Candelariella medians

It is important to clarify that Candelariella medians is not a plant disease in the pathological sense. It is a species of crustose lichen that acts as an epiphyte, colonizing surfaces such as tree bark, rocks, and concrete. It does not extract nutrients from the tree, meaning it is not a parasite, but rather a commensal organism living on the host.

This lichen is typically found in established, older orchards or wooded areas where the bark texture and moisture levels are suitable for colonization. While it does not kill the tree, its presence often indicates that the tree is growing slowly or that the canopy is too dense, preventing sufficient air circulation and sunlight exposure.

Symptoms of colonization include yellow to orange-colored crusty patches appearing on the trunk and primary branches. These patches are visually distinct and can expand over time. The primary concern with these lichen growths is that they provide a humid habitat that can hide insect pests and facilitate the accumulation of moisture, potentially harboring secondary fungal pathogens.

Environmental conditions that favor the spread of this lichen include high atmospheric humidity and lack of pruning. Trees with overgrown or neglected canopies that stay damp for long periods are the most frequent hosts. Improving the tree's overall vigor through proper fertilization and watering is key to naturally preventing widespread colonization.

Management involves basic horticultural hygiene, such as regular thinning of the canopy to increase light and airflow. If the growth is heavy, mechanical removal by gently scraping the bark during the dormant season, followed by appropriate orchard treatments, can help keep the bark clean and prevent the buildup of unwanted moisture-retaining layers.