It is probable that this disease is of foreign origin, the first report coming from Europe in 1884. Shortly thereafter the trouble became common in the United States. Special attention was given to the disease about 1890, at which time it was fairly well known to American nurserymen. It undoubtedly now prevails in all regions of the globe where the cherry is under cultivation, and is a pest both in the nursery and in the orchard. Sweet cherries, particularly mazzard seedlings, are very susceptible, whereas mahaleb seedlings are usually less so. It may be that the absence of the proper species of the pathogene accounts for the escape of the mahaleb seedlings. Early varieties are often much less affected than later - maturing ones.
The damage caused by the cherry Leaf Blight disease is not easily measured. It is believed to be one of the most important diseases affecting cherries in the nursery. Under conditions most favorable to the development of the causal pathogene the most susceptible types are completely defoliated. This results in an early loss of vigor by the young trees. Such trees are not able to mature their wood properly, and thus they withstand the dormant period with more difficulty and they enter the next season with decreased vitality. A case is on record where 40,000 young cherry trees were lost on account of Leaf Blight alone. The loss in Ohio in 1905 is estimated at $25,000. The preceding year it is estimated to have caused a loss of 8 per cent, in Maryland. One nursery company in Nebraska claims to have lost $40,000 in 1903 on account of this disease. These examples serve to show the possible destruction which may be wrought by Leaf Blight.
The fruit and pedicels are liable to show the disease, but the foliage (Fig. 48) is by far the most common seat of the trouble. Toward the last of May or early in June affected leaves exhibit slightly discolored, dark-blue areas on the upper surface (Fig. 48, left). These are not more than one-eighth of an inch in diameter, and they may be scattered over the whole blade or confined to one portion. Within a week or so the affected tissue becomes dark-red or reddish brown in color. Later developments may be one of two types; either the affected portions drop out, leaving circular areas in the leaf, or the whole leaf turns yellow (Fig. 48, center). A single leaf may show both types of symptoms, but with cherries, the yellowing of the leaves is the more common, whence the common name yellow leaf. An allied trouble affects plums, and while the leaves show both a yellowing and a shot-hole effect, the latter symptom is the more common. During periods of wet weather whitish masses appear on the lower surface of the leaf - lesions (Fig. 48, center). Sometimes these pustules are found in the center of the spot on the upper surface. Any time after the last of June premature defoliation is likely to occur on affected trees.
Affected pedicels show spots one-fourth of an inch or less in length, which extend one - third or more of the way around, often girdling the pedicels. The presence of such lesions causes the fruit to ripen unevenly. Lesions on the fruit are unusual and probably are never important. They manifest themselves in the form of dead, brown spots with whitish fruiting bodies in their centers.
The Leaf Blight of sweet cherry is caused by Coccomyces hiemalis, a fungus with a not unusual life-history. The Leaf Blight on mahaleb seedlings is caused by a very similar organism, Coccomyces lutescens. These fungi hibernate in the old fallen leaves as sexual structures called apothecia. In the spring ascospores are ejected from the apothecia, and are carried by the wind to the new foliage on which spore-germination occurs. About a week or ten days later signs of Leaf Blight are visible to the naked eye. The reader will recall that this has occurred by June 1 at least. The spore on germination sends its tube into the young leaf and mycelium rapidly develops. The mycelial threads grow between the leaf-cells, but send haustoria into the cells, thus obtaining food for the invading pathogene. Soon the cells of the affected portion are killed, and they exhibit characteristic color - changes, as already described under Symptoms.
During the summer asexual spores are developed. The mycelium amasses near the surface of the leaf and there results a fruiting body - an acervulus. From the interior of this structure conidia arise in great numbers. They push out on the leaf in large quantities, finally piling up until they are visible to the naked eye as whitish masses. These spores are carried by natural agencies to other leaves, where they germinate, and infection results in a manner similar to that described above. As new growth appears it is exposed to infection throughout the growing-season. When the leaf falls, the fungus is carried with it. Here on the ground, within the leaf - tissues, it passes the winter as apothecia formed from the mycelium in late autumn.
The elimination of the old leaves and protection of the developing leaves during the growing-season constitute the known measures of control of Leaf Blight. Plowing under fallen leaves removes a large portion of the source of the inoculum. This practice, although highly commendable, is not reliable in itself as a means of combating this disease. A few leaves always remain on the surface of the soil and are sufficient to cause trouble.
The healthy foliage may be protected by the application of a sulfur fungicide. For the past few years lime sulfur solution has been used, but recently it has been shown that sulfur dust is very satisfactory and effective and may soon replace the liquid fungicide. Bordeaux mixture, 5-5-50, is also used. In treating orchard trees the applications should be made as follows: (1) when the fruit is free from the calyx; (2) two weeks later; (3) just after the fruit has been picked; (4) three weeks later, if necessary. If lime sulfur solution is used, the strength advised is 1 to 50, with iron-sulfate one and one-fourth pounds to fifty gallons of the diluted solution. This mixture is used with safety, particularly with the addition of iron - sulfate, which reduces the burning qualities and increases adhesiveness. Sulfur dust, used at the rate of ninety parts finely ground sulfur to ten parts powdered lead arsenate, has been found to be effective.
In the nursery the same fungicides are recommended. The first application should be made when the cherry buds are about eight to twelve inches high. Subsequent applications depend on the weather. As a rule, five to seven applications at intervals of two weeks are sufficient. The earlier applications often can be made advantageously at shorter intervals in order to keep the new growth covered.
Stewart, V. B. The yellow leaf disease of cherry and plum in nursery stock. Cornell Univ. Agr. Exp. Sta. Circ. 21: 1-10. 1914. Stewart, V. B. Some important leaf diseases of nursery stock. The yellow leaf disease of cherry and plum. Cornell Univ. Agr. Exp.
Sta. Bul. 358: 184 - 192. 1915. Stewart, V. B. Dusting nursery stock for the control of leaf diseases.
Experiment for the control of leaf spot of the cherry. Cornell Univ. Agr. Exp. Sta. Circ. 32: 5-6. 1916. Higgins, B. B. Contribution to the life history and physiology of Cylindrosporium on stone fruits. Amer. Journ. Bot. 1: 145 - 173.
1914. Stewart, F. C, and Eustace, H. J. Notes from the botanical department. Shot-hole fungus on cherry fruit-pedicels. New York (Geneva) Agr. Exp. Sta. Bul. 200: 85-87. 1901. Beach, S. A. Treatment of leaf spot in plum and cherry orchards in 1896. New York (Geneva) Agr. Exp. Sta. Bul. 117: 135 - 141.
1897. Hein, W. H. Two prevalent cherry diseases. Cherry shot hole.
Nebraska Insect Pest and Pi. Dis. Bur. Bot. Div. Circ. 2: 2 - 4.
1908. Fairchild, D. G. Bordeaux mixture as a fungicide. Cherry leaf blight. U. S. Agr. Dept. Veg. Path. Div. Bul. 6: 38 - 39. 1894.