BLACKSPOT CANKER OR APPLE TREE ANTHRACNOSE.

By W. H. LAWRENCE, Plant Pathologist and Horticulturist for Hood River Apple Growers, Hood River, Oregon, and County

Fruit Inspector for Hood River County.

More than a decade has passed since the disease of the apple tree earlier known under a variety of names as canker, black canker, blackspot canker, blackspot apple canker, blackspot, deadspot, apple tree anthracnose, sour sap disease, now widely known as blackspot canker or anthracnose, made its appearance in apple orchards in various riaces throughout the Pacific Northwest. In the face of the fact that the disease is one of the most destructive of all the fungous disease of the apple throughout the sections in which it occurs, there are orchards in all of the localities that do not receive the proper attention to control the disease, principally due to neglect on the part of the orchardists, but also in part due to a lack of interest in the business or ignorance or both. Were it true that this disease had proven to be an extremely difficult one to combat, there would be some excuse for the presence of trees well plastered with cankers and in all conditions of health from the apparently healthy to the weak one loaded with a full crop of slowly developing apples-last effort to reproduce its kind before its death. Since this is the easiest to control of all of the diseases of the apple induced by a fungus, there can be no explanation other than neglect or indifference on the part of the person in charge of the property. The fact that the trouble has continued year after year, causing more or less damage, has been an incentive for studying this disease. Enough has been learned to appreciate how destructive the fungus causing the disease really is, and that the problem of its control or eradication is far more important than suspected at an earlier date.

Briefly stated, the history covering these investigations gives to Professor A. B. Cordley of the Oregon State Experiment Station, credit due him for investigations proving the relation of the fungus which he gave the name gloeosporium malicorticis, to the disease termed apple tree anthracnose. The writer later studied this disease with particular reference to cankers occurring on other trees and the fungi causing decay of apples placed in common storage. The fact that the fungus will attack the prune, plum and pear, causing surface cankers; will attack the cherry causing gummosis, and also causes a serious rot of apples in common storage, was established. The results of these studies were published in Bulletin 66 of the Washington Experiment Station under the title of Blackspot Canker, a name in keeping with the names in common use in the section in which the study was made as well as being a good descriptive name. More recently Professor H. S. Jackson of the Oregon Agricultural Experiment Station has proven that the fungus causing this disease, has, like many other well known forms of fungi, an ascigercus or perfect stage as well as a conidial or summer stage.

This fact has a very important bearing on the late investigations made on this disease by Mr. B. B. Pratt of the Bureau of Plant Industry

and the writer since the fungus has been found to be the cause of serious losses of apples kept in both common and cold storage. Pathological work with rots of the fruit of the quince done the past season established the relation of one of the rots of the quince to blackspot canker rot.

The results of the investigations relative to the development and action of the parasite, gives numerous important facts concerning the disease which may be summed up as follows:

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Young cankers appear on the bark of the apple tree during November and later, first appearing on the tender twigs, water sprouts and more succulent two-year-old stems, later becoming conspicuous on the older portions, appearing between early November and early February, although the greater number do appear late in November to the middle of December or a little later. The source of the infection is spore forms. The spores, both ascospores (winter) and the conidia (summer), apparently responsible for these infections. A large series of germination tests with the conidia show, while they may mature and will germinate the latter part of May and later throughout the season, that germination seldom takes place under ordinary field conditions but may be induced to do so at almost any time after the conidia are mature in size by providing the necessary moisture and maintaining a suitable temperature, which it is apparent these spores must have. The temperature and moisture conditions present in November and later are apparently the conditions best suited to the fungus as is evidenced by the appearance of a greater or lesser number of minute, round, somewhat depressed and dark colored areas which extend through the bark into the sap wood beneath. The investigations of Professor Jackson have also shown that the ascospores of this fungus are matured in the same cankers in which the conidia develop, but following the development of the conidial stage. It was proven that both conidia and ascospores thus developed in their order do induce the typical cankers. The relative value of the two kinds of spores as distributive agents has not been determined but it is evident that both play their part and are jointly effective in the continuation of the disease on infested trees, the spread to the bodies and fruit of other apple trees, the fruit of the quince, the trunks and limbs of the prune, plum, pear and cherry.

Of the facts discovered in the recent investigations, the occurrence of the disease on stored fruit from orchards in which anthracnose canker do not occur is the most surprising.

The following count made by Mr. B. B. Pratt, of the Bureau of Plant Industry, United States Department of Agriculture, on eight boxes of EXTRA FANCY SPITZENBURGS gives a very good idea of how severe the disease may become even upon fruit taken from well kept orchards as are the orchards from which these apples were taken.

When it is understood that the eight boxes used in this experiment were selected and handled with more than ordinary care and that at the time they were examined were perfect except the areas where the fungus had gained an entrance, it is easily seen that greater care must be taken in preventing the spread of the disease. Data is not available which shows the relative abundance of the spots upon the apples given in the above statement. During the winter of 1910 the writer, however, had an opportunity to observe the apple rot developed on a quantity of Rhode Island Greening, which had been stored in boxes in a cellar. Of this lot of fruit 197 apples were inspected on January 30, 1911. There were a total of 1,023 decayed areas, 221 of which were larger than onehalf inch in diameter, 139 areas were producing spores, 24 of which were slimy, the remainder being either or only slightly waxy. The fungus had entered through 68 scabby areas, eight injuries done by the bud moth, 15 mechanical injuries, four through the blossom end and five through the stem end, the remainder passing through the uninjured epidermis.

