VIEW ARTICLE

Disease Control and Pest Management

BLIGHT-ALERT: A Weather-Based Predictive System for Timing Fungicide Applications on Onion Before Infection Periods of Botrytis squamosa. P. C. Vincelli, Research assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853; J. W. Lorbeer, professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Phytopathology 79:493-498. Accepted for publication 4 August 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-493.

A weather-based predictive system named BLIGHT-ALERT was developed for timing applications of protectant fungicides to control Botrytis leaf blight of onion after it has reached the critical disease level (CDL) of 1.0 lesion/leaf and after the first fungicide spray has been applied. The system incorporates two components to forecast infection periods of Botrytis squamosa: a model called the inoculum production index (IPI), which forecasts the production of inoculum by B. squamosa based on temperature, relative humidity, and crop development; and National Weather Service forecasts of precipitation probability (PP), which have been associated with conditions favorable for infection of onion leaves by B. squamosa. Field experiments were conducted in commercial onion fields in New York from 1985 through 1987 in which the fungicide was first applied at the CDL in all treatments and subsequent applications were made according to IPI forecasts, PP forecasts, and combinations of these. Use of either the 30-IPI treatment (PP =30% and an IPI forecasting concurrent sporulation) or the 30-IPI₂ treatment (incorporating crop age rather than calendar date) for timing fungicide applications resulted in two to three fewer applications each season as compared to a weekly spray program. These timed treatments controlled Botrytis leaf blight as effectively as did the weekly spray program. We conclude that the 30-IPI₂ treatment (BLIGHT-ALERT) can be used as a predictive system for forecasting infection periods of B. squamosa on onion, thereby warning of periods when fungicide protection may be necessary after the CDL has been reached and the first fungicide spray has been applied.