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

Abstract

Weather variables, leaf blight, and airborne spores of Botrytis squamosa were monitored in onion field plots at the Holland–Bradford Marsh, Ontario, in 1976. Incidence of spores showed marked daily periodicities with peaks normally between 0900 and 1200 hours Eastern Standard Time (EST). Daily spore counts were low (< 85 spores) during 7 to 28 July, often high (> 1000 spores) during29 July to 16 August, and moderate (100 to 1000 spores) between 17 and 28 August. 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 to 20 °C), and by leaf dieback but was restricted by brief wetness periods [Formula: see text] and by cool temperatures [Formula: see text]. Linear correlation analyses of 2- to 4-day running means of data indicated that important limiting factors in spore production were wetness duration during 7 to 28 July, temperature during 29 July to 28 August, and severity of dieback during 7 July to 16 August. Lodging appeared to suppress incidence of spores after 16 August. 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 to 4 km/h).