Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew (CDM), is a destructive disease of cucumber resulting in blighting of the foliage. In 2004, isolates of P. cubensis overcame the genetic resistance in pickling cucumber varieties leaving growers to rely on the intensive use of fungicide to protect their crop. P. cubensis is a high-risk pathogen for developing resistance to fungicides. Field studies were conducted in 2019 and 2020 to test 14 fungicides when applied weekly for their ability to limit CDM. In both years, oxathiapiprolin + chlorothalonil held disease to <1% foliar symptoms at the end of the monitoring periods. In 2019, cyazofamid was similar to oxathiapiprolin + chlorothalonil on the last monitoring date with CDM symptoms observed on 10% of the foliage. In 2020, treatment with oxathiapiprolin + chlorothalonil resulted in less disease than all other treatments. In 2019, treatments including ametoctradin + dimethomorph (premix), fluazinam, propamocarb and zoxamide + chlorothalonil (premix) limited disease to < 20.25% and provided control similar to cyazofamid. Results from trials comparing treatment programs consisting of tank mixes and alternating fungicides indicated that chlorothalonil alone or as part of a program with other fungicides but did not include oxathiopiprolin were the least effective in limiting CDM. The timing of fungicides is also a factor in reducing the infecting of disease. This trial had 5 treatments: Untreated, 5-day, 7-day, BLITECAST, and TOMCAST. In both years of the study, the calendar sprays were the most effective. In 2019, the 5-day calendar spray was the most effective, but not different from BLITECAST holding disease pressure to 30%. In 2020, the 7-day calendar treatment was the best treatment holding disease to 31.3%, and no treatment was similar.
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