Abstract
The seasonal occurrence of loquat canker caused by Pseudomonas syringae pv. eriobotryae in spring buds and seasonal leaves in a field was surveyed for twenty years from 1970 to 1989. Severe occurrences of the disease were in 1981, 1985 and 1987 whereas the slight ones were in 1975 and 1977. The severity of disease development in spring leaves paralleled that in spring buds and the disease development in buds and leaves was affected by the precipitation and number of rainy days for about one month before and after the disease occurrence. The mean temperature in February and cold injury due to the occurrence of low temperatures during the sprouting season were the main factors for the development of canker lesions on spring buds. The disease infection did not decrease until 7 days after injury under high humidity conditions whereas the infection did not occur one day after injury under dry conditions. Branches and leaves of loquat were susceptible to the disease until five or six days after injury under high humidity conditions. The precipitation for five days after injury was closely related to the disease development, so that, much precipitation stimulated the occurrence of the disease. The density of phage of P. svringae pv. eriobotryae in rainwater was highest in the season from November to May and lowest from June to October based on three years of observations. There was a close relationship between the spread of canker lesions and the number of oriental fruit moth (Grapholita molesta Busck) living in lesions. From the result that the pathogenic bacterium was isolated from the body of the oriental fruit moth, it was suggested that the insect is able to propagate the canker disease.
