Downy mildew of cucumber.
Downy mildew and powdery mildew are very different diseases of cucurbits but they have in common that they are obligate pathogens that require a living plant for survival and thus don’t overwinter in Ohio. Downy mildew appears from late June to August in northern Ohio (but not observed yet!) and August to September for central and southern Ohio. Powdery mildew appears without fail in the first half of July throughout Ohio. Both mildews are managed by resistant varieties when available and fungicides. For gardeners, fungicide options are limited. For downy mildew the best option is a chlorothalanil product applied preventatively. You can follow me on Twitter @OhioVeggieDoc for alerts on downy mildew appearance in Ohio, or check this blog or the national cucurbit downy mildew forecasting site: https://cdm.ipmpipe.org.
Advanced powdery mildew on pumpkin.
Powdery mildew will show up on pumpkins and other cucurbits soon if not already here. Fungicides should be applied as soon as powdery mildew is observed. Sulfur products are most effective of those listed. Potassium bicarbonate and Serenade Garden also have some efficacy against powdery mildew. Note these are all protectants and only work on the leaf surface. Therefore full coverage is needed throughout the canopy (top and bottom of leaves, petioles).
