Apatococcus lobatus

Apatococcus lobatus is a common terrestrial unicellular green algae that colonizes the bark of various tree species. Often perceived as a plant disease by amateur gardeners, it is biologically an epiphyte, meaning it lives on the surface of the plant without penetrating or extracting nutrients from the living tissues of the host tree.

This organism primarily targets deciduous trees, especially those with rough or deeply fissured bark. It is most frequently found in humid environments, including orchards with poor ventilation, dense forest edges, and urban parks where air moisture levels remain consistently elevated for extended periods.

The primary symptom is the appearance of a thin, powdery, or velvet-like layer of bright green color on the trunk and branches. This growth is typically more prominent on the north-facing side of the tree, where sunlight is scarce and moisture evaporation from the bark surface is much slower than on the sunlit side.

The development of Apatococcus lobatus is driven by ecological factors rather than parasitic activity. High relative humidity, lack of sunlight, and stagnant air are the main triggers. Once established, the colony can spread rapidly across the bark, as it is highly resilient to changing environmental conditions, including moderate temperature fluctuations.

While the algae itself is not directly lethal to the host, it can be problematic as it traps moisture against the bark, potentially creating a micro-environment for wood-decay fungi or concealing insect pests. Preventive management involves improving orchard aeration through thinning cuts, maintaining tree vigor through proper fertilization, and cleaning the bark during the dormant season with appropriate fungicidal or cleansing washes.