Downy mildew on cucumbers

Cucumber downy mildew is one of the most destructive foliar diseases of cucurbits in the eastern US, capable of reducing a healthy planting to brown, wilted defoliation within 2–3 weeks once the pathogen arrives. For much of the eastern US, downy mildew arrives as airborne spores from overwintering.

Cucumber downy mildew is one of the most destructive foliar diseases of cucurbits in the eastern US, capable of reducing a healthy planting to brown, wilted defoliation within 2–3 weeks once the pathogen arrives. For much of the eastern US, downy mildew arrives as airborne spores from overwintering populations in the South and Florida each spring and summer. It is not present year-round in northern gardens, but it arrives dependably, and the arrival timing differs each year.

I don't grow cucumbers at my Long Island property, so this guide draws on University Extension research and the USDA Disease Forecasting Network publications.

The pathogen

Cucumber downy mildew is caused by Pseudoperonospora cubensis, an oomycete (water mold) in the same pathogen group as late blight of potato and tomato. Per Cornell Cooperative Extension, P. cubensis does not survive northern winters in most years — it requires living cucurbit tissue to survive. Spores (sporangia) are produced on infected plants in the Southeast and South Florida and are wind-carried northward each spring and summer, initiating new infections in gardens and commercial fields.

The pathogen is not a true fungus; oomycete fungicides (phosphonate-based and mefenoxam) are active against it while traditional fungicides are less effective.