What it is
Downy mildew of cabbage and other cole crops is caused by the oomycete Hyaloperonospora parasitica. It shows as small, angular, pale green to yellow lesions on the upper leaf surface, with a white to grey downy growth on the underside. It is especially damaging on seedlings, which it can kill, and can also cause internal darkening of cabbage heads. It thrives in cool, moist conditions and is often followed by secondary soft rot.
Symptoms to look for
- Small, angular, pale green to yellow lesions on the upper leaf surface, often bounded by veins
- White to grey downy (fuzzy) mould on the underside directly below the lesions
- Lesions turn tan or brown and necrotic; leaves may wither and die
- Seedlings can be badly stunted or killed
- Internal darkening or purplish spots inside cabbage heads in severe cases
When it spreads
Cool, moist conditions from rain, heavy dew, or fog, especially during transplant production and cool parts of the season. Dense plantings and wet foliage promote it. Spores are windborne and released as leaves dry during the day; the pathogen survives as oospores in debris and soil and on living crucifers and weeds.
Treatment
Field practices
Grow resistant or tolerant varieties, use clean seed, reduce leaf wetness during transplant production, avoid overhead irrigation, use wider spacing for airflow, control cruciferous weeds and volunteers, rotate out of crucifers for at least two years, and remove crop debris after harvest.
Organic options
Reduce leaf wetness first: improve airflow, avoid overhead watering, and use wider spacing. Remove and destroy infected plants and debris. Copper fungicides give fair protection when applied preventively and repeated every 7 to 10 days in cool, wet weather; resistant varieties are the strongest organic tool.
Active ingredients
- copper hydroxide
- copper oxychloride
Chemical options
Apply protectant fungicides such as chlorothalonil or mancozeb preventively, and oomycete-specific systemics such as products with mefenoxam/metalaxyl or phosphorous acid where downy mildew pressure is high. Start at first sign or before, repeat every 7 to 10 days in favourable weather, and alternate modes of action to slow resistance.
Active ingredients
- chlorothalonil
- mancozeb
- mefenoxam (metalaxyl)
- phosphorous acid (phosphonate)
Safety
Wear gloves, a mask, and eye protection when spraying fungicides, and follow the label pre-harvest interval before harvest. Keep children and animals away from treated plants until dry, avoid drift onto food crops, and store products safely out of reach. Wash heads before eating.
Preventing it next season
Use resistant varieties and clean seed, keep transplants and plants as dry as possible (airflow, spacing, no overhead water), control weeds and volunteers, rotate crops, and scout closely in cool, wet spells since seedlings can be killed quickly. Reducing leaf wetness is the key lever.