particularly marked in cankers on old limbs where bark growth is not very active (fig. 6) . Professor of Plant Pathology and Plant Pathologist in the Experiment Station. California Agricultural Extension Service [CiR. 67 DESCRIPTION The gross symptoms of the disease consist in a pronounced thickening of the bark upon the trunk and main limbs. This thickened bark has a rough, black, corky appearance and has a tendency to split in a longi- tudinal direction and to a certain extent crosswise, especially at the F}g. 2. — Typical diamond canker. bases of twigs and over pruning cuts (fig. 3) . In this w^ay the diamond- shaped cankers are formed. The thickening and cracking of the bark proceeds upward on the limbs in such a regular manner as to suggest that the disease is of a systemic nature, that is, that the causative agent spreads internally through the tissue. At all the pruning cuts, spur bases, and wounds, the normal callus (wound-healing) tissue becomes thickened and cracked and even at the lenticels a protrusion of corky tissue develops (fig. 2). On the older branches the bark between the 1932] Diamond Canker of the French Prune definite cankers becomes more or less abnormal, swollen, hardened, or dead. After a tree is once affected with diamond canker the diseased condition usually spreads completely through it so that certain trees in the orchard show the trouble in an advanced stage while the surrounding ones look normal. If the disease begins near the base of one limb above the main crotch it may develop extensively in that branch before the rest Fig. 3. — Typical diamond canker at pruning cuts. of the tree becomes affected. Diamond canker resembles scaly bark (pso- rosis) of orange trees in this and other respects (see Fawcett and Lee) ." Thrifty young suckers often come out in abundance from around the main forks of prune trees affected with diamond canker. In such suck- ers, when they grow out directly from the thick, corky, diseased bark, symptoms sometimes develop which suggest even more strongly a sys- temic disease. In these cases, if the suckers are allowed to remain on the tree until they are two or three years old, the bark at the base where it joins the thick, diseased bark of the main limb may be decidedly thick- ened and show swelling and longitudinal cracking in the typical man- ner (fig. 4) . Further out on the sucker the bark shows a more superficial 2 Fawcett, H. S., and H. A. Lee. Citrus diseases and their control. 582 p. Mc- Graw-Hill Book Co., Inc., New York. 1926. 6 California Agricultural Extension Service [CiR. 67 roughening. On cutting into the bark at this point it is found that there is an abnormal thickening of the cork with a somewhat waxy texture. Still further out, the superficial bark layers show a longitudinal streak- ing or mottling in color and a tendency to flake off in a scurfy condition with narrow, longitudinal, purplish lines (fig. 5.4). These symptoms Fig. 4. — Diamond canker on base of a three-year-old sucker taken from trunk of old diseased tree. usually extend in a more or less continuous streak on one side of the twig and may be visible to a slight extent almost to the extremity of the current season's growth (fig. 5B). There can be little doubt that they are the effect of the same disease which causes the thick, corky, out- growths on the trunk and main limbs. They have every indication of having developed by direct, continuous, systemic progress in the peri- derm, or outermost tissues, of the bark. Upon old trees, especially on the larger branches where growth is not very vigorous, the condition shown in figure 6 is sometimes observed. This perennial-canker-like con- 1932] Diamond Canker of the French Prune dition was at first thought to be the effect of a different disease. Further observation has shown, however, that all intermediate stages and con- ditions between those shown in figures 2 and 6 can be found on the same trees, and that the type shown in figure 6 probably results from a weak- ened condition of old trees where the abnormal bark tissue dies before Fig". 5. — Ay Diamond canker on two-year-old portion of French-prune sucker, showing scurfy bark and purple streaks. B, Blister-like swellings on bark of young shoots from diamond-canker French-prune tree. it can completely cover that underneath. This is the type of the disease which particularly suggests the name diamond canker. Also on such older trees may be found the condition shown in figure 7, where an ex- cessive thickening of the bark occurs at isolated spots on some of the limbs. Diamond canker is not a virulently destructive disease, and in fact many affected trees continue to live for almost the normal time with the disease in a chronic condition. Usually, however, the tree becomes gradually weakened, borers and wood-decay fungi attack the affected limbs and trunks, and the tree gradually succumbs (fig. 8) For several California Agricultural Extension Service [CiR. 67 Fig. 6. — Diamond canker on old French-prune tree. (Photograph by H. J. Baade.) 