Downy Mildew
Hyaloperonospora parasitica
Yellow patches on upper leaf surface with grey-purple mould on the underside. Favoured by cool wet weather.
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Identify from a photoBackground
Downy mildew is caused by several oomycete (water mould) species, each largely specialised to a plant family. Pseudoperonospora cubensis on cucurbits, Bremia lactucae on lettuce, and Peronospora species on brassicas, onions and spinach are the most important. Unlike powdery mildew, downy mildew requires extended leaf wetness and cool-to-mild temperatures.
Cucurbit downy mildew can collapse a zucchini or cucumber planting within a fortnight once conditions are right.
How to identify
- Pale yellow or angular patches on upper leaf surface bounded by leaf veins
- Greyish or purple-grey fuzzy mould directly underneath the yellow patch
- Leaves turn brown and die; heavy infection collapses the plant
- In lettuce, whole heads can rot in wet weather
Life cycle
Spores need leaf wetness for 4-6+ hours at 10-25C to infect. New spores in 5-7 days. Oospores can persist in soil and crop debris over dry seasons.
Weather triggers
- Temperature: 10-25°C
- Humidity: >85%
- Rainfall trigger: >5mm
- Long dew periods and overnight leaf wetness drive infection. Runs of cool damp weather are peak risk.
Peak season (southern hemisphere)
Red = active season · Dark red outlined = this month
Affected vegetables & crops
Click any crop to see current prices and growing info.
Climate zones at risk
Organic & low-impact control
- Copper-based fungicides (Bordeaux mixture, copper oxychloride) as a protectant
- Remove affected leaves at first sign
- Improve airflow and spacing
Chemical control
- Systemic fungicides (metalaxyl) are available to commercial growers; home gardeners should rely on copper and cultural controls
- Rotate active ingredients to prevent resistance
Always read product labels. Registrations change.
Prevention
- Choose resistant cultivars (especially lettuce and cucurbits)
- Avoid overhead watering in late afternoon
- Space plants for airflow
- Rotate susceptible crops through at least 3 years
Companion planting
No specific companion plant prevents downy mildew.
Biosecurity
Widespread, with new strains appearing periodically in lettuce and cucurbits.
Know when Downy Mildew is actually about
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