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Incidence, Distribution, and Development of Pruning Wound Cankers Caused by Phytophthora syringae in Almond Orchards in California. M. A. Doster, Graduate research assistant, Department of Plant Pathology, University of California, Davis 95616; R. M. Bostock, Associate professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 78:468-472. Accepted for publication 21 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-468.

Almond trees in three orchards in California were surveyed for pruning wound cankers caused by Phytophthora syringae. In the orchard with the most disease, 23% of all pruning wounds examined had cankers, and some sections of the orchard had more than 50% of the wounds infected. The number of infections increased linearly as the diameter of pruning wounds increased (r2 > 0.93). Pruning wound cankers were frequently observed high in the trees, and, in one orchard, the percentage of pruning wounds with cankers increased with height, while in a second orchard there was no clear trend. Cankers resulting from inoculations with P. syringae expanded throughout the fall, winter, and spring but ceased expansion in summer. In the orchards surveyed, there was no sign of canker expansion during or after summer. P. syringae was isolated from 18 to 26% of recently fallen almond leaves on the orchard floor. Abundant oospores were formed in the leaves. Pruning wound cankers occurred most frequently in the parts of the orchards pruned during periods of heavy rainfall.