Replacing low-risk fungicides with high-risk fungicides for control of powdery mildew in a resistant and susceptible summer squash cultivar in New Jersey

From 2016 to 2018, six fungicide programs for the control of cucurbit powdery mildew (Podosphaera xanthii) on a powdery mildew resistant summer squash ‘Reward F1’ and powdery mildew susceptible ‘Senator’ were evaluated with an emphasis on reducing the number of chlorothalonil applications. In all three years, cucurbit powdery control was greatest (based on lowest AUDPC values) when fungicides with a high-risk (HR) for resistance development and with different modes of action were applied in rotation season long or when HR fungicides were applied before and following the onset of disease development, rather than weekly following symptom observation in the susceptible and resistant cultivars. For a disease such as cucurbit powdery mildew, where there are multiple HR fungicides with different modes of action available and where their use could be limited to one or two applications per growing season, these fungicides might be suitable replacements for protectant fungicides in cucurbit crops that have stable, genetic resistance to cucurbit powdery mildew.