Disease measurement in a study of apple scab epidemics

Abstract

SUMMARYAn epidemic of apple scab in an unsprayed planting at East Mailing was monitored during the summer of 1979. Different rates and patterns of scab development on six apple cultivars were observed. Of several disease measures investigated the number of colonies/extension shoot best represented the epidemic.More comprehensive disease measurements on leaves, fruits and shoots were made toward the end of the summer. Cultivar differences were demonstrated for all disease measures investigated. The proportion of leaves with scab (incidence) and the number of colonies on leaves (intensity) were the measures with the lowest coefficients of variation. Consistent relationships between incidence and intensity were obtained.The implications of these findings for studies concerned with fungicide evaluation, cultivar susceptibility, disease—quality loss relationships and monitoring for disease management are discussed.