Grey mildew (cotton)
Pale angular spots bounded by the veins with a frosty grey-white mould underneath — a late-season cotton leaf disease that sheds the leaves.
- Pathogen
- Fungus (Ramularia areola)
- Spreads by
- Air- & rain-splashed spores; debris
- Favoured by
- Humid, cloudy, dewy late season; dense crop
- Main risk
- Late-season leaf shedding; poor boll fill
Overview
Grey mildew (also called areolate mildew, Ramularia areola) is a common fungal leaf disease of cotton, typically appearing in the second half of the crop. It makes pale, angular spots on the leaves that are bounded (limited) by the leaf veins, so the spots look sharp-cornered; on the underside of these spots a frosty, greyish-white powdery mould (the fungal growth) develops, which is the diagnostic sign.
As the spots multiply and merge, the affected leaves yellow and shed, and heavy grey mildew can defoliate the crop in the late season — just when the plant needs its leaves to fill and mature the bolls. That loss of leaf area cuts boll development, yield and fibre quality. It is favoured by humid, cloudy, wet weather and heavy dew late in the season, and by dense, over-nitrogen crops. It carries over on debris. Management combines balanced nutrition and canopy management, resistant varieties, and a protective fungicide from the first spots in a humid late season.
How to identify it
- Pale, angular spots on the leaves that are bounded (limited) by the veins, giving a sharp-cornered look.
- A frosty, greyish-white powdery mould on the underside of the spots — the diagnostic sign.
- Spots multiplying and merging so leaves yellow and shed, especially in the second half of the crop.
- Late-season defoliation just when leaves are needed to fill and mature the bolls.
What causes it and when
- The fungus Ramularia areola, spread by air- and rain-splashed spores and carried over on debris.
- Favoured by humid, cloudy, wet weather and heavy dew, mostly in the second half of the crop.
- Worse in dense, over-nitrogen crops with a closed, slow-drying canopy.
- Builds late in the season, so it hits during boll development and maturation.
Cultural & organic control
- Grow tolerant varieties and avoid over-dense, over-nitrogen crops with a closed canopy.
- Use recommended spacing and balanced nutrition so the canopy is open and dries after dew/rain.
- Remove and destroy infected debris that carries the fungus over.
- Keep the crop vigorous and balanced so it holds its leaves through boll fill.
Chemical control
- Spray a CIB&RC-registered fungicide from the first spots in a humid late season, before heavy leaf shedding.
- Cover the underside of the leaves, where the frosty mould forms, and rotate fungicide groups.
- Repeat as recommended through the humid, dewy late-season period, observing the pre-harvest interval.
- Confirm the current product, dose and timing for your area with your local KVK.
Prevent it coming back
- Use tolerant varieties, recommended spacing and balanced nitrogen for an open, drier canopy.
- Scout the lower/older leaves from mid-season and protect the crop before heavy shedding in a humid year.
- Destroy infected debris and avoid the dense, over-fed crops grey mildew favours.
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 cotton leaves have pale angular spots with a frosty grey mould underneath, and are shedding. What is it?
Pale angular spots bounded by the veins, with a frosty greyish-white powdery mould on the underside, that yellow and shed the leaves late in the season, are grey mildew (Ramularia areola). The late defoliation cuts boll fill and yield. Grow a tolerant variety, use recommended spacing and balanced nitrogen for an open, drier canopy, destroy debris, and spray a registered fungicide from the first spots in a humid late season, covering the leaf undersides.
Why is late-season timing so important for grey mildew?
Because grey mildew mostly appears in the second half of the crop, exactly when cotton needs its leaves to develop and mature the bolls. If it defoliates the crop then, the plant cannot fill the bolls properly and yield and fibre quality drop. So the key is to protect the canopy through boll development — scouting the older leaves from mid-season and spraying from the first spots in a humid late season — rather than reacting once the leaves are already shedding.
How do I reduce grey mildew without over-relying on sprays?
Focus on the canopy, since grey mildew loves a dense, humid, slow-drying crop. Grow a tolerant variety, use recommended spacing and balanced nitrogen (avoiding an over-fed, closed canopy), and keep the field open so leaves dry after dew and rain. Destroy infected debris to reduce carry-over. These steps lower the disease pressure so that a protective fungicide from the first spots in a humid late season works better and may be needed less.
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.