Among plums the European varieties are notably susceptible to Leaf Blight; these may be completely defoliated in wet seasons. On the other hand, Japanese plums and Myrobalan seedlings are comparatively resistant, the disease being of little consequence thereon.
Affected leaves show dead spots at first (Fig. 105); but soon the dead tissue falls away, leaving a circular hole, whence the name shot-hole (Fig. 105). This type of symptom is common on the plum. At times leaves are completely riddled with holes. Sometimes affected leaves turn yellow, whence the name yellow leaf. The similar disease on cherry shows less shot - holing and more yellowing of the leaves than in the case of plum. In most seasons if sufficient moisture is present, white, velvety pustules appear on the lower surface. These are coils of spores belonging to the conidial form of the causal fungus Coccomyces Prunophorce (commonly known as Cylindrosporium Padi).
Bordeaux mixture 5-5-50 is effective in the control of plum Leaf Blight, but this fungicide may injure the foliage, particularly in nursery-stock. Lime sulfur, on the contrary, diluted 1 - 50, may be used without fear of such injury. In the orchard, spray as follows: (1) ten days after the blossoms fall; (2) three weeks after the first application; (3) three to four weeks after the second application. In relatively dry seasons the third spraying may be omitted. Treatment for nursery trees should be given as recommended under Cherry, page 175.
Fig. 105. Leaf Blight of plum. Note the shot - hole effect. Upper surface (left), lower surface (right).
See fuller discussion of the symptoms and cause under Cherry, page 172. The Leaf Blight of plum and cherry, while not caused by identical fungi, are so similar that the statements made for cherry Leaf Blight apply to the plum.
Hesler, Lex R. Diseases of the plum. Leaf spot. In The Fruit Industry in New York State. New York Agr. Dept. Bul. 79: 1194 - 1198. 1916.
See further references to literature under Cherry, page 176.
Japanese plums behave much as do peaches when injured by frost. Domestica plums, on the other hand, are more hardy, except the variety Reine Claude, which is highly susceptible to sun - scald. (See Apple for fuller discussion, page 35.)