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Epidemiology and management of stemphylium leaf blight on onion in the Holland Marsh, Ontario.

Tayviah, S.C., Gossen, B.D., and McDonald, M.R. 2017. Epidemiology and management of stemphylium leaf blight on onion in the Holland Marsh, Ontario. Can. J. Plant Pathol. 39: 109.

Abstract

Stemphylium leaf blight, caused by Stemphylium vesicarium Wallr Simmons (telemorph; Pleospora allii (Rabenh) Ces. & de Not.), is a new disease of onion in Ontario. It is presently managed with multiple applications of fungicide. To understand its epidemiology, air-borne spore concentrations were monitored continuously for 120 days in 2015 using a Burkard 7-day volumetric sampler. Hourly spore counts revealed a diurnal pattern, with 48% of ascospores and 73% of conidia captured between 0500–1200 hours. The highest concentrations of ascospores (54 ascospores m^-3 air day-1) were captured prior to disease onset, but capture of conidia was highest (97 conidia m^-3 air day^-1) during epidemic development. Spore concentrations increased dramatically 24–72 hours after precipitation. The first appearance of blight symptoms coincided with high conidia numbers, rainfall, and warm days (temp ≥ 18°C for ≥ 9 hr). Conidia appear to be important in the development of blight epidemics, but the role of ascospores is not yet clear. A foliar fungicide (fluopyram 12.5%, pyrimethanil 37.5%) was applied with spray timings based on calendar application, initiation of calendar application based on spore trapping, spray prediction models (BOTCAST threshold 1, TOMCAST DSV 15, and a model modified specifically for stemphylium leaf blight) and a non-sprayed control. TOMCAST prompted six sprays compared to ten with the spore trapping treatment and eight applications with the other models. Early spray applications reduced foliar blight severity, but no treatment improved marketable yield. More effective fungicides and spray timings are required to provide effective management of stemphylium leaf blight.

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