Carrot leaf blight

Cercospora carotae · Fungal

Carrot leaf blight is a fungus (Cercospora carotae) that attacks the leaves and leaf-stalks of carrots, not the root itself. It is one of the two common carrot leaf blights — the other is Alternaria — and it favours warm, damp weather. Its spores blow and splash from spotted leaves and old carrot debris onto healthy plants, and in a wet spell they can spot and brown the whole top of the crop. Because the leaves are the engine that fills the root, losing them means small, poorly-coloured carrots and tops too weak to grip when you lift.

Why it matters

The carrot's leaves are the engine that feeds the swelling root — so a disease that scorches the tops steals the crop straight from the ground. A bad leaf blight can strip the tops and cost you up to 4 in every 10 carrots (40% of your crop): the roots stay small, pale and poorly flavoured because the leaves died before they could fill them. It spreads fastest in warm, damp weather, jumping plant to plant on splashing rain and dew, and it comes back year after year from infected seed and old carrot leaves left in the soil. Worst of all, weakened, rotted-off tops snap when you try to pull the roots, leaving carrots stuck in the ground at harvest.

When to act

first leaf spots found — about 1 in 10 plants showing spots — or warm, wet, humid weather with long dew forecast; start protectant fungicide early, before the spots multiply

This guidance is general, compiled from published agronomic references including FAO and CABI. Thresholds and timing shift with your crop, your variety and the season.

In much of the region a spray is simply called mushonga — medicine. But a chemical group you cannot name is one you cannot rotate, and a product you did not write down has no pre-harvest interval you can observe. Mushonga, and other farming words →

The full Carrot leaf blight field guide

Identify it

The first thing you see

small grey-tan to brown spots scattered on the leaves, each ringed by a darker border and often a thin yellow edge. They are small — about 2–3 mm, the size of a pinhead — round to angular, and they start on the younger inner leaves and the leaf-stalks first.

As it spreads

the little spots multiply and run together, so whole leaflets brown, curl and shrivel, and the ferny tops look scorched, as if singed. On the leaf-stalks you get long tan-brown streaks that can ring the stalk right round, so the whole leaf flops and dies — badly hit crops look brown and collapsed from a distance.

Don't confuse it

with the other carrot leaf blight, Alternaria — the two often grow together and are treated the same way, but they start differently. Alternaria starts on the older, outer leaves with bigger dark brown-to-black blotches, often at the leaf edges with a yellow halo, scorching the tops from the outside in. Cercospora (this one) starts on the younger, inner leaves and the leaf-stalks, with smaller, greyer spots that have a clear dark ring.

Also don't confuse it

with plain browning from frost, drought or a shortage of feed — those brown the whole leaf evenly, with no small ringed spots. Leaf blight always shows as many separate spots that spread and join up.

Scout for it

When to look hardest

from about 3 weeks after sowing (when the tops are growing fast) right through root-swelling — the development and mid-season stages. Step up your checks whenever the weather turns warm and damp — muggy days, heavy dew, fog, or after rain or overhead watering — because that is when this fungus explodes.

How to walk the field

twice a week, walk a big "W" shape so you cross all four corners and the middle — not just the edge by the path. Make about 5 to 10 stops along the way.

What to check at each stop

look closely at the younger inner leaves and the leaf-stalks first — that is where Cercospora starts — hunting for the small grey-tan spots with their dark ring. Check in the early morning while the dew is on, when the spots show up best.

The number that means act now

the threshold is the very first spots you find — about 1 sick plant in every 10 (10%) showing spots is enough to start treating, because the disease multiplies fast in warm, damp weather. And when warm, wet, humid weather is forecast to settle in, protect the crop before any spots appear rather than waiting to find them.

Control it

Prevent it first

start clean (your strongest tool):
the fungus rides in on the seed itself and on old carrot leaves left in the soil, so use fresh, certified disease-free seed every time, and rotate at least 3 years off the carrot family (no carrot, parsnip, celery, parsley or coriander on that ground) so last year's spores starve out. Where you can find one, plant a leaf-blight-tolerant variety. Starting clean is worth far more than any spray later.
grow an airy, dry crop:
thin early and keep the recommended spacing so air moves through and the leaves dry quickly after rain or dew — the fungus needs long leaf wetness to take hold. Weed the bed so the carrots aren't sitting in a damp thicket. Water at the base early in the day, not over the tops, so the leaves dry before night. After harvest, plough in or clear away all the old carrot tops — they are where the disease overwinters and starts again.

Act early — knock back the source:

the moment you find the first spots, pick off and remove the worst-spotted leaves and carry them right out of the field to bury or burn — never leave them on the bed or the compost. Do this on a dry day so you don't spread wet spores by hand, and this is the moment to begin protectant spraying (below) before the spots multiply.

Chemical as a last resort — protectant fungicide:

if the weather is against you and the spots are spreading, use a protectant (contact) fungicide such as mancozeb or chlorothalonil. "Protectant" means it coats the leaf and stops new spores taking hold — it shields healthy leaves but cannot cure a leaf already spotted, so it must go on at the very first spots, and be re-applied on a tight schedule (about every 7–14 days) through warm, wet spells and after rain washes it off.

Spray safely

cover your skin, eyes, nose and mouth; spray in the cool of the morning or evening with the wind at your back; keep the spray off drinking water and rivers, and away from children and bees. Respect the pre-harvest interval — the number of days the label says you must wait between the last spray and pulling or selling the carrots. Wash yourself and your clothes afterwards, and store the product locked away from food, water and children.

Fight resistance

don't lean on one product forever — rotate to a different chemical group each time you spray so the fungus can't get used to it. Protectant sprays used early and preventively are the backbone; a curative product used late and alone soon stops working.

If it keeps spreading →

keep the protectant schedule tight (spray on time, and again after every rain) and keep removing the worst leaves. If the tops collapse anyway and the roots are near full size, lift the crop rather than lose it — small blighted tops won't fill the root further. Then at season's end, bury or burn every last carrot leaf, and give that field a longer break (3–4 years) off the carrot family, and never save your own seed from a blighted crop — that is how it comes back next year.

Catch Carrot leaf blight before it costs you the harvest

This guide is the general answer. Divisi works out yours — the week to scout your own field for Carrot leaf blight, and what to do the day you find it.

See what grows on my farm

Crops it affects

Carrot

Plan the crops it attacks

When to plant Carrot in Plahn Nyarn

All planting calendars, country by country

Frequently asked questions

How do I identify Carrot leaf blight?

The first thing you see. small grey-tan to brown spots scattered on the leaves, each ringed by a darker border and often a thin yellow edge. They are small — about 2–3 mm, the size of a pinhead — round to angular, and they start on the younger inner leaves and the leaf-stalks first.

When should I take action against Carrot leaf blight?