What it is
Alternaria leaf spot (also called black spot or leaf blight and head rot) is a common fungal disease of cabbage and other brassicas, caused mainly by Alternaria brassicicola and A. brassicae. It causes dark brown to black circular spots with target-like concentric rings and yellow halos, usually on older leaves first. Centres crack and drop out, giving a shot-hole look. It can also infect heads, and even small infections can make the crop unmarketable.
Symptoms to look for
- Small black specks first, enlarging into dark brown to black circular spots
- Target-like concentric rings within spots, surrounded by a yellow halo
- Older, lower leaves affected first; spots may be angular where limited by veins
- Spot centres crack and fall out, giving a shot-hole appearance
- Dark spots and rot can develop on cabbage heads, especially in storage
When it spreads
Cool to warm temperatures (about 20 to 28 degrees C) with high humidity (around 87%) and long periods of leaf wetness from rain, dew, or fog - often worst in humid summer and autumn conditions. The fungus survives on crop debris and seed and can be seedborne internally for years. Flea beetles can also carry the spores.
Treatment
Field practices
Use clean, certified seed or hot-water-treated seed, choose tolerant cultivars, rotate away from brassicas for two to three years, use drip irrigation and wider spacing to keep foliage dry, control cruciferous weeds and flea beetles, and remove crop debris after harvest.
Organic options
Remove and destroy infected leaves and crop debris. Copper fungicides and Bacillus subtilis biofungicides protect healthy foliage when applied preventively and repeated every 7 to 10 days in humid weather. Good airflow and dry foliage are important alongside any spray.
Active ingredients
- copper hydroxide
- copper oxychloride
- Bacillus subtilis
Chemical options
Apply protectant fungicides such as chlorothalonil or mancozeb preventively before disease establishes, on a 7 to 14 day schedule. Products with azoxystrobin, pyraclostrobin, or difenoconazole are also effective. Alternate modes of action to slow resistance and cover lower and inner leaves and heads.
Active ingredients
- chlorothalonil
- mancozeb
- azoxystrobin
- pyraclostrobin
- difenoconazole
Safety
Wear gloves, a mask, and eye protection when spraying fungicides, and follow the label pre-harvest interval before harvest (varies by product). Keep children and animals away from treated plants until dry, avoid drift onto food crops, and store products safely out of reach.
Preventing it next season
Start with clean, certified or hot-water-treated seed, use tolerant cultivars, rotate crops, keep foliage dry with drip irrigation and good spacing, control cruciferous weeds and flea beetles, and remove debris promptly. Preventive fungicides work far better than trying to cure established spots.