Downy mildew
Pale angular patches above, greyish downy mould below — a cool, wet-weather disease that can strip a crop fast in a rainy spell.
- Pathogen
- Oomycete (downy mildews)
- Spreads by
- Wind & rain-splashed spores
- Favoured by
- Cool, wet, humid weather
- Main risk
- Fast defoliation in rainy spells
Overview
Downy mildew is a group of fungus-like (oomycete) diseases whose classic sign is a two-sided one: pale yellow, angular patches on the upper surface of the leaf (bounded by the leaf veins, so they look blocky), with a greyish, purplish or white downy mould growing on the underside directly beneath each patch.
It affects cole crops, cucurbits, onion and garlic, grapes, sunflower, and — as distinct but related diseases — pearl millet ("green ear") and sorghum ("crazy top"). Unlike powdery mildew, downy mildew needs cool, wet, humid conditions and leaf wetness: it explodes in rainy, overcast, humid weather with heavy dew, and can defoliate or destroy a susceptible crop within days once it starts, so early, protective action matters.
How to identify it
- Pale yellow, angular patches on the upper leaf surface, bounded by the veins so they look blocky rather than round.
- A greyish, purplish or white downy mould growth on the leaf underside directly beneath each patch, especially in the morning.
- Patches that brown and dry; heavy infection browns and kills whole leaves and can defoliate the crop.
- In pearl millet, deformed leafy "green ear" heads; in cucurbits and grapes, rapid leaf death in wet weather.
What causes it and when
- Downy-mildew oomycetes, spread by wind- and rain-splashed spores; infection needs leaf wetness (rain, dew, fog).
- Favoured by cool, wet, overcast, humid weather with prolonged leaf wetness — the opposite of powdery mildew.
- Worsened by dense, poorly-drained, poorly-ventilated crops and overhead irrigation that keeps leaves wet.
- Some downy mildews are seed-borne (e.g. in pearl millet), so infected seed carries the disease.
Cultural & organic control
- Grow resistant/tolerant varieties and use clean, treated seed, especially for seed-borne downy mildews like pearl millet.
- Give wide spacing and good drainage and ventilation so leaves dry quickly; avoid overhead irrigation late in the day.
- Rogue out and destroy the first infected plants (e.g. green-ear heads in pearl millet), and destroy crop residue after harvest.
- A copper-based or recommended bio-fungicide spray, applied protectively before rainy weather, helps hold it.
Chemical control
- Downy mildew moves fast, so spray protectively ahead of, or at the very first sign in, cool wet weather rather than waiting.
- Copper-based fungicides and CIB&RC-registered downy-mildew fungicides (including systemic/oomycete-specific ones like metalaxyl-based products) are used for the crop and rotated.
- Cover the leaf undersides where the mould grows, and observe the pre-harvest interval, especially on grapes and vegetables.
- Confirm the current recommendation and rotation for your crop and area with your local KVK.
Prevent it coming back
- Use resistant varieties and clean, treated seed, and rotate crops to reduce carry-over.
- Manage the canopy and irrigation to keep leaves dry, and improve drainage.
- Scout closely in cool, wet, humid spells and act protectively, rotating fungicide groups to avoid resistance.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My leaves have pale angular patches on top and greyish mould underneath. What is it?
That two-sided sign — pale yellow angular patches (bounded by the veins) on the upper surface, with a greyish or purplish downy mould directly beneath them — is downy mildew. It needs cool, wet, humid weather and leaf wetness, and can spread fast in rainy spells. Improve airflow and drainage, use resistant varieties, and spray protectively before wet weather.
How is downy mildew different from powdery mildew?
They are opposites in weather and appearance. Powdery mildew is a white powdery coating on the upper leaf surface that thrives in dry weather. Downy mildew shows as angular patches on top with a downy mould underneath and needs cool, wet, humid conditions with leaf wetness. So downy mildew flares in the rains, powdery mildew in dry spells — and the fungicides differ.
Why does downy mildew spread so fast?
Because in cool, wet, humid weather its spores are produced in huge numbers on the leaf undersides and spread by wind and rain-splash, infecting through leaf wetness within hours. A susceptible crop can be defoliated within days once it starts. That is why protective spraying ahead of wet weather, resistant varieties, and a dry, airy canopy matter so much.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.