Alternaria leaf blight on carrots — symptoms and treatment

Severity: medium

What is Alternaria leaf blight

Alternaria leaf blight is caused by the fungus Alternaria dauci. It attacks the carrot tops rather than the root directly, causing dark brown lesions on older leaves first and working up into younger foliage as the season goes on. The damage matters because the root is built from what the leaves produce: strip a carrot of its canopy and the root is weakened and never bulks up as it should. It is the reason a row of tops can go from lush to ragged in a wet spell.

Symptoms

The disease starts low and old. Dark brown to black lesions appear on the leaf margins and on the petioles — the stalks that carry the leaflets — beginning on the oldest, outermost leaves nearest the ground. On the leaflets the lesions eat in from the edges; on the petioles they show as dark streaks that can girdle the stalk. The lesions expand, run together, and the leaf dies. From a distance the effect is of the outer ring of tops browning while the centre still looks green. The roots are weakened as the foliage is lost.

• Early: dark brown to black lesions on the margins of the oldest, lowest leaves.
• Progressing: lesions on the petioles; lesions expanding and merging; leaf death.
• Advanced: the canopy thins from the outside in; weakened roots.

The oldest-leaves-first pattern is a useful clue — it separates the disease from damage that strikes the whole canopy at once, such as frost or spray injury.

Causes and conditions

The fungus can arrive on the seed, which is why an outbreak can appear in a bed with no carrot history, and it survives on infected carrot debris left in the soil from a previous crop. Once a lesion is producing spores, they spread to healthy foliage by wind and splashing rain, and infection depends on the leaves staying wet — prolonged leaf wetness from rain, dew, fog or overhead irrigation lets the spores germinate and get in. Warm, humid, wet weather drives the disease hardest, and a dense canopy that traps moisture and dries slowly extends the wet period.

Treatment

Crop rotation and clean seed — cultural

Timing: pre-planting. Use disease-free seed to keep the fungus out of the bed to begin with. Rotate with non-umbelliferous crops for 2-3 years — that means avoiding not just carrots but their relatives such as parsnip, celery, parsley, dill and fennel. Remove crop debris, which is where the fungus survives to infect the following crop.

Chlorothalonil — chemical

Timing: at first symptoms. Apply the fungicide when symptoms first appear on older leaves — this is the moment that counts, while the infection is still confined to the outer canopy. Repeat every 7-10 days. Pre-harvest interval: 14 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Sow disease-free seed from a reliable source.
• Rotate with non-umbelliferous crops for 2-3 years; do not follow carrots with parsnip, celery, parsley or dill.
• Thin carrots properly and avoid crowding so air moves through the canopy and the foliage dries quickly.
• Water at the base rather than over the tops, and water early in the day.
• Clear and destroy crop debris after harvest instead of leaving old tops in the bed.

Frequently asked questions

Are the carrots still edible if the tops are blighted? Yes. The disease is on the foliage, and the roots themselves remain edible — they will simply be smaller than they should have been because the plant lost its canopy. If you have sprayed, respect the pre-harvest interval given above.

When should I start spraying? At first symptoms, as soon as lesions show on the older leaves — not on a calendar. Once started, repeat every 7-10 days while conditions stay favourable.

Can it spread to my other vegetables? It affects carrots and their umbelliferous relatives — parsnip, celery, parsley, dill, fennel. Unrelated crops such as tomatoes, beans or brassicas are not at risk, which is what makes rotation with non-umbelliferous crops effective.