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Plant diseases
Alternaria leaf blight
Alternaria dauci · Fungus
Also called carrot leaf spot (the Pacific).
Illustration
A fungus that leaves dark brown to black spots with yellow edges on carrot leaves, starting on the oldest ones, until the tops blacken and wither in warm, wet or humid weather. Clean seed, dry leaves, thinner sowing and a three-year break from carrots keep it in check.
How to spot it
- Small, dark brown to black spots with a yellow halo, starting at the edges of the oldest, outer leaves.
- Spots that join up until the leaflets yellow, blacken, wither and die.
- Dark streaks on the leaf stalks, which can kill whole leaves and leave the stalks brittle.
- In a bad outbreak the whole patch looks scorched, as if hit by frost.
- The roots themselves are rarely attacked, but they stay small when the tops die early.
Easily confused with
Cercospora leaf spot
Pale tan, round spots, usually on the young leaves of young, fast-growing plants. The two often turn up together and are hard to tell apart.
Aster yellows
Yellow, twisted, bunched new growth and hairy roots, with no spots.
Powdery mildew
A white, dusty coating on the leaves rather than dark spots.
Where it's a problem
- Worldwide: Found wherever carrots are grown, including Australia and New Zealand. It is worst in warm, wet or humid seasons.
What to do
- Buy good-quality seed from a reliable supplier: seed is the main way it reaches a new garden.
- Sow thinly and thin the seedlings, so the leaves dry quickly after rain.
- Water the soil, not the leaves, and do it in the morning so the tops dry during the day.
- Pull out badly affected plants as soon as you see them.
- After harvest, clear away or dig in the old tops so they rot down quickly: the fungus dies once they break down.
- Keep carrots, parsnips, celery and parsley out of that bed for at least three years.
- Where it keeps coming back, grow tolerant varieties.
If you use a fungicide, choose one labelled for alternaria leaf blight and follow the label and local rules. Official guides start sprays at the first spots, and they are hard to get onto the leaves once the tops have closed over.
Follow your local council's guidance for disposing of diseased plant material.
Vegetables it affects
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Sources
- UMass Extension: Carrot, Alternaria leaf blight
- University of Wisconsin–Madison Vegetable Pathology: Carrot Alternaria and Cercospora leaf blights
- Pacific Pests, Pathogens and Weeds: Carrot leaf spot
- University of California IPM: Leaf spots on carrots
- Utah State University Extension: Alternaria leaf blight
Facts checked Wednesday 21 October 2026. General guidance only: follow your local council, agency and product label.