In the examination of this fruit which had been stored in two boxes, it was observed that apples with large discolored spots on them were distributed throughout the boxes, indicating that the spores had lodged upon the apples before the same were gathered. It was also noticeable that the apples in the bottom of the boxes were literally covered with decaying spots. In one case one group of 30 apples had eight large spots and 406 smaller ones. The observations made confirm previous observations indicating that as the spores mature the gelatinous substance surrounding the spores absorbs a large quantity of water from the atmosphere, finally going into solution. These masses of liquid containing spores finally run off, falling on the fruit beneath. Many times these drops splash from one apple to another. A line of small areas down the side or a group of spots show how the liquid splashed, scattering the spores.

The recognition and control of the diseases caused by fungus which is now known under the scientific name of neofabrea malicorticis (Cordley) Jackson is of great importance to all the growers of the apple, since the disease of the body of the tree directly threatens the life of the plant and serious losses of apples held in storage will occur in cases where the spores of the fungus lodge on the fruit before the apples are gathered. It is apparently true that spores lodge on the fruit some days before the general harvest of the apple crop takes place, since the early maturing apples are as subject to the rot as the later maturing varieties that may hang on the trees until the advent of the early autumn rains and later.

The discovery that there are two spore forms developed in each canker, and that the conidial matures the first year and the ascigerous the second year, should make the orchardist doubly cautious in attempting to eradicate the trouble, since the distribution of spore forms continues through so long a period and that damage to the body of the tree as well as the loss of the fruit after the crop has been placed in storage is apt to occur.

Owing to the fact that tests with lime-sulphur and Bordeaux mixture made on mature fruit shortly before picking time has at least given good indications that the fruit may be sprayed without injury, it seems advisable to resort to spraying in order to prevent loss of fruit in storage, especially in all cases where fruit is taken from diseased trees. Past seasons' experiences also indicate that such an application would no doubt be of great value in preventing the spread of late infection of apple scab which frequently takes place after the fruit has been wrapped in paper and placed in storage.

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Owing to the late date at which apple harvest continues, it is sometimes true that the autumn rains make the soil so loose and muddy that it is impossible to apply a spray following the gathering of the fruit. In such cases there are other practices which may be followed with good A thorough spraying of Bordeaux mixture before the buds begin to open during the spring and while the entire tree is exposed to the direct drive of the spray the trees have been so thoroughly coated that the amount of Bordeaux mixture remaining on the trees throughout the seasons, including the following autumn, was sufficient to give as good results as could be gotten by spraying at any other date. These phenomenal results are less wonderful since it is now known that the old cankers two years of age are the seat of a second set of spores thrown off the second season and capable of starting new infections. This early spraying admits of thoroughly soaking these old cankers, which if in a dry condition have been observed to become a deeper blue color on the application of Bordeaux mixture, the dry and dead bark absorbing the liquid like a sponge.

In the cleaning up of any orchard of trees that have become badly infested with the disease it is advisable to tear out the bark from all cankers after the cankers of the present year are mature in size. Each wound should also be coated with thick Bordeaux mixture, since through experiments conducted by the writer, it is evident that the fungus will thrive in the wood as a saprophyte as well as in the old and dead bark previously killed by the fungus. In case time will not admit of removing the bark the cankers should be thoroughly plastered with the spray. Cutting out the growing cankers may also be done and is advisable when the condition of the orchard will admit of such a practice. In cutting out the cankers on badly infested trees very frequently more injury is done in doing the work than the fungus would do if left to complete the season's growth.

The timing for spraying infested trees depends upon several important orchard practices and it is therefore very important that the timing should be carefully considered. Anticipating loss from the rot, which is sure to follow when fruit is exposed to the spread of spores during the last part of summer and early autumn, the most logical method of meeting the situation is to apply a spray before the fruit is gathered in order to kill the spores adhering to the apples. In making this application it is not necessary to use a very strong spray, since the weaker mixture (4-4-50 Bordeaux) carries a far greater per cent of the killing properties than are actually required to do the work; besides the coating of the apple is only temporary, serving to protect the surface of the fruit until it has been placed in the warehouse or packinghouse. from which the fruit is packed following the wiping of the same, a practice which apparently pleases the consumer, although not to be considered at all necessary when considering the action of the spray ingredients on the keeping qualities of the fruit. Another consideration relative to the application of autumn sprays to control this disease is the partial destruction of the scab fungus which at this time is apt to be very abundant on the foliage. It is apparent from field observations and experiments that the coating of spray left on the leaves during the fall spraying that the active principles of the ingredients applied do act favorably in at least lessening the spread of the fungus the following spring.

Since it is less difficult to spray a tree after the fruit has been gathered, it is generally advisable that the autumn spray follow the gathering of the fruit. In cases of light infestation only one spraying is necessary. When the disease is abundant, however, two applications

should be made. For these applications 6-4-50 Bordeaux mixture is advised. Of the two applications, the first should immediately follow the gathering of the fruit. This is necessary in order that the work may be done before rainy weather sets in. The second application should be made two to three weeks later, depending upon weather conditions-if heavy rains occur the spraying should be done within two weeks, otherwise the work may be delayed one to two weeks later. All spraying done to control anthracnose must be done thoroughly in order to get the desired results.

Owing to the fact that even following the application of double strength Bordeaux (12-8-50) mixture, as is recommended for use in Western Washington, much of the active principles in the spray are rendered worthless by being removed from the tree early during the season by heavy showers or during long rainy periods. In order to meet this situation the writer prepared a mixture known as Bordeauxpetroleum emulsion, spraying the trees twice before the heavy rains began. Although only four trees, all badly infested with the disease, as shown by the presence of a large number of cankers from which spores were being discharged, three of which were sprayed with the mixture, good results in the control of the disease were secured as well as discovering that the petroleum contained in the mixture had a toxic effect, the leaves being much darker and healthier than those on the unsprayed tree or those which had only received coatings of Bordeaux mixture and standing in the same part of the orchard.

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