Bacterial blight of cotton: symptoms, spread and control

Photo at the top: Angular leaf spots of bacterial blight on cotton leaves: small dark spots with sharp corners, some joining into larger patches, Photo: Kurt C. Showmaker and others (Standards in Genomic Sciences), CC BY 4.0, cropped

Quick answer

Bacterial blight of cotton is a disease caused by the bacterium Xanthomonas citri pv. malvacearum. It makes small, dark, angular spots on leaves, black streaks on stems ("black arm") and black, sunken spots on bolls. It comes in with seed and old crop waste and spreads in rain. No spray cures it. Use clean seed, resistant varieties and field hygiene.

How to identify bacterial blight of cotton: symptoms

In Hindi the disease is called kapas ka jivanu jhulsa (कपास का जीवाणु झुलसा), and in Telugu patti konakara aakumachcha tegulu (పత్తి కోణాకార ఆకుమచ్చ తెగులు). In English it also goes by angular leaf spot and black arm. Older books name the germ Xanthomonas axonopodis pv. malvacearum or Xanthomonas campestris pv. malvacearum. It is the same disease.

The national IPM package for cotton from NIPHM (National Institute of Plant Health Management) says this disease can attack cotton at every stage, from the seedling to the boll. The signs get different names depending on the plant part:

• Seedling blight: small, dark green, water-soaked spots on the seed leaves (cotyledons), often along the edge. They show first on the underside. The seed leaves can twist, and the infection can move down into the stem. Badly hit seedlings stay small or die.
• Angular leaf spot: small spots that look wet or greasy at first, most clearly on the underside of the leaf. The spots stop at the small veins, so they get sharp corners, like small triangles or squares. They turn brown, then black. Some spots have a yellow ring around them.
• Vein blight: black streaks run along the main veins of the leaf. Cotton Incorporated describes them as looking like lightning bolts.
• Black arm: the disease moves down the leaf stalk into the branches and main stem. It leaves long, sooty black marks. If a mark goes all the way round a branch, the branch can break in strong wind or under the weight of the bolls.
• Boll spots: round, water-soaked, greasy spots on the bolls. They sink in and turn black. They often start at the base of the boll, under the leafy bracts, or along the lines where the boll later opens. Infected bolls can rot, drop early or open badly, and the lint gets stained.

Badly spotted leaves look torn and fall early.

Look-alikes and how to tell them apart

• Alternaria leaf spot: a fungal disease. Its spots are more round, brown, and often show rings inside like a target, with a purple edge on green leaves. Bacterial blight spots have sharp corners, look wet on the underside when young, and turn black. See our guide to Alternaria leaf spot and blight in cotton and other crops.
• Grey mildew: also makes angular spots between the veins, and NIPHM warns that the two are easy to mix up. Grey mildew starts on older, lower leaves after the first bolls set. Turn the leaf over: grey mildew shows a white or grey powdery growth on the underside. Bacterial blight does not.
• Bollworm damage: bolls entered by bollworms often rot too, because germs get in through the hole. Look for an entry hole or a larva inside. Read our guide to pink bollworm symptoms and control in cotton.
• Weedkiller drift: spray blown in from a nearby field can spot or burn leaves. It shows on many plants at once, on the side facing that field, and does not make greasy, water-soaked spots.

How it spreads and when it attacks

The bacteria live in the fuzz and on the seed coat, and in the leaves and stems left in the field after harvest. NIPHM notes that the disease is carried by seed. So a field can get the disease in two ways: from seed that carries the germ, or from old cotton waste left in the soil.

Once a few plants are sick, rain does most of the spreading. Raindrops and wind-driven rain splash the bacteria from spot to spot and plant to plant. Sprinkler or flood water, tools, tractors and people walking through a wet crop can also carry them. The bacteria enter the leaf through its tiny breathing pores and through wounds, for example where wind-blown sand has scratched the leaf.

Cotton Incorporated, a research body for cotton growers, reports that the disease grows and spreads more when heavy rain is followed by warm, humid weather with humidity above 85%. It names day temperatures of about 32 to 38 °C, with warm nights, as the risky conditions. Rain soon after sowing can start an outbreak early. When the disease starts at the seedling stage, the damage later in the season is greater.

In Telangana and Andhra Pradesh, cotton is mostly a kharif crop sown with the monsoon. So the young crop grows through the rainy months, which are the risky time. Across India, NIPHM lists this disease as a major cotton disease in all cotton zones. It notes losses of 5 to 20% are often seen where cotton is grown with irrigation.

Bacterial blight disease of cotton at each crop stage

The disease is a problem of cotton, and every cotton crop in a wet season can be at risk. How much harm it does depends on the stage at which it starts.

Seedling stage: spots on the seed leaves and the young stem can kill seedlings or leave gaps in the field. Early infection also gives the disease a start, so it has the whole season to spread.

Vegetative stage: angular leaf spots and vein blight reduce the green leaf area. Heavy spotting makes leaves fall early, so the plant has less food for flowers and bolls.

