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A Revision of Millsís Criteria for Predicting Apple Scab Infection Periods. William E. MacHardy, Professor, Department of Botany and Plant Pathology, University of New Hampshire, Durham 03824; David M. Gadoury, Research associate, Department of Plant Pathology, Cornell University, New York Agricultural Experiment Station, Geneva 14456. Phytopathology 79:304-310. Accepted for publication 26 September 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-304.

A review of published investigations of the relationship between leaf wetness, temperature, and infection of apple leaves by Venturia inaequalis indicated that infection by ascospores requires approximately 3 hr less than the interval reported by Mills and Laplante (Cornell Ext. Bull. 711, rev. 1951). Conidia require approximately 2.5 hr more than ascospores to infect apple foliage, rather than two-thirds the time required by ascospores, as stated by Mills. The discrepancy with ascospore infection could be explained by the daily periodicity of ascospore discharge, in which nearly all ascospores are released during the daytime. A revision of Millsís warning system is proposed, which computes primary infection periods from 0700 hr when the rain begins at night and utilizes a new polynomial equation for predicting infection. The impact of the revised criteria on scab warning systems is discussed.

Additional keywords: epidemiology, spore trapping.