Relationship of Precipitation Probability to Infection Potential ofBotrytis squamosaon Onion

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Abstract
An infection index for Botrytis squamosa was developed that uses temperature and leaf wetness duration to categorize leaf wetness episodes as insignifiant, 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 8, 18, 24, and 84% of the 36-hr forecast periods in which the maximum PP for the forecast period was < 30, 30-40, 50-60, and .gtoreq. 70%, respectively. It was concluded that a PP .gtoreq. 30% could be used to forecast severe potential infection periods of B. squamosa. A standard meteorological score for assessing forecasting skill precipitation .gtoreq. 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 .gtoreq. 30% as a decision role for forecasting severe potential infection preirods of B. squamosa should have equal predictive value for up to 36 hr in advance.