Koleroga (fruit rot / mahali)
A monsoon fungal rot of immature nuts that can drop a bad year's crop by the thousand — prevented with a timed prophylactic spray, not cured after the rot appears.
- Pathogen
- Phytophthora meadii / P. arecae
- Spreads by
- Rain-splashed spores in continuous wet weather
- Favoured by
- Heavy, continuous monsoon rain and high humidity
- Main risk
- Mass immature-nut drop in a bad wet year
Overview
Koleroga, also called mahali or fruit rot, is one of the two defining diseases of arecanut alongside yellow leaf disease. It is caused by the fungus Phytophthora (mainly P. meadii, sometimes P. arecae), which thrives in the continuous heavy rain and high humidity of the monsoon in the wetter western coastal growing tracts of Karnataka and Kerala.
The disease attacks immature nuts on the bunch, starting as a water-soaked patch that spreads into a soft rot, often with a visible white fungal growth in very wet weather. In a bad monsoon year, affected bunches can drop rotted nuts by the dozen or hundred, causing a real, direct loss of that season's crop. There is no cure once nuts are already rotting — the entire management strategy is built around preventing the outbreak with a timed prophylactic spray programme rather than treating it after it appears.
How to identify it
- A water-soaked patch on an immature nut that spreads into a soft rot.
- A visible white fungal growth on affected nuts in very wet weather.
- Rotted nuts dropping from the bunch — by the dozen or hundred in a bad year.
- Symptoms concentrated in the monsoon months, worst in the wettest western coastal tracts.
What causes it and when
- The fungus Phytophthora (mainly P. meadii, sometimes P. arecae), spread by rain-splashed spores.
- Favoured by continuous heavy monsoon rain and high humidity — the same weather that suits the palm itself favours this disease.
- Worst in the wetter western coastal growing tracts of Karnataka and Kerala, where monsoon rainfall is heaviest and most continuous.
- Risk builds through the monsoon months and is highest in years with unusually heavy, unbroken wet spells.
Cultural & organic control
- Ensure good garden drainage so water does not stand around the base of the palms, since this compounds the wet conditions the fungus favours.
- Maintain reasonable canopy airflow across the garden rather than dense, unmanaged overcrowding.
- Once nuts on a bunch are rotting, there is no cure for that bunch — the priority shifts to protecting the remaining healthy bunches on the same and neighbouring palms.
Chemical control
- A prophylactic 1% Bordeaux-mixture spray on the bunches is the standard and most effective defence, timed to start just before the monsoon begins.
- Repeat the spray through wet spells during the monsoon rather than applying it only once.
- Confirm the current recommended spray schedule and concentration for your tract with your local KVK or the Directorate of Arecanut and Spices Development.
Prevent it coming back
- Build the prophylactic Bordeaux-mixture spray programme into the garden's routine annual calendar, not as a reaction to an outbreak.
- Keep drainage and canopy airflow in good order year-round, not just during the monsoon.
- Start spraying just before the rains rather than waiting for the first wet spell to pass.
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 arecanut nuts are rotting and dropping during the monsoon. What is this, and can I still save them?
This is almost certainly koleroga (fruit rot / mahali), caused by Phytophthora fungus that thrives in continuous wet monsoon weather. Once a nut is rotting, there is no cure for that nut or bunch — the priority now is protecting the remaining healthy bunches with a Bordeaux-mixture spray and improving drainage, and starting the prophylactic spray programme earlier next season, just before the monsoon begins.
How do I prevent koleroga next year?
Start a 1% Bordeaux-mixture spray on the bunches just before the monsoon begins, and repeat it through wet spells during the rains rather than spraying only once. Good garden drainage and canopy airflow also help reduce the humid conditions the fungus favours. This is a prevention-only disease — there is no effective treatment once nuts are already rotting.
Why does this disease seem worse in some years than others?
Koleroga is directly driven by how continuous and heavy the monsoon rain is — years with unusually persistent, unbroken wet spells give the fungus the sustained humidity it needs to spread fast through a garden, while a monsoon with more breaks in the rain gives lower disease pressure even with the same spray programme.
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.