1932] Diamond Canker of the French Prune 9 years, while the tree is in the earlier stages of the disease, its production is actually improved both in quantity of fruit and size of the individual prunes. The effect is like that of girdling. For this reason it is difficult to persuade growers to remove affected trees in the earlier stages of diamond canker. It is doubtful if an affected tree ever recovers from this disease. In almost every case the tree gradually weakens and either dies from secondary causes or lingers along in a stunted condition. Fig. 7. — A large canker on a secondary branch of an old. French -iprime tree. Anatomical study of diamond canker shows that the abnormal tissues consist entirely of bark and arise from an excessive formation of cork. The condition is practically the same as that in the normal process in rough-barked trees. Successive layers of cork develop, pushing out- ward the ever-increasing mass of tissue. The thick, rough, cankered covering of trunks and limbs consists of tough, leathery cork heavily impregnated with waxy material. The section shown in figure 9 repre- sents a one-year-old twig such as those shown in figure 55; the black 10 California Agricultural Extension Service [CiR. 67 areas correspond to the little bark protuberances. By sectioning at intervals from the base of such a sucker toward the extremity, areas or strands of diseased tissue may be detected extending near the surface throughout the length of the sucker. ^^^i'"' M'* "•^' ^m ^ ^^i«», ««««w».^>— *^*^ ,- sp^-^W^ Ni^^^^-*^\^^^" * **"., ■^-..•iJr- «^ * ■^ Mg. 8. — Typical effect of diamond canker in gradually weakening a Frencli- prune tree. Some of the surrounding trees have been removed on account of this disease, which in this case developed in a typical, definite area in the orchard at this point. The surrounding trees are healthy. DISTRIBUTION Diamond canker is most abundant in two areas of central California about equally distant to the north and south respectively from San Francisco Bay, where prunes are extensively grown. The disease pre- vails to a serious extent in some parts of these districts, orchards being found in which more than 50 per cent of the trees are affected. It is a common idea that such orchards occur mainly on shallow, poor soil. While this is true in many cases, it is certainly far from being a general rule. In addition to the districts mentioned, diamond canker is not uncommon in a few sections immediately adjoining them. In the other 1932] Diamond Canker of the French Prune 11 prune-growing districts of the state the disease is extremely rare. It has never been observed in the San Joaquin Valley and only in a very few instances in the Sacramento Valley, although French prunes are grown extensively in these districts. HISTORY Diamond canker has attracted attention only within comparatively recent times. The first mention of the disease occurred not much over fifteen years ago. It seems that previous to about 1915 the disease had been observed only in a few isolated trees. Since that time it has developed much more extensively, mainly in new plantings. Until recently the idea was very common that diamond canker was exclusively propagated and distributed by one nursery through the belief that trees of this type represented a valuable sport, or strain, of the French prune. ^ This idea originated from the fact that the nursery alluded to actually did, according to the statements of reputable per- sons, propagate nursery trees with buds taken from diamond-canker trees, but without realizing that they were diseased. It would appear from the history of the present affected orchards that previous to 1915 there existed, here and there, in some of the older French-prune orchards, an occasional tree suffering from a chronic form of diamond canker. Such trees, even though they were somewhat smaller in size, produced more regular crops and larger prunes than the normal trees. A close study of figure 5B will show the possibility of bud transmission of diamond canker, since affected bark may be seen close to the base of many of the buds. About 1925, diamond canker began to be evident in a large and increasing number of young French prunes which had been planted after 1915. Many of these younger trees had been obtained from the nursery mentioned, and the idea then became current that all the diamond-canker trees came from that one nursery. Very little investi- gation is needed, however, to prove that this is not true. In fact, it is self-evident that not enough buds could have been obtained from one or two of these old, stunted, diamond-canker trees to propagate any considerable amount of nursery stock. Trees from many different nurseries have developed diamond canker and the evidence is conclusive 3 Baade, H. J. Diamond canker in Napa County. California Cult. 71:494. 1928. Baade, H. J. Diamond canker situation. Napa County Farm Bureau Monthly 10:3. 1930. Baade, H. J. Diamond canker in prunes. California Cult. 75:125, 139, 1930. Davis, J. E. [Diamond canker.] Pacific Eural Press 113:207. 