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Disease Control and Pest Management

Relationship of Precipitation Probability to Infection Potential of Botrytis squamosa on Onion. 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 78:1078-1082. Accepted for publication 7 March 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1078.

An infection index for Botrytis squamosa was developed that uses temperature and leaf wetness duration to categorize leaf wetness episodes as insignificant, slight, moderate, or severe potential infection periods, given the presence of inoculum. Using this infection index, it was found that the frequency and severity of potential infection periods of B. squamosa in a commercial onion field in Orange County, NY, increased with increasing National Weather Service (NWS) precipitation probability (PP). Severe potential infection periods occurred in 0, 18, 24, and 84% of the 36-hr forecast periods in which the maximum PP for the forecast period was < 30, 3040, 5060, and ≥ 70%, respectively. It was concluded that a PP ≥ 30% could be used to forecast severe potential infection periods of B. squamosa. A standard meteorological score for assessing forecasting skill was used to evaluate the accuracy of PP forecasts in forecasting measurable precipitation (≥ 1.3 mm) in a commercial onion field. In contrast to a reported trend among NWS forecast offices of declining forecasting skill with increasing time after issuance of a forecast, skill of PP forecasts for the Lower Hudson Valley in New York (which includes Orange County) remained high for 36 hr following the forecast. This indicated that the use of PP ≥ 30% as a decision rule for forecasting severe potential infection periods of B. squamosa should have equal predictive value for up to 36 hr in advance.