Algal spot

Algal spot is a plant disease caused by the parasitic green alga Cephaleuros virescens. Unlike typical fungal pathogens, this organism is a specialized alga that colonizes the leaves, stems, and fruits of various plants, often leading to significant cosmetic and physiological damage in tropical and subtropical regions.

The pathogen has a wide host range. It is particularly problematic for crops such as avocado, where it affects leaf health, and for ornamental species, including orchids like Cattleya. The presence of the pathogen on these plants is often an indicator of environmental conditions that favor excessive humidity and poor air circulation.

Symptoms typically manifest as circular, raised, or crust-like spots on the foliage. The color can range from greenish-grey to a rusty red or brown, depending on the stage of algal development. Under humid conditions, the colonies may produce fuzzy or velvety patches, which are the reproductive structures of the alga.

The development and spread of Cephaleuros virescens are strictly governed by moisture. Spores are disseminated by splashing rain or irrigation water. Plants kept in dense canopy environments or greenhouses with insufficient airflow are at a much higher risk of severe infestation compared to well-spaced, ventilated plants.

Management of algal spot focuses on reducing humidity and increasing aeration. Key strategies include:

• Regular pruning to improve light and air penetration.
• Sanitation by removing and destroying heavily infected plant parts.
• Application of copper-based fungicides when environmental conditions are conducive to disease outbreaks.