Development of an Advisory System for Grapevine Powdery Mildew in Eastern North America: A Reassessment of Epidemic Progress

Abstract

Historical data indicated that year-to-year variation in severity of powdery mildew fruit infection in New York was not directly related to annual variations in temperature or relative humidity through their known effects upon conidial germination, colony expansion, or latent period. Although severity of fruit infection varied from 0.02 to 61% among years, these environmental factors generally remained in the optimal range for the pathogen, with only slight year-to-year variations during the brief period that fruit were susceptible to infection. Vineyard experiments in New York and Australia indicated that colony growth was slower and sparser than would be expected from previous lab studies. Additional experiments indicated that transient exposure to temperatures near 4°C for a few hours had a debilitating effect on mildew colonies and produced growth similar to that observed on ontogenically-resistant tissues: slow and sparse with epidermal necrosis. Additional studies on seasonal release of ascospores from overwintering cleistothecia indicated that a large proportion of inoculum was often released prior to local grapevine budbreak at multiple sites in the eastern USA. These findings may represent key elements in forecasting potential severity of fruit infection in cool climates such as New York. Accepted for publication 28 January 2010. Published 26 May 2010.