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Plant diseases
Alternaria leaf spot
Alternaria brassicicola and Alternaria brassicae · Fungus
Also called black spot, grey leaf spot (USA), black leaf spot (the Pacific).
Illustration
A fungus that leaves round brown to black spots with target-like rings on the older leaves of cabbage-family plants, and dark rots on broccoli and cauliflower heads. It isn't the Alternaria that blights carrot leaves; clean seed, good spacing, dry leaves and clearing away old plants keep it in check.
How to spot it
- Small dark or yellow spots, on the older leaves first, that grow into round brown to grey spots up to 25 mm (1 in) across.
- Target-like rings in the spots, often with a dark rim or a yellow halo.
- Sooty black spores on the spots; old spots go papery and their centres fall out, leaving shot holes.
- Spots that merge until the leaves dry out, look scorched and drop.
- Dark, sunken spots or rots on broccoli and cauliflower heads, and dark streaks on the stems and leaf stalks.
Easily confused with
Alternaria leaf blight
On carrots, parsnips, parsley and celery: a different Alternaria species that attacks the carrot family rather than the cabbage family. The spots look much alike, so the plant tells you which it is.
Black rot
Yellow V-shaped wedges from the leaf edge with blackened veins inside them, not round, ringed spots.
Bacterial leaf spot
Small spots that look water-soaked at first, without target-like rings or sooty black spores.
Where it's a problem
- Worldwide: Found in temperate and tropical regions. Black leaf spot likes warm, wet weather of about 20–30°C (68–86°F); grey leaf spot can infect in cooler weather.
What to do
- Buy good seed from a reliable supplier: the fungus is carried on and inside seed.
- Space plants so air moves between them and the leaves dry quickly.
- Water the soil, not the leaves, especially once spots appear.
- Pick off spotted older leaves early, and pull out cabbage-family weeds and self-sown plants between crops.
- Don't grow spring and autumn crops of the cabbage family side by side, where the old crop can infect the new one.
- After harvest, clear away or dig in the old plants: infected leaves keep making spores until they rot.
- Keep the cabbage family out of that bed for at least three years.
If you use a fungicide, choose one labelled for alternaria leaf spot and follow the label and local rules.
Follow your local council's guidance for disposing of diseased plant material.
Vegetables it affects
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Sources (4, from Pacific Northwest Plant Disease Management Handbook, Pacific Pests, Pathogens and Weeds and UConn Extension)
- Pacific Northwest Plant Disease Management Handbook: Kale, Alternaria diseases
- Pacific Pests, Pathogens and Weeds: Cabbage black leaf spot · Cabbage grey leaf spot
- UConn Extension: Alternaria leaf blight and head rot of brassicas
Facts checked Wednesday 28 October 2026. General guidance only: follow your local council, agency and product label.