Dimethomorph activity against oomycete fungal plant pathogens

Dimethomorph (DMM) was effective in controlling late blight in potato and tomato caused by either metalaxyl-sensitive (MS) or metalaxyl-resistant (MR) field isolates of Phytophthora infestans and downy mildew in cucumbers and melons caused by MS or MR isolates of Pseudoperonospora cubensis. The fungicide did not affect zoospore discharge from sporangia of P. infestans but strongly inhibited zoospore encystment, cystospore germination, and mycelial growth in vitro. DMM showed translaminar activity and local systemic activity in intact plants but failed to translocate from one leaf to another in either acropetal or basipetal direction. DMM applied as a soil drench strongly protected tomatoes against late blight but failed to protect cucumbers against downy mildew. When applied in lanolin paste to the stem surface, but not when applied to the hypocotyl, it provided excellent control of both diseases in the foliage. DMM applied 1 day postinoculation partially protected potato against late blight and cucumber against downy mildew, and in both systems reduced sporulation of the pathogen. DMM diminished the sporulation of P. infestans and P. cubensis when applied to normally developed infected tissue. In P. cubensis it induced enhanced callose-encasement of haustoria. It showed a remarkable persistence on foliage subsequent to excessive washing with water as well as a high residual activity lasting for 9 days. DMM seems to be a good candidate for the control of oomycete diseases in the field especially in growing areas where phenylamide-resistant fungal populations prevail.