This is a disease which occurs on the peach, apricot and nectarine as well as the plum, and which is known as Black Spot, shot-hole, bacterial Leaf Spot and bacterial - crack. Of all fruits affected, plums show the greatest variation in susceptibility of varieties. The larger part of American plums suffer but little, whereas Japanese varieties, especially Abundance and Burbank, are severely injured. The Chabot and Red June are among other susceptible varieties. The more resistant plums include Arkansas, Clifford, Cumberland, Diamond, Damson, Lombard, Wild Goose and Yellow Egg.
Black Spot of plums has been noted in various sections of eastern United States during the past ten or twelve years. It was first observed in Georgia in 1904, in Nebraska and Missouri in 1906, in Delaware in 1907, and on Long Island about 1908. Subsequent records have been made east of the Mississippi River.
The leaves (Fig. 106), fruits (Fig. 107) and twigs are diseased. The leaves show a spot in which the tissue is brown and dead (Fig. 106). Soon the affected portion falls away, leaving a shot-hole. The holes are comparatively large and irregular in outline (Fig. 106). In severe cases premature defoliation occurs. Fruits of susceptible varieties are badly distorted and cracked, and are thus rendered liable to rot-producing organisms. Black spots are developed thereon (Fig. 107). On plum - twigs it is characteristic for the black spots to persist from year to year, forming perennial cankers. Thus old neglected trees are particularly a constant source of trouble and danger to other trees. These cankers are relatively large and deep on the Abundance and Burbank varieties. Cankers on the plum are somewhat different in appearance from those on the apricot, nectarine and peach. Open cankers are the more common type observed on the last three fruits, whereas on the plum the affected bark clings to the twig for some time. Watersprouts of plums may at times show twenty or more cankers in various stages of development.
See more complete account of Black Spot under Peach, page 306.
Jackson, H. S. Bacteriosis of plums. Delaware Agr. Exp. Sta.
Ann. rept. 16-18: 75 - 76. 1907. Lewis, I. M. A bacterial canker of plum twigs. American Micros.
Soc. Trans. 31: 145l - 149. 1912. (Additional references to literature under Peach, page 311.)