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Poster Session: Biology and Disease Management - Chemical Control


First report of resistance to azoxystrobin in Phytophthora cactorum on strawberry.
T. SEIJO (1), E. Zuchelli (2), N. A. Peres (3)
(1) Univ of Florida - GCREC IFAS, Wimauma, FL, U.S.A.; (2) Universidade de Passo Fundo, Passo Fundo, Brazil; (3) University of Florida, Wimauma, FL, U.S.A.

Phytophthora cactorum causes leather rot of fruit or Phytophthora crown rot (PhCR) in strawberry. Leather rot occurs sporadically, but can be severe during conducive weather conditions. Epidemics with up to 50% fruit loss have been reported. In Florida’s annual strawberry winter production system, PhCR can be severe when transplants of a susceptible cultivar infected with P. cactorum are overhead irrigated for 10 days to promote establishment during warm weather in October. Mortality and stunting of >50% of transplants can lead to entire fields being replanted. Both diseases are controlled with chemicals. Mefenoxam alternated with phosphites are used to control PhCR. Currently azoxystrobin is labelled for control of leather rot but not PhCR. P. cactorum isolates from both leather rot and PhCR were collected from multiple strawberry fields in Florida between 1997 and 2015. Isolates were tested for sensitivity to azoxystrobin at concentrations of 0, 0.01, 0.1, 1.0, 10, and 50 mg/L amended with SHAM (100 mg/L). The isolates were separated into two groups, sensitive isolates, with EC50 values of less than 1.0 mg/L, and resistant isolates all having EC50 values greater than 50 mg/L. Resistance was found among both leather rot and PhCR isolates collected after 2010. This is the first report of resistance to azoxystrobin in P. cactorum from strawberries.