Flowering and boll stage: black arm can weaken or break branches that carry bolls. Boll spots lead to boll rot, early boll drop, poor opening and stained lint, which lowers the price you get.

When to act

We could not find an economic threshold (the level of disease at which control starts to pay) for bacterial blight of cotton from a recognised public source, so we do not give one here. NIPHM advises acting on the first appearance of the disease in the field.

Walk the field once or twice a week from the time the seedlings come up, and more often after heavy rain. Turn leaves over and look for small, wet-looking spots with sharp corners. Check the leaf stalks and branches for black streaks, and the base of young bolls for greasy spots. Note which part of the field shows it first.

How to control bacterial blight in cotton without chemicals

• Use clean seed: buy certified seed from a trusted dealer. Seed that is acid-delinted (the fuzz removed in the factory) carries much less of the germ. Cotton Incorporated reports that acid delinting has greatly cut the losses from this disease. Do not sow seed saved from a field that had bacterial blight.
• Grow resistant or tolerant varieties: Cotton Incorporated names resistant varieties as the cheapest way to cut losses. Ask your KVK (Krishi Vigyan Kendra), the agriculture officer or your seed dealer which hybrids sold in your area have tolerance to bacterial blight. Use them in fields that had the disease before.
• Clean up crop waste: after the last picking, remove and destroy cotton sticks and fallen leaves from fields that had the disease, or plough them deep into the soil so they rot. Remove cotton plants that come up on their own between seasons.
• Rotate crops: do not grow cotton after cotton in a field with this disease. A season with a crop other than cotton, such as sorghum, maize or a pulse gives the old waste time to rot and the bacteria time to die out.
• Do not crowd the crop: NIPHM advises avoiding dense planting. Use the recommended spacing so leaves dry faster after rain.
• Manage water: drain standing water after heavy rain. Where you can, avoid sprinkler irrigation on a crop with the disease, because the splash spreads it.
• Keep out of a wet crop: avoid weeding, spraying or walking through the field while leaves are wet. Clean tools and tractor tyres before moving from a sick field to a healthy one.

Chemical control: no spray cures bacterial blight

No spray cures leaves, stems or bolls that already have spots. Bacterial blight is caused by bacteria, not a fungus, so most fungicides do not work on it. A spray can at most protect new growth and slow the spread to healthy plants.

No product on our store has bacterial blight of cotton on its label, so we do not suggest one here. Ask your local agriculture officer or KVK for a product labelled for your crop and this disease. Then use only that product, at the dose and water volume printed on its label.

If you do spray:

• Use one product at a time. Do not mix products in one tank.
• Follow the waiting period before harvest printed on the label, counted from the last spray.
• Wear gloves, a mask, full-sleeve clothes and boots. Spray in the cool part of the morning or evening, not in wind or just before rain.
• Do not spray over flowers when bees are working.
• Read the label and leaflet before use.

Frequently asked questions

What is bacterial blight disease of cotton?

It is a leaf, stem and boll disease of cotton caused by the bacterium Xanthomonas citri pv. malvacearum. It makes dark, angular leaf spots, black streaks on stems and branches (black arm), and black, sunken spots on bolls. It is also called angular leaf spot.

What causes black arm in cotton?

Black arm is a later stage of bacterial blight. The bacteria move from the leaf, through the leaf stalk, into the branches and stem, and leave long black marks there. Branches marked all the way round can break.

Will a fungicide cure bacterial blight in cotton?

No. It is a bacterial disease, so most fungicides do not act on it, and no spray heals spots that are already there. Clean seed, resistant varieties, crop rotation and removing old crop waste are the main control. For a spray, ask your agriculture officer or KVK for a product labelled for this disease on cotton.

Does bacterial blight of cotton come from the seed?

Yes, it can. The bacteria can live on and in the seed and its fuzz, so infected seed can start the disease in a new field. Cotton Incorporated reports that under the right weather, one infected seed in 6,000 can start an outbreak. Sow acid-delinted, certified seed.

Is it the same as bacterial leaf blight of paddy?

No. The names sound alike, but the paddy disease is caused by a different bacterium and does not attack cotton. Read our guide to bacterial leaf blight of rice for paddy.

Related reading

• Alternaria leaf spot and blight: symptoms and control
• Cotton leaf curl virus: symptoms and how to stop its spread
• Pink bollworm in cotton: symptoms, when to spray and control
• Jassids in cotton, okra and brinjal: identify and control leafhoppers
• Bacterial blight of rice: symptoms, spread and control in paddy
• Products for cotton

Sources

• National Institute of Plant Health Management (NIPHM), Directorate of Plant Protection, Quarantine and Storage, and National Centre for Integrated Pest Management, Government of India: Integrated pest management package for cotton
• Cotton Incorporated and US university plant pathologists: Identification and management of bacterial blight of cotton
• Showmaker and others, Standards in Genomic Sciences (2017): The genome of the cotton bacterial blight pathogen Xanthomonas citri pv. malvacearum strain MSCT1 (source of the photo)