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Temporal distribution pattern of Sclerotinia sclerotiorum apothecia is modulated by row spacing and weather variables in soybean
Mamadou L. Fall: Michigan State University; Jaime Willbur: University of Wisconsin - Madison; Damon Smith: University of Wisconsin; Adam Byrne: Michigan State University; Martin Chilvers: Michigan State University
<div><p>Identifying the best fungicide application timing is crucial to maximize Sclerotinia stem rot (SSR; caused by <i>Sclerotinia sclerotiorum</i>) control. In this study, the impact of canopy closure and soil temperature on apothecia development was investigated to optimize fungicide application timing. Replicated soybean plots were established with 0.36- and 0.76-m row spacing’s in 2015 and 2016 at the MSU Montcalm Research Center. The number of apothecia, ascospores and SSR were monitored two times per week. In both row spacing experiments, apothecia were observed earlier in 2016 (before R1 growth stage) than in 2015 (R2). The maximum number of apothecia was 50 times higher in the 0.36-m vs. 0.76 m row spacing in 2015 whereas it was 2.5 times higher in the 0.76-m than in the 0.36-m row spacing in 2016, though the overall numbers were much lower in 2016. The apothecia distribution pattern was synchronized with the canopy closure pattern and the soil temperature profile. The peak number of apothecia was observed when the canopy closure reached 60% and soil temperature in row was between 21.5 and 23.5 <sup>o</sup>C. In 91% of the cases, apothecia presence was observed when the shade factor was at 70% and no apothecia germinated when the shade factor was at 0%, 30% and 100%. In both row spacings, the rate of the canopy closure was two times higher in 2016 than in 2015 and the first diseased plant was observed earlier in 2016 (R2) than in 2015 (R5). These results can explain the inconsistent efficacy of fungicide applications based on soybean growth stage and help fine tune fungicide application strategies.</div>

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