Problem: Bacterial Leaf Spot (Plum)
This plant disease, caused by bacteria (Xanthomonas campestris pv. pruni), also attacks apricots and peaches. The disease is common east of the Rocky Mountains. The bacteria spend the winter in lesions on the twigs and in buds, oozing out in the spring to be carried by splashing rain to young leaves, shoots, and developing fruit. Periods of frequent rainfall favor the infection. Trees that defoliate early in the summer become weakened and produce few, poor-quality plums.
Treating when the flower buds open in the spring may help suppress the disease but will not eliminate it.