Pertusaria pertusa

Pertusaria pertusa is a species of crustose lichen that is commonly found on the bark of deciduous trees and shrubs. In an agronomic context, it is not considered a true disease-causing pathogen but rather an epiphyte that utilizes the bark as a substrate for growth, deriving its nutrients from the air and rain rather than the tree itself.

This lichen is frequently observed on older fruit trees such as apple, pear, and plum, especially in humid or shaded locations. While it does not directly consume plant tissues, its presence is often an indicator of poor tree vigor or environmental conditions that favor excessive moisture retention on the bark surface.

The visual signs of this organism include the formation of thalli, which are crust-like patches that appear greyish-green or whitish. The identifying feature is the presence of numerous small pores, or perithecia, distributed across the surface, which create a pitted or perforated appearance characteristic of this specific lichen species.

The development of the lichen is largely dependent on environmental factors such as high atmospheric humidity, lack of direct sunlight, and poor airflow. Trees with rough, aging bark provide the perfect anchor point for these organisms, allowing them to expand their surface area over time as the tree remains in a neglected or overly dense state.

The main concern for growers is that dense lichen colonies can provide a hiding spot for overwintering pests, such as scales or mites, and may potentially block gas exchange through the bark lenticels. Control measures focus on preventative pruning to ensure adequate sunlight and air circulation, alongside mechanical removal of the lichen from the bark to support tree health.