Red algae (Cephaleuros)

Red algae (Rhodophyceae), specifically the genus Cephaleuros, are unique organisms that act as plant pathogens. While most algae are free-living, certain species have adapted to a parasitic lifestyle. This condition, often referred to as algal leaf spot or red rust, involves the algae penetrating the host plant's tissues, primarily through the stomata, where they establish their thallus beneath the cuticle.

This pathogen targets a wide array of woody and perennial crops. Common hosts include tea plants, citrus trees, coffee shrubs, avocado, and various ornamental species like magnolias. The infection is predominantly found in tropical and subtropical climates where humidity levels remain high for extended periods, providing a perfect environment for the pathogen to thrive.

The clinical signs of infestation present as distinct circular spots on the leaf surface. Initially, these may be green or gray, but as the algae mature and produce spores, they turn a characteristic rusty-red or brownish-orange color. The surface of these spots often appears velvety or crusty. Severe infestations can cause leaf necrosis, premature chlorosis, and significant defoliation.

Development and spread are heavily dependent on environmental conditions. High humidity, heavy rainfall, and insufficient airflow within the plant canopy are the primary drivers of an outbreak. The spores are typically dispersed by splashing rain droplets or wind currents, which carry them from infected foliage to healthy parts of the plant or adjacent trees in the nursery or orchard.

Managing red algae requires an integrated strategy to reduce moisture and stress:

• Pruning to increase light penetration and air circulation within the canopy.
• Reducing relative humidity levels through proper site selection and spacing.
• Applying copper-based fungicides when conditions are conducive to disease.
• Sanitary measures, including the removal of infected leaves and debris to lower inoculum levels.