Abnormal leaf fall
Monsoon defoliation of rubber by a Phytophthora fungus spread on wind-driven rain — managed by spraying the canopy before the rains, not after symptoms.
- Pathogen
- Fungus-like (Phytophthora spp.)
- Spreads by
- Wind-driven monsoon rain
- Favoured by
- Prolonged heavy monsoon rain and humidity
- Main risk
- Heavy defoliation; up to a fifth of annual yield lost
Overview
Abnormal leaf fall is the single most destructive disease of natural rubber in the traditional belt of Kerala and adjoining tracts. It is caused by Phytophthora fungi, which are splashed and blown onto the canopy by the wind-driven rain of the south-west monsoon. Infected green leaves develop dark, water-soaked lesions, a white fungal bloom appears on the leaf stalks and green seed pods, and the leaves and young fruit fall in quantity, so a badly affected tree is half-bare by mid-monsoon.
The loss of a large part of the canopy through the growing season cuts latex yield, and on highly susceptible clones such as RRII 105 the annual crop can drop by a fifth or more. Because the disease moves with the monsoon rain, the only effective approach is prophylactic — spraying the canopy with a protectant fungicide before the rains set in. Once defoliation has started, a spray largely wastes the cost.
How to identify it
- Sudden heavy fall of green (not yellow) leaves and immature green seed pods during the monsoon.
- Dark, water-soaked lesions on the leaf blade and, characteristically, on the leaf stalk (petiole).
- A white, powdery fungal bloom on infected petioles and green pods.
- Badly affected trees stand half-bare by the middle of the monsoon.
What causes it and when
- Phytophthora fungi (several species), which survive in the soil and on plant debris and are carried up onto the canopy by wind-driven rain.
- Active through the south-west monsoon; worst in prolonged, heavy, wind-driven rain and continuous high humidity.
- Highly susceptible clones (e.g. RRII 105) are hit hardest; some clones are far less affected.
Cultural & clonal control
- Plant less susceptible clones where the disease is severe, following the current Rubber Board / RRII recommendation.
- Keep trees vigorous with balanced nutrition so they can carry and refoliate through an attack.
- Maintain drainage and remove heavy plant debris that harbours the fungus.
Prophylactic spraying
- Spray the canopy with a protectant fungicide — an oil-based copper fungicide or mancozeb — to cover the leaves before the monsoon rain sets in.
- Use a high-volume sprayer or mist blower; large estates use aerial spraying to cover tall canopy in time.
- Complete the round before the rains, not after symptoms — a spray applied once defoliation has begun largely wastes the cost.
- Confirm the current product, dose and spray timing for your area with the Rubber Board or your local RRII office.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Varieties to consider
Named seed varieties whose resistance or susceptibility to this problem is documented on their own variety page.
Frequently asked questions
My rubber trees are dropping green leaves in the monsoon. Is this abnormal leaf fall?
Very likely. Abnormal leaf fall drops green (not yellow) leaves and immature green pods during the monsoon, with dark water-soaked lesions and a white bloom on the leaf stalks. It is caused by Phytophthora spread on wind-driven rain. There is no useful spray once defoliation is under way — plan the prophylactic fungicide round for before next year's monsoon, and consider a less susceptible clone when replanting.
Why does the spray have to go on before the monsoon?
The fungus is carried onto the canopy by the wind-driven monsoon rain itself. A protectant fungicide only works if it is already coating the leaves when the infective rain arrives. Once the rain has spread the disease and the tree is defoliating, spraying does little — the crop and canopy are already lost for that season.
How much yield can abnormal leaf fall cost?
On a highly susceptible clone such as RRII 105, an unmanaged attack can cut the annual latex crop by a fifth or more, because the tree loses much of its canopy through the growing season. Prophylactic spraying and, over the long term, planting less susceptible clones are what keep that loss down.
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.