Fungicide efficacy and timing for the management of Stemphylium vesicarium on onion
2021
Stricker, Sara M. | Tayviah, Cyril S. | Gossen, Bruce D. | Mcdonald, Mary Ruth
Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium (Wallr.) E.G. Simmons, has become an important disease of onion (Allium cepa L.) in Ontario, Canada, and the northeastern USA in recent years. The disease presents as elongated lesions on the leaves and severe leaf dieback. The effect on yield is unclear, but the extensive leaf dieback limits uptake of sprout inhibitors that are applied to onion foliage prior to harvest. This can result in high losses in storage. There are no resistant commercial onion cultivars and growers apply foliar fungicides at 7–14-day intervals to manage the disease. Field trials to evaluate fungicide efficacy and disease-forecasting models were conducted at the Muck Crops Research Station, Holland Marsh, Ontario, from 2011 to 2019. Fungicide efficacy declined over the years. The disease-forecasting models reduced the number of fungicide spray applications, but none of the models, including calendar-based applications, reduced SLB severity. Seed treatments containing penflufen, combined with calendar-based fungicide applications, reduced SLB severity. Assessments of fungicide insensitivity in isolates collected locally in 2018 and 2019 demonstrated that 90% of isolates (n = 48) were insensitive to azoxystrobin (a QoI fungicide, FRAC 11) and 57% (n = 47) were insensitive to pyrimethanil (an AP fungicide, FRAC 9). Both fungicides are used extensively on onion in the Holland Marsh but are no longer effective against the population of S. vesicarium that is present in the region. Additional studies on the efficacy of seed treatments, fungicide insensitivity, and potential of biological fungicides are required.
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