Thread blight
Fine black silky threads of fungus spread through the nutmeg canopy in the monsoon, blighting leaves and causing defoliation. Pruning for air circulation and copper sprays during wet weather are the main defences.
- Pathogen
- Marasmius pulcherrima / M. equicrinus
- Spreads via
- Black silky threads (rhizomorphs) through the canopy
- Favoured by
- Monsoon rain, dense unpruned canopy
- Best defence
- Pruning, debris removal, copper sprays
Overview
Thread blight is caused by the fungi Marasmius pulcherrima and M. equicrinus, which spread through the canopy as fine, dark, silky threads (rhizomorphs) that look like horse hair or black thread strung between branches. These threads contact leaves and stems, causing blight — the affected tissue dies and the leaves dry and fall. In severe cases, large sections of the canopy can defoliate.
The disease is most active during the monsoon months when continuous rain keeps the canopy wet and humidity is high. Dense, unpruned trees with overlapping branches are most vulnerable because the threads spread by physical contact through the canopy. The fungus survives the dry season on dead leaves and twigs on the ground or trapped in the canopy. Management centres on pruning to open the canopy, removing and burning fallen infected debris, and preventive copper sprays during the wet season.
How to identify it
- Fine, dark (black or dark brown), silky threads appear on stems, branches and the undersides of leaves — they look like horse hair or fine black string running between branches.
- Wherever the threads contact a leaf, the tissue dies: you see irregular brown-black blighted patches that dry out.
- Affected leaves dry up and fall, often still attached to the threads, creating hanging mats of dead leaves in severe cases.
- The threads may form a loose, irregular network across sections of the canopy.
- In severe attacks, large parts of the canopy defoliate, weakening the tree and reducing yield the following season.
Why it happens
- The Marasmius fungi produce rhizomorphs (thread-like structures) that grow through the canopy from branch to branch and leaf to leaf, spreading by physical contact.
- Continuous wet weather during the monsoon provides the moisture these fungi need to grow actively.
- Dense, overlapping canopies trap moisture and provide the physical contact points the threads need to spread.
- Dead leaves and twigs — on the ground or caught in the canopy — harbour the fungus through the dry season.
How to manage it
- Prune overlapping branches well before the monsoon to improve air circulation and reduce the physical contact the fungus needs to spread. This is the most important structural measure.
- Collect and burn fallen infected leaves and trapped debris from the canopy. Do not let them accumulate.
- Spray copper oxychloride at 0.25% or 1% Bordeaux mixture during the monsoon — TNAU advises this for tree-spice crops in the wet season. Confirm the current recommendation with your KVK.
- Avoid overcrowding — maintain proper spacing (8 × 8 m for nutmeg) and thin out any volunteer trees growing too close.
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.
- IISR ViswashreeNutmeg (Jaiphal)
- IISR KeralashreeNutmeg (Jaiphal)
- Konkan SugandhaNutmeg (Jaiphal)
- Konkan SwadNutmeg (Jaiphal)
- Konkan SreemanthiNutmeg (Jaiphal)
- VTLCH-1 (Vittal Cocoa Hybrid 1)Cocoa Bean
- VTLCH-2 (Vittal Cocoa Hybrid 2)Cocoa Bean
- VTLCH-5 / Nethra CenturaCocoa Bean
- VTLCC-1 (Vittal Cocoa Clone 1)Cocoa Bean
- VTLCS-1 (Vittal Cocoa Selection 1)Cocoa Bean
- VTLCS-2 (Vittal Cocoa Selection 2)Cocoa Bean
Frequently asked questions
Does thread blight affect crops other than nutmeg?
Yes. Thread blight caused by Marasmius species is reported on several tropical tree crops including tea, cocoa, rubber and various fruit trees. The management approach — pruning for air circulation and copper sprays in the monsoon — is similar across crops.
Will the tree recover after a thread-blight attack?
Yes, if the attack is managed. The tree will refoliate after the monsoon once new growth starts. Removing the source of infection (dead debris, overlapping branches) and spraying during the wet season prevents repeat attacks. Severe defoliation may reduce the next season's yield but does not usually kill a mature nutmeg tree.
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.