Plant disease
Downy mildew
Pale angular patches on top, gray-purple fuzz underneath, in cool damp weather.
Downy mildew is in its active season now — scout your plants this week.
How to identify downy mildew
Downy mildew makes pale yellow or light-green patches on the tops of leaves that are oddly angular — squared off and boxed in by the leaf veins, not round. Those vein-bounded blocks are the tell.
Flip the leaf over in damp weather and you'll see a gray, purplish, or fuzzy growth on the underside beneath each patch. As it worsens, the patches brown and the whole leaf dies.
It is the cool, wet mirror image of powdery mildew. Where powdery mildew is white dust on top and shrugs off dry weather, downy mildew needs cool, humid, damp conditions and shows its fuzz underneath. Basil, cucumbers, and greens are frequent targets.
Attacks: Cucumbers, Melons, Squash, Basil, Onions, Lettuce, Spinach
Life cycle: Spores spread on wind and splashing water and germinate only on wet leaves in cool, humid weather (heavy dew, fog, rainy cool spells). It moves fast when nights are damp and can arrive on the wind from far away.
Signs of downy mildew
What you actually see on the plant — usually before you spot the pest itself.
- Pale yellow, angular patches on the leaf tops, boxed in by veins
- Gray, purple, or fuzzy growth on the underside in damp weather
- Patches browning and leaves dying as it spreads
- On basil, yellowing leaves with a dusty gray underside
Organic control, least-toxic first
Start at the top and only move down if you need to. Physical and cultural fixes come before any spray.
- Open up airflow
Wide spacing, thinning, and staking let leaves dry between damp spells. Downy mildew needs leaf wetness, so faster drying directly slows it.
- Water at the base in the morning
Water the soil early in the day, never over the leaves, so foliage isn't sitting wet through cool, humid nights when the spores germinate.
- Plant resistant varieties
Downy-mildew-resistant cucumbers, basil, and spinach are widely available and are the surest defense in a wet climate. The seed packet or catalog will name the resistance.
- Remove infected leaves promptly
Snip off spotted leaves as soon as you notice them and bag them for the trash to take the spores out of the patch before they spread.
- Apply a copper fungicide preventively
In a cool, wet stretch or once downy mildew is reported nearby, a registered copper fungicide can protect healthy leaves. It prevents new infection rather than curing sick tissue — start early and reapply per the label and after rain.
Some links on this page are affiliate links. If you buy through them we may earn a small commission, at no extra cost to you. We only point to seeds and gear we would use ourselves. See our full affiliate disclosure.
One rule for any product you spray: follow the label. The label is the law, and it is the tested, safe rate for your plants — homemade mixes and dish-soap sprays are not, and can scorch foliage.
Prevent it next season
- Space, thin, and stake so leaves dry quickly after rain and dew
- Water at the base in the morning, never over the leaves
- Choose downy-mildew-resistant varieties, especially for cucumbers and basil
- Scout during cool, damp, humid spells when it moves fastest
- Clear and trash infected debris so it can't harbor spores
Questions about downy mildew
How do I tell downy mildew from powdery mildew?+
Downy mildew makes pale angular patches on the leaf tops with gray-purple fuzz underneath and needs cool, damp weather. Powdery mildew is white powder on the leaf tops and tolerates dry conditions. Check the leaf underside — fuzz means downy.
Why are my basil leaves yellowing with gray fuzz underneath?+
That's basil downy mildew. Improve airflow, water only at the base in the morning, remove infected leaves, and next time plant a downy-mildew-resistant basil variety.
Can I still eat crops with downy mildew?+
Yes, the unaffected leaves and fruit are fine to eat. Just remove and trash the diseased foliage and wash what you harvest.
Plan a garden that fights back
Healthy, well-spaced plants shrug off pests that flatten a crowded bed. PlotToTable sizes your beds, spaces every crop, and flags the pests that hit what you grow.