Abstract
Four years of surveying walnut blight and the causal pathogen in Tehama County California (Ca.) and three years in Butte County (Ca.) revealed a linkage of a history of Xanthomonas arboricola pv. juglandis populations in orchards with current season disease and temporal dynamics explained by differences in disease control measures. In the Tehama County (Ca.) orchards, walnut blight control has been very good and aggressive spray programs are maintaining bud pathogen populations at low levels. In the Butte County (Ca.) locations, although there are only three years of data, less effective spray programs have allowed disease damage and the initial inoculum overwintering in dormant buds to increase. The incidence of walnut blight damage differed greatly between different trees in a linear transect through a ‘Vina’ orchard and disease was strongly related to the population of pathogen in the developing buds, suggesting that the disease history of individual trees also varies, and is predictive of the risk of disease on those trees and that spatial segregation of disease occurs within an orchard. Assessments of the population sizes of the pathogen in dormant buds is thus a good predictor of the risk of walnut blight in a given year since the walnut buds are the primary source of initial inoculum that can spread to susceptible leaf and nut tissues after they open.
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