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Investigating spatiotemporal and genotypic characters of Phytophthora infestans, causal agent of late blight, in Wisconsin during 2009-2017

Tina Wu: University of Wisconsin-Madison

<div><em>Phytophthora infestans</em> causes late blight on tomatoes and potatoes and is a major agricultural concern worldwide. In Wisconsin (WI), where vegetable and potato production is ranked 2<sup>nd</sup> and 3<sup>rd</sup> in the nation, respectively, monitoring for disease risk each season is a top priority. Our objectives included 1) diagnose, genotype, and track location of <em>P. infestans </em>in WI from 2009 to 2017, and 2) investigate the timing of initial late blight incidence per WI county and compare to accumulation of Blitecast disease severity values (DSV) in each year from 2009 to 2017. Blitecast is a disease forecasting tool which utilizes units of DSVs based on temperature and relative humidity promotive to late blight. When DSVs surpass the threshold level of 18 in WI, growers initiate preventative fungicide regimes. The use of Blitecast as an integrated management tool has been useful with an average of 47 days between 18 DSV thresholds and first identification of late blight in central WI. During the 8 years of this investigation, between 3 to 22 counties had confirmed late blight each year. First detections by county varied from 7 July to 26 August. While genotypes included US-8, -22, -23, and -24, during 2009-2017, US-23 (mating type A1) was the most predominant genotype. Recent re-emergence of US-8 (mating type A2) suggests a continued need for monitoring of the pathogen to limit potential interaction of these two compatible mating types to avoid risk of sexual reproduction and resulting soilborne oospores and increased population variation.</div>