1927. 12 California Agricultural Extension Service [Cir. 67 that the increase in the disease during the last twenty years has not been due, to any extent, to propagation with buds from affected trees or distribution by any one nursery. ^^^^ ^HHBgl^S^^^fly ,^^J^'*^(f- ^xMi. ^^^^^^^^kC' /'^^^^^^SrSS^Bfc^ M^^^^^^^^^^^^^'^^^^^^M^^^ ^^'" ''^^^m B^^^r^^^^ " ^^ Sm '^^^^^l^k ^m%m ^ r^^^^^» Mwft '' ■*• '^4 ^#^^H I^E" jSm^K^ "5^*^% "tKS(^*^S^» M"^W; '^^^^M ^P^^^p, >*^ -4r>^S^iS 1^ %*" wk7^ '-^^^M ^^»^9 ^^< W%^^"^ '* '^'' ^ ^totoS^ Ir*'^^'' I,^K^^^Hb ^.'^R'- "^^^.^^^B mS^-* ^ ' ^^^^ vl^^^^^^' ^^1^^ ■^'"r^^^^t^'^'^ -cWf^m ^^^ WmM ^^^^^^^p^ Fig. 9. — Section of a one-year-old French-prune sucker showing irregular swelling and lesions due to diamond canker. VARIETIES AFFECTED Diamond canker is limited almost entirely to the French prune. The only exception to this which has been observed is the case of the Standard variety of prune, in which one minor case of apparent dia- mond canker has been observed. Other prune varieties and trees of all the common stone fruits have been frequently observed growing in close proximity to diseased French-prune trees without becoming infected. 1932] Diamond Canker of the French Prune 13 Many French-prune scions grafted npon almond, peach, plum, apricot, and myrobalan trees have become badly affected with the disease, but the stocks in such cases have never become infected. The reverse condi- tion has also been seen: various other stone fruits have been grafted upon French prune and the disease has developed upon the stock without running up onto the scions.^ The evidence may be considered conclusive that the Sugar, Imperial (Imperial Epineuse) and Robe de Sergeant (Sergeant) prunes, together with all varieties of plum, apri- cot, almond, and peach, are practically immune to this disease. Some of the selected strains of the French prune, of which several have been planted rather widely, seem to be more susceptible to diamond canker than average trees of the variety. The fact that the nursery referred to above was particularly active in selecting and propagating such strains may have some connection with the establishment of their reputation for distributing diamond canker. Diamond canker has considerable resemblance to the tumor disease of apricot described by Khazanoff (1923)^ and attributed by him to the fungus Monockaetia 7'osenwaldia. The effects of the present disease are also very similar to those on prune, cherry, and other hosts of the exanthema, or die-back disease (see Smith and Thomas, 1928).^ Famil- iarity Avith these diseases, however, shows that they are entirely distinct from diamond canker. DEVELOPMENT IN THE ORCHARD In orchards where diamond canker is now most abundant, in trees planted from about 1918 to 1925, the most conspicuously affected trees have had the disease for a number of years and are now in a chronic or very much weakened condition. These badly affected trees have a tendency to occur in rather definite areas, especially in larger plantings. In some cases very definite diamond-canker areas occur here and there with large blocks of normal trees between. In these badly affected trees the disease occurs rather uniformly throughout the bark of the trunks and larger limbs and runs out systematically towards the extremities. In the vicinity of these diamond-canker areas there is usuallv some 4 A number of eases have been seen of diamond canker in trees of selected strains of the French prune topg^rafted on ordinary trees of the same variety. In such trees the disease alwavs runs from the g^ra.ft union up much more than down. This may be because the scion tissue is young-er than that of the stock. ^'^ Kliazanoff, A. A new tumor of the apricot. Jour. Agr. Eesearch 26:45-60. 1923. fi Smith, Ealph E., and Harold E. Thomas. Copper sulphate as a remedy for exanthema in prunes, apples, pears, and olives. Phytopathology 18:449-454. 1928. 14 California Agricultural Extension Service [CiR. 67 evidence of recent development of the trouble in new trees. In such recent cases the original infection appears to have taken place at prun- ing cuts or wounds, since the disease begins to develop at such places. There is no particular indication, however, that it is spread with pruning tools. For instance, in the case of normal trees adjoining diseased ones, diamond canl^er begins to appear at pruning cuts or other wounds, but with no relation to proximity of previously aifected Fig. 10. — Young Frencli-prune tree showing beginning of diamond canker at injury in the bark. trees. From the primary infections the diseased condition spreads gradually throughout the tree, extending up to all the branches if it begins in the trunk, or first becoming extensive in one branch if it begins above the main forks. As the disease spreads through a branch the pruning cuts or spur bases usually first show the cankers. These begin as slightly increased roughenings in the callus tissue or the bark at the base of spurs in lateral