Thelotrema

Thelotrema is a genus of lichenized fungi that colonize the bark of trees and shrubs. While often perceived by gardeners as a plant disease, it is technically an epiphyte—an organism that grows on the surface of another plant without causing direct harm to its internal tissues or nutrient supply systems.

The organism consists of a fungal partner and an algal partner living in symbiosis. The fungus provides structural support and protection, while the algae produce nutrients through photosynthesis. Because it stays on the exterior of the bark, it does not extract water or minerals directly from the tree, unlike true pathogenic fungi or parasitic plants.

This lichen is commonly found on trees with thick, rough, or slow-growing bark. It favors humid environments and shaded areas where moisture persists on the bark surface. It can be found on a wide range of deciduous and tropical trees, often indicating that the tree has a stable, undisturbed bark surface that has not been shed for some time.

The primary visual symptom of Thelotrema is the development of apothecia, which appear as small, crater-like structures embedded in the thallus. These structures are distinct, usually circular with a noticeable rim, giving the bark a textured, speckled appearance that differs significantly from moss or other common lichen species.

While usually benign, excessive colonization can have negative impacts on orchard health.

• Sheltering grounds for insect pests and their eggs during winter.
• Potentially trapping excessive moisture, which may encourage wood rot pathogens.
• Compromising the bark’s ability to perform gas exchange effectively.

Management strategies include physical cleaning of the bark, pruning for better air circulation, and ensuring the tree is kept in a vigorous state of growth, which encourages natural bark renewal.