Collemaceous lichens

The family Collemataceae consists of gelatinous lichens that are frequently mistaken for plant diseases. While they are not parasites that derive nutrients directly from the host plant, their colonisation on tree trunks and branches is often considered an indicator of tree decline. They thrive in environments where tree maintenance is insufficient, leading to high humidity and poor light penetration within the canopy.

The biology of these organisms is based on a symbiotic relationship between a fungus and a cyanobacterium. Unlike typical lichens, species in the Collemataceae family have a unique gelatinous thallus structure that absorbs significant amounts of moisture. While they do not penetrate the bark tissue, they create a physical barrier that restricts gas exchange through the lenticels, potentially impacting the bark's metabolic functions.

These lichens predominantly affect fruit trees with rough or aged bark, such as apples, pears, and plums. They prefer shaded, humid sites where the bark surface remains moist for extended periods. In well-pruned and managed orchards, these lichens struggle to establish themselves, as the trees maintain vigorous growth and the bark surface is less conducive to colonization by spores.

Symptoms are easily identifiable: look for dark, jelly-like patches on the trunk that swell when wet and become brittle when dry.

• Reduced shoot vigor due to bark suffocation.
• Presence of micro-cracks where pests can overwinter.
• A decline in the overall photosynthetic capacity of the tree.
• Premature shedding of bark flakes.

Management of these lichens is primarily focused on improving the orchard's microclimate and physical removal. Pruning is the most critical cultural practice to improve light and air circulation, making the habitat unfavorable for lichen growth. In severe cases, scraping the old bark and applying lime wash or iron sulfate solutions during the dormant period is an effective way to sanitize the trunks and branches.