White thread disease of cacao

White thread disease, caused by the fungus Marasmiellus scandens, is a notable fungal infection affecting cocoa trees in various tropical regions. While it is often considered a minor pathogen, it can cause localized damage in plantations, particularly where environmental conditions favor the development of fungal growth.

The disease is characterized by the presence of white, thread-like mycelial strands that grow on the surfaces of stems, twigs, and leaves. Marasmiellus scandens is a necrotrophic pathogen that utilizes the plant surface as a substrate, subsequently invading tissue and causing blight symptoms. The infection can lead to leaf necrosis and the eventual death of branches if left unmanaged.

Key symptoms of white thread disease include:

• Visible white, silky threads or cords on twigs and leaves
• Rapid wilting and death of leaves in contact with the mycelium
• Formation of small, stalked, white mushroom-like fruiting bodies under humid conditions

Development and spread of the disease are highly dependent on high humidity and poor ventilation. Rain splash is the primary mechanism for the dispersal of spores throughout the tree canopy. Consequently, the disease is most prevalent in areas with high rainfall and in plantations where the trees are planted too closely together or lack proper pruning.

Control strategies for white thread disease focus on modifying the microclimate within the plantation. Pruning is the most effective cultural practice, as it opens the canopy and allows better airflow, which reduces humidity levels and hinders the survival of the fungus. Additionally, the removal and destruction of infected materials help to reduce the primary inoculum load in the field.