This disease has a very general geographical range in Europe and in portions of the United States. It has been reported from Michigan, Iowa, South Dakota, North Dakota, Ohio, Colorado, New Jersey, North Carolina, New York, Kentucky, Kansas and other states. The disease was observed as early as 1593, but it attracted little attention in America before 1888, at which time it occurred commonly in certain of the states already listed. The history of the disease indicates that it may never be expected to cause serious trouble over wide areas. It has never swept over any country, but rather affects individual trees here and there in isolated localities. In many cases, however, affected trees may not have a single healthy plum. Moreover, once a tree is diseased it seldom wholly recovers, but is attacked from year to year. The nature of the losses is as follows: (1) plums are distorted (Fig. 109) and may be rendered unmarketable; (2) affected fruits drop prematurely; (3) leaves are curled; (4) twigs are swollen, distorted and finally killed.
All kinds of plums are attacked, although the trouble is more abundant on the red and purple varieties. Wild species are said to be more commonly affected than cultivated ones. It is to be noted in connection with susceptibility that there is an apparent individual resistance which may be confusing in pointing to resistant and susceptible varieties. Certain individuals of a given variety may be seriously affected, whereas others of the same variety, in close proximity to the affected trees, remain unaffected.
In the case of affected fruits, there results, instead of normal plums, peculiarly enlarged, hollow deformities (Fig. 109). It is this condition which has given rise to the name plum-pockets. These consist merely of a thin shell, and in most cases there is no evidence of a seed. Wherever a stone is present at all, it is only rudimentary. The disease makes its appearance soon after the flowers have fallen. Toward the last of June the disease attains its full course of development and affected fruits drop from the tree. Affected plums are at first globoid in shape, but as they grow older they become oblong or oval, and more or less curved (Fig. 109). When young they are smooth, and can be distinguished from the healthy fruit by their pale-yellow or reddish color. Later, the surface is gray and is wrinkled considerably. Finally, diseased fruits turn dark-brown or black, and rattle like bladders when brought in contact with any hard substance; whence the name plum bladders. In this condition they hang to the tree two or three days, then fall to the ground.
The younger branches and leaves are affected, which increases the damage done to the tree. The growing limbs and leaflets become distorted and swollen. In June the diseased branches turn gray, the tissues soften, dry up and die. Shoots arising the next year just below these dead extremities are most frequently affected by the disease. As the new leaves unfold they turn red or yellow, when affected by the disease, and show an arching of the leaf-blade. In general a curl disease, similar to peach Leaf Curl, is developed.
The cause of these interesting malformations is the fungus Exoascus Pruni. Its growth in the plum-tissues causes their peculiar development, which finally result in the so-called pockets. For a long time the cause of the disease was not known, but was attributed to the work of some insect, to improper fertilization, and to a superabundance of moisture in the atmosphere at the time the fruit was forming. The real cause of plum - pockets was discovered in 1861.
Fig. 109. - Plum - pockets.
The parasite spends the winter as mycelium in the smaller branches, as evidenced by the fact that it is found there before diseased fruit appears. Moreover, the annual recurrence of the disease strengthens this opinion. From the affected branches the mycelium grows out into the young ovaries of the fruit, stimulating them to form a remarkable hypertrophy. After the mycelium develops to a considerable degree internally, the threads pass towards the surface. Some of the hyphae push up between the epidermal cells and spread out between these and the cuticle. Here they form a net-work of short cells, which soon grow erect on the surface of the fruit and become asci with ascospores. The ascospores are discharged through the apex of the ascus, but their further history is unknown. Conditions favorable to plum-pockets are similar to those favoring peach Leaf Curl, namely cool, wet weather in the early season of the year.
Satisfactory control measures have not been established for all sections of the country. It is advised that severe pruning to remove diseased twigs be practiced. It has been found in Montana that trees sprayed with lime sulfur showed very few diseased plums, whereas more than half of the fruit on un-sprayed trees was destroyed. These results were obtained in 1915 by making two applications as follows: (1) just before the flower - buds open, and (2) just after the petals fall. This treatment is advised for other regions where the disease is troublesome.
Galloway, B. T. Plum pockets. U. S. Agr. Comm. Rept. 1888: 366369. 1889. Hesler, Lex R. Diseases of the plum. Plum pockets. In The Fruit Industry in New York State. New York Agr. Dept. Bul. 79:
1198 - 1199. 1916. Swingle, Deane B. The botany and bacteriology department. In Report of Director. Montana Agr. Exp. Sta. Rept. 22: 241.
1916. Atkinson, G. F. Leaf curl and plum pockets. Exoascus Pruni Fckl.
Cornell Univ. Agr. Exp. Sta. Bul. 73: 329 - 330. 1894. Selby, A. D. Some diseases of orchard and garden fruits. Plum pockets. Ohio Agr. Exp. Sta. Bul. 79: 117. 1897.