branches. This roughening then develops into a gradually increasing corky swelling until finally a typical canker is produced. The disease spreads more rapidly and is more injurious in young trees than in older ones. More information as to the inception and spread of diamond canker in the orchard is to be gained by examining young French-prune 1932] Diamond Canker of the French Prune 15 orchards (four to seven years of age) growing in localities where the disease is common. In many young orchards of this sort the develop- ment of diamond canker can be traced, and here it is very evident that the disease has no relation to trees from any particular nursery but apparently is an infection of wound-healing callus tissue. Figures 10 to 12 inclusive show very plainly how diamond canker first begins in young Fig* 11. — Young French-prune tree showing beginning of diamond canker near crotch. French-prune trees. The original infection can, in almost every case, be located at some point on the trunk in callus tissue surrounding a pruning cut, crotch crack, mechanical injury, sunburn, bark crack, or some other place where wound-healing tissue is formed. A very common condition is that shown in figures 12A and 12B where the tree has suffered and the trunk has been partly killed by a previous attack of the disease called bacterial gummosis, or sour sap. In the healing of the trunk lesions which follow this disease, diamond canker frequently develops. The examination of a large number of young trees at an age when the disease first appears confirms the opinion that the trouble practically always begins in wound-healing tissue. 16 California Agricultural Extension Service [CiR. 67 Diamond canker is hardly ever seen in trees less than about five years of age and develops most actively during the period covered by the following five years. Up to an age of about four years the trunk bark Fig. 12. — A, Young French-prune tree showing diamond-canker infection m bacterial-gummosis (sour sap) lesion. B, Young French-prune tree showing diamond- canker infection in bacterial-gummosis lesion. Note how canker stops at union with myrobalan-root base. is smooth in French-prune trees, pruning cuts are few, small, and quick healing and the opportunities for wound infections are not abundant. It is just at the period when this condition changes to that of the older trunk that diamond canker first appears. The occurrence of diamond canker in certain rather definite areas in the orchard, which is often a marked feature of the disease, might 1932] Diamond Canker of the French Prune 17 be explained on the basis of the spread of the disease from certain primarily infected trees. This does not seem very probable, however, since most of the trees in these areas seem to have had the disease for about the same length of time, and there has been very little continuous spread. Careful examination of young orchards described above, also gives little evidence of a spread of the disease from tree to tree. On the other hand, diamond canker does not appear to be the direct effect of any peculiar soil conditions located in certain spots in the orchards as in the case of exanthema. Rather, it seems that diamond canker may attack any tree in the orchard at any time when favorable conditions exist. That the disease develops in certain areas, seems more probably to be due to the occurrence of conditions favorable to infection in those areas during certain seasons. With the assumption that the disease usually begins in healing wounds, the idea suggests itself that any influence which might be the cause of lesions in the young trees might result secondarily in the devel- opment of diamond canker when such lesions had begun to heal. One of the most suggestive conditions in this connection is the occurrence of the bacterial-gummosis (sour sap) disease previously referred to. This trouble is of a very sporadic nature as to season and also inclines very strongly to attacking groups of trees in certain localized areas in the orchard. Such areas usually represent spots with poor drainage or other unfavorable soil conditions. Observation of young orchards often suggests the possibility that the trees in these areas are the ones which are most likely to develop diamond canker. French-prune orchards are often planted in places where the young trees are attacked by bacterial gummosis. During the attack the trunks of many of the trees are not entirely killed but afterward heal over with the formation of a large amount of callus tissue. In every young orchard where these definite areas of diamond-canker trees have developed there has been a correspondingly large amount of bacterial gummosis and this disease was apparently most abundant in the same areas and in the same trees where now the diamond canker is so conspicuous. In other words, it appears to be true that the attack of bacterial gummosis was the primary development, whereas diamond canker came on later and largely as a secondary matter. If the French-prune trees from one nursery were