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Fungicides are relatively ineffective against Stemphylium leaf blight of onion.

McDonald, M.R., Tayviah, C.S., and Gossen, B.D. 2016. Fungicides are relatively ineffective against Stemphylium leaf blight of onion. Proc. 18th Intern. Reinhardsbrunn Symposium, April 24 28, 2016, Friedrichroda, Germany. Abstract 47.

Abstract

Stemphylium leaf blight of onion, caused by Stemphylium vesicarium, is a damaging foliar disease on onion in the Holland Marsh, Ontario, Canada ((44’02’’ N 75’ 35” W). Genetic resistance is not available, so frequent application of fungicides is required to manage this disease. Replicated small-plot field trials were conducted to identify effective fungicides (2012 2015) and optimum application timing (2014 and 2015). The fungicides were: Pristine (pyraclostrobin 25.2%, boscalid 12.8%), Bravo 500 (chlorothalonil 50%), Manzate 750 F (mancozeb 75%), Switch 62.5 WG (cyprodinil 37.5%, fluodioxinil 25.0%), Fontelis 20 SC (penthiopyrad 20%), Inspire (difenoconzole 23.2%), Luna Tranquility (fluopyram 11.3%, pyrimethanil 33.8%), and Quadris Top (azoxystrobin 18.2%, difenoconazole 11.4%). Spray timing trials used Quadris Top with a non-ionic surfactant in 2014 and Luna Tranquility in 2015. In 2012 and 2014, disease severity was moderate, with 44 55% incidence by mid-August. In 2012, the first of 5 sprays was applied mid-July. Each of the fungicides reduced severity slightly compared to the untreated check (12 24% vs. 33%). Quadris Top was most effective. In 2013 and 2015, disease pressure was high, with 96 100% disease incidence by mid-August. Four sprays were applied in 2015 beginning mid- July. Only Luna reduced disease incidence compared to the untreated check (81 vs. 97%), and at only one sample date. In the spray timing trial in 2014, initiating the spray schedule between 3 and 17 July resulted in 3 to 5 fungicide applications and no differences in disease incidence (77 86%). In 2015, all of the spray timing treatments (6 10 sprays) resulted in a slight reduction in disease severity (42 46%) compared to the untreated check (65%). Lack of efficacy may indicate a need for very early fungicide application to prevent initial infection, and/or the role of toxins produced by the pathogen in season-long symptom development.

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