Alternaria blight / leaf spot
Concentric-ring brown spots on leaves, stems and pods — worst on mustard, where pod infection shrivels the seed and cuts oil, and it rides on the seed.
- Pathogen
- Fungus (Alternaria)
- Spreads by
- Seed, wind & rain-splashed spores
- Favoured by
- Warm, humid, cloudy weather
- Main risk
- Pod spotting; shrivelled seed, low oil
Overview
Alternaria blight (or leaf spot) is a common fungal disease of mustard/rapeseed, cole crops, sunflower, pigeonpea, sesame and others. It shows as dark brown to black spots, often with concentric rings and a surrounding yellow halo, on the leaves, and it also blights the stems, flower stalks and pods.
On mustard it is one of the most damaging diseases: heavy leaf spotting defoliates the plant, and — most costly — spots on the pods shrivel the developing seed and lower both yield and oil content. Alternaria is favoured by warm, humid, cloudy weather with dew or light rain, and it is seed-borne, so infected seed both carries the disease into a new crop and reduces germination. Management therefore centres on clean/treated seed, rotation, and protective fungicides at the leaf and pod stages.
How to identify it
- Dark brown to black spots, often with concentric rings and a yellow halo, on the leaves — merging in a heavy attack to blight and dry them.
- Spots and blight on the stems, flower stalks and, in mustard, on the pods.
- In mustard, pod spots that shrivel the seed inside, lowering yield and oil content.
- Favoured in warm, humid, cloudy weather; worse on late-sown and dense crops.
What causes it and when
- Alternaria fungi, spread by wind- and rain-splashed spores and — importantly — carried on the seed.
- Favoured by warm, humid, cloudy weather with dew or light rain, and by dense, late-sown crops.
- Seed-borne infection both introduces the disease and reduces germination of infected seed.
- Carries over on infected crop debris and volunteer/related host plants.
Cultural & organic control
- Use clean, disease-free, treated seed (a seed treatment is a key step against this seed-borne disease) and grow tolerant varieties.
- Sow at the recommended time (timely-sown crops escape the worst) and avoid over-dense stands and excess nitrogen.
- Rotate with non-host crops, and remove and destroy infected debris and volunteers that carry it over.
- Treat seed with a bio-agent such as Trichoderma where recommended.
Chemical control
- Spray protectively from the first leaf spots and again at flowering–pod formation to protect the pods, which decide yield in mustard.
- CIB&RC-registered fungicides for the crop (e.g. mancozeb and other recommended protectants) are used and rotated.
- Cover the foliage and pods, and observe the pre-harvest interval.
- Confirm the current recommendation and seed treatment for your crop and area with your local KVK.
Prevent it coming back
- Always sow clean, treated seed and rotate crops — the two most important steps against a seed-borne, debris-carried disease.
- Sow on time, manage density and nitrogen, and destroy infected debris and volunteers.
- Protect the pod stage in mustard with a timely spray, rotating fungicide groups.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My mustard leaves and pods have brown spots with rings. What is it?
Those dark brown spots with concentric rings and yellow halos on the leaves, stems and pods are Alternaria blight/leaf spot. On mustard the pod spots are the costly part — they shrivel the seed and cut oil content. Use clean, treated seed, sow on time, rotate crops, and spray protectively from the first leaf spots and again at pod formation.
Why does treating the seed matter for Alternaria?
Because Alternaria is seed-borne — the fungus rides on infected seed into the new crop and also lowers the seed’s germination. So a seed treatment (with a recommended fungicide or bio-agent) is one of the most important steps: it protects germination and reduces the disease being reintroduced. Combine it with clean seed, rotation and timely protective sprays.
When should I spray mustard for Alternaria?
Spray protectively from the first leaf spots, and — crucially — again at flowering to pod formation, because in mustard the spots on the pods shrivel the seed and cut oil, which is where the yield loss really happens. Use a registered fungicide, cover the foliage and pods, rotate groups, and observe the pre-harvest interval.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.