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Poster: Biology & Disease Mgmt: Chemical Control


G143A mutation in cytochrome b is found in azoxystrobin resistant Phytophthora cactorum but not in resistant Phytophthora nicotianae from strawberry.
T. SEIJO (1), E. Zuchelli (2), N. Peres (1) (1) University of Florida - GCREC, U.S.A.; (2) Universidade de Passo Fundo, Brazil

Phytophthora cactorum and P. nicotianae cause leather rot of fruit and Phytophthora crown rot (PhCR) in strawberry. Leather rot occurs sporadically, but can cause up to 50% fruit loss during conducive weather. In Florida’s annual strawberry winter production system, PhCR can be severe resulting in stunting, mortality and severe yield loss. Currently, azoxystrobin is labelled for control of leather rot but not for PhCR. P. cactorum and P. nicotianae isolates from both leather rot and PhCR were collected from multiple strawberry fields in Florida between 1997 and 2016. Isolates were tested for sensitivity to azoxystrobin at 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 having EC50 values greater than 50 mg/L. Resistant P. cactorum and P.nicotianae isolates were found among both leather rot and PhCR isolates collected after 2010. The first 450 bases of the mitochondrial cytochrome b gene were sequenced from a selection of resistant and sensitive isolates of both species. The G143A mutation reported to cause azoxystrobin resistance was found in all three resistant P.cactorum isolates but not in resistant P.nicotianae isolates or in any sensitive isolate. This is also the first report of resistance to azoxystrobin in P. nicotianae from strawberry.