Extended control of downy mildew and Phytophthora root rot on ornamentals with the novel fungicide oxathiapiprolin

Oxathiapiprolin (piperidinyl thiazole isoxazolin class fungicide; Syngenta Crop Protection, LLC) was first registered in the United States on ornamentals in 2015. This novel, reduced-risk fungicide targets oomycete plant pathogens that cause downy mildew and root rot, economically important diseases of greenhouse ornamentals worldwide. Impatiens (Impatiens walleriana), a flowering annual bedding plant, is highly susceptible to downy mildew incited by Plasmopara obducens. Residual plant protection beyond the greenhouse is necessary as landscape sites may be infested with the pathogen. A two-year study was conducted whereby drench or spray applications of oxathiapiprolin, either alone or in a program with other fungicides were applied to impatiens in the greenhouse prior to transplanting them into infested landscape sites. The drench applications of oxathiapiprolin either completely prevented or significantly reduced infection more than 120 days after treatment. The foliar applications of oxathiapiprolin limited downy mildew initially however, disease incidence was statistically similar to the untreated control 90 days after transplanting. To determine the longevity of control against root rot caused by Phytophthora drechsleri, African daisy (Osteospermum ecklonis) was treated with oxathiapiprolin and inoculated 0, 14, 28, and 42 days post application. Plants inoculated 42 days after the fungicide application were asymptomatic for all observation dates more than 60 days after treatment. These trials demonstrate that oxathiapiprolin offers extended plant protection against downy mildew and Phytophthora root rot. When used in an integrated program including environmental control and cultural strategies, the extended protection afforded by oxathiapiprolin may reduce the amount of fungicides introduced to the environment.