Blister blight (tea)
Translucent spots on the tender pluckable leaves that swell into white, powdery blisters — the key wet-season leaf disease of tea.
- Pathogen
- Fungus (Exobasidium vexans)
- Spreads by
- Air-borne spores from the blisters
- Favoured by
- Cool, wet, cloudy weather; long leaf wetness
- Main risk
- Spoiled pluckable leaf; lower yield & quality
Overview
Blister blight, caused by the fungus Exobasidium vexans, is the most important leaf disease of tea, attacking the young, tender, harvestable leaves and shoots — exactly the part that is plucked and made into tea. It begins as small, pale, translucent (oily-looking) spots on the young leaves; these enlarge and the underside swells into a characteristic white-to-greyish, velvety, powdery blister (bearing the spores), while the upper side becomes concave.
The blistered shoots become brittle and break, and heavy infection reduces both the quantity and the quality of the made tea and weakens the plucking table. It is strictly a wet-season disease — favoured by cool temperatures, long leaf wetness, high humidity, persistent cloud and low sunshine during the monsoon/high-rainfall period. Because it attacks the crop that is plucked every few days, management combines shorter plucking rounds (removing infected tissue), keeping the bushes open, and a protective fungicide programme through the wet season.
How to identify it
- Small, pale, translucent (oily-looking) spots on the young, tender leaves.
- Spots enlarging so the underside swells into a white-to-greyish, velvety, powdery blister, with the upper side concave.
- Blistered shoots becoming brittle and breaking, spoiling the pluckable crop.
- Appears and spreads only in the cool, wet, cloudy monsoon/high-rainfall period.
What causes it and when
- The fungus Exobasidium vexans, whose spores form on the white blisters and spread on air and moisture.
- Strictly favoured by cool temperatures, long leaf wetness, high humidity, persistent cloud and low sunshine.
- A wet-season disease — it builds during the monsoon/high-rainfall period and fades in dry, sunny weather.
- Attacks the young pluckable leaves and shoots, so it directly hits the harvested crop.
Cultural & organic control
- Pluck at shorter rounds in the wet season so infected tender leaf is removed before the fungus sporulates and spreads.
- Keep the bushes and field open — good spacing, pruning and drainage, and reduced shade — so the canopy dries.
- Remove and manage excessive shade during the wet season to raise light and lower humidity.
- Maintain vigorous, healthy bushes with balanced nutrition to withstand infection.
Chemical control
- Run a protective fungicide programme through the wet season — CIB&RC-registered products for tea blister blight (e.g. copper-based and recommended systemics), rotated to avoid resistance.
- Time and repeat sprays to protect each new flush between plucking rounds, observing the plucking/pre-harvest interval strictly (the leaf is eaten as tea).
- Adjust spray frequency to the weather — more often in prolonged wet, cloudy spells.
- Confirm the current fungicide programme and intervals for your area with your local KVK/tea research body.
Prevent it coming back
- Make shorter wet-season plucking rounds and an open, well-drained, less-shaded canopy the routine.
- Plan the protective fungicide programme for the wet season, matched to the weather and plucking cycle.
- Keep the bushes vigorous and remove excess shade to reduce the cool, wet, low-light conditions it needs.
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 tea leaves have translucent spots that swell into white powdery blisters. What is it?
Those pale, translucent spots on the young pluckable leaves that swell into white-to-greyish powdery blisters underneath are blister blight, the key wet-season disease of tea. It spoils exactly the tender leaf you pluck and weakens the bush. Pluck at shorter rounds to remove infected leaf, keep the canopy open and drier, and run a protective fungicide programme through the wet season — observing the plucking interval strictly.
Why does plucking more often help control blister blight?
Because blister blight attacks the young, tender pluckable leaves, and the fungus needs those leaves to form its white spore-bearing blisters. Plucking at shorter rounds in the wet season removes the infected tender tissue before it sporulates and spreads to the next flush, so it both harvests the crop and lowers the disease. Combined with an open, drier canopy and a protective fungicide programme, it keeps blister blight in check.
Is blister blight a problem all year round?
No — it is strictly a wet-season disease. It needs cool temperatures, long leaf wetness, high humidity, cloud and low sunshine, so it builds up during the monsoon/high-rainfall period and fades in dry, sunny weather. That is why control is concentrated on the wet season: shorter plucking rounds, an open drier canopy, and a protective fungicide programme timed to the wet, cloudy spells when the disease is active.
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.