in any way different from those from another this differ- ence may have related primarily to bacterial gummosis rather than to any latent infection with diamond canker. In a number of cases wiiere stone-fruit trees of different varieties were topgrafted to French prune, diamond canker has been observed 18 California Agricultural Extension Service [Cir. 67 to develop in the callus bark tissue at the graft union. In some instances a large percentage of the trees have become infected in this manner. The development of the disease in some of these cases may be accounted for by the fact that scions for such grafting were chosen from diamond- canker trees under the impression that such trees represented a desir- able sport. In numerous other cases, however, evidence is available that the trees from which the scions were taken were not affected with diamond canker, and the development of the disease can only be explained by assuming that infection from some outside source took place in the susceptible graft-union tissue. These cases confirm the observation that diamond canker is due to an infection beginning in wound-healing bark tissue. In these graft unions again, bacterial gum- mosis almost always precedes or accompanies diamond canker. CAUSE During the past five years large numbers of cultures have been made by several persons connected with the Division of Plant Pathology in an endeavor to isolate a microorganism which might prove to be the cause of diamond canker. This work has been done according to the usual technique of surface sterilization with mercury bichloride, hydro- gen peroxide, alcohol, and heat; removal of affected tissue without sterilization; and the employment of various culture media, both liquid and solid. In all of this work the only organism that has com- monly been isolated is a polymorphic fungus similar to, if not identical with, Bematium pullulans de By (fig. 13) . This organism appears to be a common saprophyte of stone-fruit trees but no evidence has been obtained to indicate that this uniform occurrence in connection with diamond canker is of any significance. Except for this fungus no organism has been isolated with consistency. Inoculations of French- prune trees with cultures of D. pullulans have given negative results. In one case, an inoculation of a pruning cut with a slice of diseased bark resulted in the appearance after two years of typical diamond canker at the lesion. This tree was in a place where no other case of diamond canker has ever been discovered. The most common ideas concerning the cause of diamond canker have been (1) that it is a virus disease, on account of its apparently systemic and internal mode of distribution in the tree; (2) that it is due to some nutritional or toxic soil factor, on account of the similarity of the pathological tissues to those in exanthema; and (3) that it is a bud sport which is transmissable only by budding or grafting. 1932] Diamond Canker of the French Prune 19 From the manner of development of this disease in the orchard, the way in which it begins in wound-healing tissue and its transmission by diseased tissue in the case described above, it is reasonable to con- clude that diamond canker is due to a wound-callus infection by some microorganism which is common in certain places and at certain times. After the infection has once begun it appears that the disease becomes Fig. 13. — Plate culture of Bematium pullulans showing development of radiating mycelium from yeast-like colonies. Natural size. chronic and spreads generally throughout the tree over long distances through the medium of the cork-forming tissue, stimulating there an abnormal production of cells. In the case of the appearance of symptoms in young suckers, as previously described and illustrated in figures 5A and 55 it is difficult to imagine that this can be due to external infection since these suckers develop during the dry time of year in California when infection con- ditions are very unfavorable. It seems rather to be the case that these symptoms progress from the old, thickened, diseased bark out into the base of the suckers, from where they may then be followed, externally 20 California Agricultural Extension Service [C'ir- ^^ and internally, in some cases almost to the extremity of the sucker. Some of these shoots grow to the length of several feet during the first season. This condition has never been seen in suckers or any branches less than several years of age except those growing out directly from cankered bark on the trunk or large limbs. In the case of the outward or upward spread of the disease on the branches of trees which have become infected in the lower parts, it is the usual occurrence that every pruning cut and spur base soon begins to develop a canker out to a point where the growth is four or five years old. Whether the disease is systemic (internal), and finds external expression only in callus- forming regions, or whether it is due to separate external infections at these points, is difficult to