Apatococcus

Apatococcus is a genus of unicellular green algae that frequently colonizes the bark of trees. While not a pathogenic fungus, it is often treated as a biological concern by arborists and agronomists because its presence indicates an ecosystem imbalance on the tree's surface, manifesting as a characteristic green film.

This organism is commonly found on a wide range of woody plants, particularly fruit trees like apples, pears, and cherries, as well as various ornamental species. It thrives on bark surfaces that remain damp and shaded for extended periods, frequently appearing on the north side of trunks or in heavily shaded orchard sections.

The primary symptom is the appearance of a bright green or yellowish-green powdery coating. Unlike lichens, which have a more complex structure, Apatococcus forms a smooth, sometimes slightly slimy layer that obscures the natural color and texture of the tree bark, often being confused with moss or fungal infections.

Environmental conditions are the main drivers of its development. High humidity, poor air circulation within the canopy, and low light intensity create the perfect environment for Apatococcus to proliferate. It is most active during mild, damp seasons when the bark stays wet enough for the algae to undergo rapid cell division.

The primary detrimental impact is the impairment of the bark's physiological functions, specifically gas exchange through lenticels. By creating a physical barrier, these algae inhibit proper respiration, potentially leading to weakened bark health. Furthermore, they can harbor pathogens and insect pests. Effective management includes:

• Pruning to increase canopy airflow and light penetration.
• Applying whitewash to tree trunks to disinfect the surface.
• Scrubbing dead bark tissue in early spring to remove growth substrates.
• Applying copper-based fungicides to inhibit algal colonization.