Shot Hole in Leaves Caused by Pseudomonas syringae, (Bacterial Canker)


Glynn C. Percival

Introduction. Bacterial canker attacks many species of Cherry and Pear. The disease is caused by the bacteria Pseudomonas syringae. The disease usually occurs as enlarging lesions or cankers on trunks and scaffolds of young, vigorous trees, especially sweet cherry. Great amounts of gum are usually associated with active cankers, but gumming is not always a positive indication of this disease.

Symptoms: The disease attacks most parts of the tree. Cankers on trunks, limbs and branches exude gum during late spring and summer (Figure 1). Leaves on the terminal portions of cankered limbs and branches may wilt and die in summer or early autumn. Leaf and fruit infections occur sporadically, but they can be of economic significance in years with prolonged wet, cold weather during or shortly after bloom. Leaf spots are dark brown, circular to angular, and sometimes surrounded with yellow halos.
The spots may coalesce to form large patches of dead tissue, especially at margins of leaves, or the centers of the necrotic spots may drop out, resulting in tattered leaves which form the distinctive shot hole symptoms (Figure 2). Infected leaves may abscise during midseason.