Protoparmelia

Protoparmelia is a genus of lichens that often colonizes the bark of mature trees. While they are not pathogens in the strict sense, as they do not invade the living tissues of the host, they are considered unwanted epiphytes in commercial and home orchards. Their presence indicates that a tree may be under stress or that the environmental conditions are overly humid and stagnant.

These organisms typically affect perennial crops, with pome and stone fruit trees being the most frequent hosts. They thrive on the rough, dead bark of older trees, where crevices provide a stable surface for the lichen thallus to attach. Over time, the lichen colonies can spread to cover large sections of the trunk and primary branches.

The symptoms of colonization include the appearance of crust-like or leafy patches on the bark, which vary in color from pale grey to brownish-green. These patches can trap moisture against the bark surface, leading to an environment that is favorable for the survival of insect pests such as scale insects and mites, and potentially harboring spores of true fungal diseases.

The development and spread of Protoparmelia are facilitated by high humidity levels, poor air circulation within the tree canopy, and low light penetration. Orchards that lack regular pruning and maintenance are the primary targets, as the lack of light and airflow creates the perfect microclimate for lichen colonization and rapid growth.

Managing lichens in the orchard involves a combination of cultural and physical methods. The primary control strategy is the physical removal of lichens using a soft-wire brush or scraper during the dormant season, taking care not to damage the healthy cambium. Maintaining tree vigor through proper fertilization, regular pruning to open up the canopy, and the application of lime-based whitewash are the most effective preventive measures.