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Resistance of Phytophthora cactorum isolates causing crown and leather rot in Florida strawberries to Mefenoxam

Marcus Marin: University of Florida

<div><em>Phytophthora cactorum</em> causes leather rot on fruit and crown rot (PhCR) of strawberry plants. Leather rot is not a common disease in Florida; however, up to 50% yield loss has been reported after high rainfall events. PhCR is an important disease during plant establishment when infected transplants from nurseries are overhead irrigated for 10 days during plant establishment. Mefenoxam is the most effective and widely used fungicide to control both diseases. <em>P. cactorum</em> isolates from both leather rot and PhCR have been collected from multiple strawberry fields in Florida since 1997 and sensitivity to mefenoxam has been monitored. Isolates (n= 230) were tested for mefenoxam sensitivity at 0, 0.05, 0.5, 5 and 100 mg/L. EC<sub>50</sub> values of sensitive isolates ranged from 0.05 to 1 mg/L. Resistance to mefenoxam (EC<sub>50</sub> values > 100 mg/L) was found among leather rot and PhCR isolates collected after 2015. During 2015-16 and 2016-17 growing seasons, 10% (n=10) of all isolates collected were resistant. These isolates were from six different commercial farms and three transplant sources. This is the first report of occurrence of <em>P. cactorum</em> resistance to mefenoxam in Florida, which suggests that alternative control strategies are needed to avoid the increase of <em>P. cactorum</em>-resistant populations in Florida fields.</div>