Chrysothricaceae lichens

Chrysothricaceae are a family of lichens that often colonize tree bark. In agricultural science, they are not considered primary plant pathogens, but rather epiphytes. Their presence is significant because they often act as bioindicators of tree health, signaling that a tree may be under stress or that the microclimate in the orchard has become overly humid and poorly ventilated.

These lichens consist of a fungal component in symbiosis with algae. They do not penetrate the plant tissues to derive nutrients, which distinguishes them from parasitic fungi. However, their physical accumulation on the bark can interfere with the tree's natural respiratory processes by covering lenticels, which are essential for gas exchange between the interior of the tree and the atmosphere.

Fruit-bearing species such as apple, pear, and various stone fruits are primary hosts. These lichens flourish in orchards that lack regular maintenance, specifically where pruning is neglected. Densely packed canopies prevent sunlight from drying out the bark, creating a permanently damp environment that supports the growth of Chrysothricaceae.

Symptoms are easily identified by the appearance of bright yellow or sulfur-colored powdery patches on the trunks and main scaffold branches. Unlike crustose lichens that grow flat and hard, members of this family are often loosely attached and powdery to the touch. Heavy colonization can provide a safe haven for over-wintering insect pests and fungal spores.

Control strategies focus on cultural practices rather than direct chemical extermination. Mechanical cleaning of the bark, especially on older trees, is the most effective approach. This should be followed by the application of copper or iron-based solutions (like Bordeaux mixture or iron sulfate) during the dormant season to sanitize the bark surface and prevent re-colonization.