Relation of weather variables and host factors to incidence of airborne spores of Botrytis squamosa

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Abstract
Weather variables, leaf blight and airborne spores of Botrytis squamosa were monitored in onion [Allium cepa] field plots at the Holland-Bradford Marsh, Ontario [Canada] in 1976. Incidence of spores showed marked daily periodicities with peaks normally between 0900 and 1200 h Eastern Standard Time (EST). Daily spore counts were low (< 85 spores) during 7-28 July, often high (> 1000 spores) during 29 July-16 Aug., and moderate (100-1000 spores) 17-28 Aug. Spore production was observed only on necrotic portions of onion leaves. Circumstantial and correlative evidence indicated that spore production was promoted by persistent leaf wetness (> 13 h), high temperatures during wetness periods (14-20.degree. C), and by leaf dieback but was restricted by brief wetness periods (.ltoreq. 12.degree. C). Linear correltion analyses of 2-4 day running means of data indicated that important limiting factors in spore production were wetness duration during 7-28 July, temperature during 29 July-28 Aug., and severity of dieback during 7 July-16 Aug. Lodging apparently suppressed incidence of spores after 16 Aug. Spore release was promoted largely by declining relative humidity (RH) and by rain but occasionally by increasing RH. Striking peaks of airborne spores occurred during rain showers. Spore counts when leaves were dry failed to correlate with wind speed, and many spores were dispersed when wind speeds were low (1-4 km/h).

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