Economic impacts of reduced fungicide efficacy against downy mildew on slicing cucumber

Fungicides remain the most important tool to manage downy mildew on cucumber. The objective of this study was to estimate yield losses that might occur if fungicides to which Pseudoperonospora cubensis has demonstrated reduced efficacy were applied to slicing cucumber. A downy mildew-susceptible hybrid cultivar was grown in spring and fall 2020, sprayed weekly with one of four oomycete-specific fungicides (cymoxanil, fluopicolide, propamocarb, or oxathiapiprolin + chlorothalonil) applied in alternation with chlorothalonil. Control treatments were chlorothalonil applied biweekly and water. The fluopicolide-chlorothalonil treatment and the water control were repeated in spring 2021. Cucumbers were harvested 12 times, sorted by size, and weighed. Profits were calculated by subtracting production and fungicide application costs from the crop values, which were calculated with prices from terminal markets in the eastern United States during the harvest periods. Treatments performed the same in spring and fall 2020. Area under the disease progress curve (AUDPC) was significantly lower with fluopicolide, propamocarb, and oxathiapiprolin + chlorothalonil than cymoxanil and both controls (P = 0.01). AUDPC was lower with fluopicolide than with oxathiapiprolin + chlorothalonil. Mean marketable weights did not differ among the four fungicide treatments but were greater than in the two control treatments (P = 0.05). Profits also were greater for the four fungicide treatments than the controls, a mean of $8,829/ha versus $2,437/ha, respectively. Results with fluopicolide-chlorothalonil in 2021 confirmed results in 2020. Fungicides with reduced efficacy, like cymoxanil, or fungicides for which resistance has been reported, like propamocarb hydrochloride or fluopicolide, may still protect hybrid slicing cucumber from yield losses due to downy mildew.