Resistance to Mefenoxam of Phytophthora cactorum and Phytophthora nicotianae Causing Crown and Leather Rot in Florida Strawberry.

Phytophthora cactorum and Phytophthora nicotianae cause 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 in harvests after intense rainfall events. PhCR is an important disease worldwide and is characterized by a sudden wilting and collapse of plants. Mefenoxam is the most effective and widely used fungicide to control both diseases. P. cactorum and P. nicotianae isolates from leather rot and PhCR have been collected from multiple strawberry fields in Florida since 1997, and the sensitivity of 185 isolates was tested at 0, 0.05, 0.5, 5, and 100 µg/ml. The 50% effective concentration (EC50) values of sensitive isolates ranged from 0.05 to 1 µg/ml. Resistance to mefenoxam (EC50 values >100 µg/ml) was found among P. cactorum isolates collected after 2015, but no resistance was found in P. nicotianae isolates. During the 2015 to 2016, 2016 to 2017, 2017 to 2018, and 2018 to 2019 seasons, resistance was detected on 9, 10, 21, and 23% of the isolates collected, respectively. Mefenoxam-resistant isolates originated from 3 of the 24 strawberry nurseries monitored. This is the first report of the occurrence of P. cactorum resistance to mefenoxam in Florida, suggesting that alternative control strategies are needed to avoid the increase of mefenoxam-resistant populations of P. cactorum in Florida fields.