Thrombium

Thrombium is a genus of lichens often observed in various ecosystems, including orchards. While lichens are not traditional plant pathogens, their presence on the bark of fruit trees is a concern for agronomists, as it suggests the tree may be lacking in vigor or that the environment has become excessively humid and stagnant.

The biological nature of Thrombium involves a symbiotic relationship between a fungus and algae. It colonizes the bark primarily to use it as a structural base. Because it is not a parasite that feeds on the tree's internal vascular system, it does not directly kill the plant; however, it can compromise bark health by retaining moisture and providing habitat for pests.

Key signs of its presence include tiny, often submerged apothecia (fruiting bodies) that appear as spots on the bark. These spots can vary in color, usually blending with the texture of the tree trunk. The development is most noticeable during rainy seasons when the lichen absorbs water, becomes more vibrant, and expands its footprint on the branches.

Conditions favoring its development include high humidity, lack of sunlight reaching the lower branches, and poor air circulation within the canopy. Aged trees with rough, scaling bark are most susceptible. These factors are common in unmanaged or neglected orchards where regular cleaning and pruning are not prioritized.

Control measures are primarily preventive and focus on tree hygiene. It is essential to remove dead bark and debris from the trunks, which reduces the surface area available for colonization. Proper pruning to allow sunlight penetration and air movement is crucial. Additionally, applying a lime wash to trunks during the dormant season remains a highly effective method to manage and discourage growth.