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Epidemiology of European Apple Canker in California. H. J. Dubin, Graduate Research Assistant, Department of Plant Pathology, University of California, Davis 95616; Harley English, Professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 65:542-550. Accepted for publication 30 November 1974. DOI: 10.1094/Phyto-65-542.

Factors affecting the incidence of European apple canker on Red Delicious, Golden Delicious, and Rome Beauty apple in California were studied. The disease is widely distributed in Sonoma County, and is present in other coastal counties. Isolates of Nectria galligena from paper birch, yellow birch, and bigtooth aspen did not infect apple trees when inoculated at leaf scars, but an isolate from apple was pathogenic on pear. European canker was found on pear for the first time in California. The major period of infection of apples was in November, with some occurring in December and none in the spring. Analysis of factors affecting development of the disease in 1970-71 showed that only leaf fall was limiting. However, in 1971-72 rain and temperature also were limiting factors. Multiple regression equations that were generated accounted for 83-94% of the variation in infection. Conidia were the major infective propagules and ascospores played an insignificant role in infection. Ascospores were water-borne or wind-borne; always in low numbers. Ascospore discharge showed no diurnal periodicity, however, it was influenced by duration of both rain and temperature.