Thread blight

Thread blight is a fungal disease caused by the pathogen Corticium stevensii (syn. Hypochnus stevensii). It is a basidiomycetous fungus known for its ability to damage the foliage and twigs of various woody plants, leading to severe defoliation and reduced productivity of the affected trees.

The primary host for this pathogen is the apple tree, though it can impact various other fruit and ornamental species. The disease is most prevalent in humid and warm environments, as moisture is a critical factor for the growth and spread of the fungal mycelium.

The characteristic symptoms include the appearance of brown, thread-like mycelial strands on the twigs and leaf petioles. As the disease advances, leaves turn dark brown or black, wither, and die. Interestingly, the dead leaves often remain attached to the twigs, held in place by the fungal threads, which is a key diagnostic feature of this condition.

The development of Thread blight is heavily dependent on high relative humidity, usually exceeding 85%, and temperatures that favor fungal metabolic activity. The fungus overwinters primarily in the form of mycelium or sclerotia on infected branches or in dead leaf debris trapped within the tree canopy.

Effective disease management relies on cultural practices and targeted chemical application. Growers should prioritize the following:

• Rigorous pruning of infected branches to reduce the inoculum load.
• Improving air circulation within the orchard by thinning the canopy.
• Removal and destruction of fallen leaves and infected plant debris.
• Application of copper-based fungicides during the dormant or early spring season to prevent new infections.