determine. The possibility of insect transmission suggests itself since the cankers shelter numerous bark- inhabiting species. The systematic spread of diamond canker both in young suckers and older branches after an initial infection is once established certainly suggests an internal mode of distribution. CONTROL Possibilities of control in this disease appear to be poor. The princi- pal lines which suggest themselves are discussed in the following sections. Surgical Methods. — It appears to be hopeless in most cases to con- trol diamond canker by this method since the cankers are so numerous on many trees and also since it is practically impossible to detect them in their early stages. Reinfection takes place very rapidly after the work is completed. In certain cases the diseased areas on the trees may be cut out and the wounds disinfected. Disinfection is accomplished by treating all cuts with a disinfecting solution of 1 part of corrosive sublimate to 1,000 parts of water, or with bordeaux paste. The work is of so critical a nature that it can be done only by an expert and the cost is usually prohibitive. In cases of one canker on a tree, or a few pruning cuts on older trees with cankers, or where only one limb is affected, cutting out may be practical. Some benefit seems to have been obtained in a few cases by scraping off the rough outer bark as thor- oughly as possible from the cankered trunks and limbs and painting liberally with heavy bordeaux mixture or a mixture of copper carbonate and water. While such treatment may actually cure some of the treated cankers it does not reach many of the younger, incipient ones and the benefit is only temporary. 1932] Diamond Canker of the French Prune 21 Some work of this nature has been done in cooperation with Mr. A. W. Appel, Deputy Agricultural Commissioner of Santa Clara County. Mr. L. H. Day, of the Division of Pomology, and Mr. H. J. Baade, County Farm Advisor of Napa County, have also done considerable work along this line. External Treatment of Affected Trees. — Many different substances have been tried with the idea of finding a fungicidal spray material Fig. 14, — Young French-prune orchard with tree trunks coated with bordeaux whitewash to prevent sunburn, bacterial gummosis, and diamond canker. which would penetrate the canker tissue, destroy its cause, and produce a healing-over with sound bark. This has not been accomplished on account of the difficulty of penetrating the thick, corky, waxy, canker tissues. Injection. — An experiment has been started along the line of inject- ing the trunks of trees infected with diamond canker with various chemicals in solution. This was suggested by the work of injecting pear trees with iron salts to cure chlorosis, and by the successful treatment of prune and pear trees for exanthema by injection with copper salts. No treatment of this sort has thus far shown any effect on diamond canker. 22 California Agricultural Extension Service [Cir. 67 Renewal of affected trees by topgrafting root sprouts. — Trees af- fected with diamond canker have a tendency to throw out sprouts from the root, especially those on myrobalan. Some growers have made a practice of cutting off the original tree at the ground when it reaches an unprofitable condition and renewing it by topgrafting one or more of the myrobalan sprouts to French prune. So far as the disease in the original tree is concerned it is possible to overcome it by this method. Some topgrafts made in this manner develop into perfectly healthy trees. Others, however, become reinfected with diamond canker in the callus tissue at the graft union and in a short time become as badlj^ diseased as the original tree. Prevention. — The only possible control of diamond canker seems to lie along the line of prevention in protecting the young trunks and main branches from unnecessary wounds, sunburn, and particularly from bacterial gummosis (sour sap). Something may be accomplished along this line by keeping the trunks of young French-prune trees covered at all times with a heavy coating of whitewash or bordeaux mixture (fig. 14). These mixtures should first be applied when the trees are about two years old and kept up continuously after that. Any method of prevention of bacterial gummosis should also tend towards the same result. Resistant Varieties. — In districts where diamond canker is most prevalent it would seem that the safest procedure for prune growers at the present time is to replace badly affected French prune trees with Sugar prunes. Imperials, or any other variety which is preferred in that locality, or with some entirely different fruit or nut tree. It ap- pears to be certain that no commercial fruit tree except the French prune is susceptible to diamond canker. ACKNOWLEDGMENTS Acknowledgment is made to Mr. A. W. Appel, Deputy Agricultural Commissioner at Gilroy, California, for his assistance and interest in this work, also to Mr. H. J. Baade, County Farm Advisor at Napa, and to Acton and Son, prune growers, of Morgan Hill. 12m-10,'32