Lecanora

Lecanora is a genus of crustose lichens that often colonize the bark of trees and shrubs. While lichens are technically a symbiotic association between fungi and algae rather than a parasitic disease, their presence on trees is frequently an indicator of poor plant vigor, high humidity, or lack of proper orchard management.

The organism attaches itself to the bark surface, using the tree merely as a substrate. Although it does not directly consume nutrients from the plant's vascular system, its dense growth can lead to physiological stress by restricting the bark's ability to breathe and by trapping moisture against the surface for extended periods.

Lecanora typically affects mature fruit trees, including apple, pear, and plum varieties, especially those with rough or aging bark. It is also commonly found in neglected orchards where canopy density prevents sunlight from reaching the trunk, creating a dark and humid environment favorable for colonization.

Symptoms are easily identifiable as small, circular, crusty patches that vary in color from pale gray to yellowish-green. As the colony matures, it may form characteristic apothecia—small, disk-like reproductive structures that stand out against the bark, eventually causing the underlying tissue to flake off prematurely.

Environmental conditions favoring the development of Lecanora include high humidity levels, poor air circulation within the tree canopy, and low light exposure. Trees that have not been pruned for years are particularly susceptible, as their shaded, stagnant interiors provide the perfect habitat for lichen to thrive.

The harm caused by Lecanora is indirect but significant. The dense cover provides a refuge for harmful insects like scale insects and mites, and the constant moisture trapped beneath the lichen can promote the development of wood-rotting fungi. Consequently, heavily infested trees may show reduced growth and decreased fruit production.

Preventative measures focus on maintaining tree health and vigor through regular pruning to ensure proper sunlight penetration and airflow. Mechanical removal of the lichen using soft-bristled wire brushes during the dormant season is effective, followed by an application of a lime-wash to maintain tree hygiene and prevent regrowth.