Prediction of Weather-Mediated Release of Conidia ofBotrytis squamosafrom Onion Leaves in the Field

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Abstract
Large catches of B. squamosa conidia with a Burkard recording spore trap in an onion field plot were almost always preceded by a 2-3 day period of fairly constant moderate (12.degree.-20.degree. C) temperatures and low (0-5 mb [millibar]) vapor pressure deficits (Edef). The more common weather pattern yielding smaller spore catches consisted of greater fluctuations in temperatures (15.degree.-30.degree. C) and Edef (0-15 mb) values. Frequent rain showers usually occurred during periods when large numbers of spores were trapped, but did not directly trigger spore release. Multiple regression analysis identified temperature and Edef as the weather variable most strongly associated with spore release. A sporulation index (based on average temperature and Edef values for the preceding 72 h period) was devised that accurately predicted the initial large release of conidia by B. squamosa about 85% of the time. Subsequent conidial releases were predicted with somewhat lower accuracy.