The Dynamics of Mefenoxam Insensitivity in a Recombining Population of Phytophthora capsici Characterized with Amplified Fragment Length Polymorphism Markers.

ABSTRACT Recent findings from Michigan suggest that recombination may play a role in the survival and evolution of sensitivity to the fungicide mefenoxam in populations of Phytophthora capsici on cucurbit hosts. In 1998, 63 mefenoxam insensitive isolates were recovered from a squash field in which mefenoxam had been applied. Additional isolates were recovered from untreated squash fields planted at this location in 1999 (200 isolates) and the spring of 2000 (34 isolates). Isolates from 1998 and 1999 were characterized using fluorescent amplified fragment length polymorphism (AFLP) markers and all isolates were screened for compatibility type and mefenoxam sensitivity. In 1998 and 1999, 92 and 71% of the isolates, respectively, had unique multilocus AFLP genotypes with no identical isolates recovered between years. Seventy-two identical AFLP markers were clearly resolved in both the 1998 and 1999 sample sets, and fixation indices for the 37 polymorphic AFLP loci indicate little differentiation between years. There was no decrease in the frequency of resistant isolates during the 2 years without mefenoxam selection. We conclude that oospores play a key role in overwintering and that the frequency of mefenoxam insensitivity may not decrease in an agriculturally significant time period (2 years) once mefenoxam selection pressure is removed.