Boll rot (cotton)
Cotton bolls rot brown-black from the inside once insects or wet weather open a way in for germs — a problem of wounds and wetness, not one single germ.
- Cause
- Several fungi/bacteria entering wounds
- Main entry point
- Insect-chewed holes, boll cracks
- Worst in
- Wet, humid weather at boll opening
- Main loss
- Stained, downgraded lint
Overview
Boll rot is not caused by one single fungus. It happens when several different fungi and bacteria that are already sitting on the plant get inside a boll through a wound — most often a hole chewed by a bollworm or sucking bug, or a crack in the boll wall — and then multiply fast in warm, humid weather.
Once inside, the germs rot the boll from the inside out. Affected bolls turn brown to black, do not open cleanly, and sometimes show visible fuzzy fungal growth if you split one open. Even where some lint does emerge, it is stained and weak, and mills pay less for it. The problem is worst in a dense, humid canopy during the wet weeks when bolls are opening, and it tracks closely with how much insect damage the crop has already taken.
How to identify it
- Bolls turn brown to black instead of the normal cream colour, and fail to open cleanly at maturity.
- A rotten boll may feel soft or show a fuzzy fungal coating if you pull it open.
- Lint that does come out of a partly-rotten boll is stained yellow-brown, weak and lower grade.
- Damage clusters in the lower, shadier part of a thick canopy and in patches where insects have been feeding.
What causes it and when
- Several common fungi and bacteria that live harmlessly on the plant surface get inside a boll through a wound and then rot it — there is no single germ to blame.
- The most common wound is a hole chewed by a bollworm or pink bollworm, or a puncture from a sucking bug; splits in the boll wall from heavy rain also let germs in.
- Warm, humid weather during the boll-opening weeks lets the germs multiply fast once they are inside.
- A thick, over-fertilised canopy stays damp longer after rain or dew, which keeps the risk high for longer each day.
Cultural & organic control
- Control bollworm and sucking bugs promptly — fewer insect wounds means far fewer bolls get infected in the first place.
- Avoid excess nitrogen and overly close spacing; a thinner canopy dries faster after rain and lets more air and light reach the lower bolls.
- Pick mature, open bolls promptly rather than leaving them in a wet field, and remove and destroy clearly rotten bolls so they do not spread germs to healthy ones nearby.
- Where the field allows, avoid irrigating right before or during the boll-opening weeks.
Chemical control
- There is no cure for an already-rotten boll — the priority is stopping the insect damage and wetness that let germs in.
- A protective fungicide spray (such as a copper-based fungicide) during a wet boll-opening spell can help on fields with a history of heavy boll rot, alongside insect control.
- Confirm the current recommended fungicide and dose for your area with your local Krishi Vigyan Kendra (KVK), since the advice can change by season and by which germs are most active locally.
Prevent it coming back
- Keep bollworm and sucking-pest numbers down through the season — this is the single biggest lever, since most boll rot starts at an insect wound.
- Grow the crop at the recommended spacing and nitrogen dose so the canopy stays open enough to dry quickly after rain.
- Pick promptly at maturity and do not let opened bolls sit wet in the field.
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
What actually causes boll rot — is it one fungus?
No, it is usually several different fungi and bacteria acting together, not one single germ. They are normally present on the plant and only become a problem once they get inside a boll through a wound, most often one chewed by a bollworm or sucking bug. That is why controlling insects matters as much as any spray.
Can I spray something to cure a rotten boll?
No — once a boll is rotten there is no cure, and the lint inside is already damaged. The useful spray is a protective one, applied before or during a wet spell to stop new infections, alongside controlling the insects that create the entry wounds in the first place.
Why does boll rot get worse in a thick, heavily-fertilised field?
A dense canopy with too much nitrogen stays wet longer after rain or dew because air cannot move through it — and germs need that wetness to multiply. Growing the crop at the recommended plant spacing and nitrogen rate keeps the canopy more open, so it dries faster and boll rot has less chance to build up.
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.