Pleurocapsa minor

Pleurocapsa minor is a species of cyanobacteria that can act as an epiphyte and, under specific conditions, a harmful agent for cultivated plants. While typically considered a soil or aquatic alga, it can colonize the stems and foliage of plants in high-humidity environments, forming dense biofilms that impair the plant's health.

This organism primarily affects crops grown in greenhouses, hydroponic systems, or nurseries where moisture levels are consistently high. It is particularly common on the stems of sensitive vegetable seedlings, such as tomatoes and cucumbers, where it creates a persistent damp layer that interferes with tissue function.

The primary symptom of infestation is the development of a dark blue-green or brown slimy coating on the plant stems and substrate surface. This biofilm is notoriously difficult to remove and has a distinct, sometimes earthy odor. Unlike fungal pathogens, this algal growth thrives specifically on moisture availability rather than tissue necrosis.

Environmental conditions that favor the spread of Pleurocapsa minor include high humidity (above 80%), poor air circulation, and stagnant water on surfaces. Furthermore, excessive nutrient runoff or fertilizer accumulation on surfaces provides the necessary chemical environment for these cyanobacteria to bloom rapidly.

The damage caused by this organism is largely indirect. By forming a physical barrier, it blocks stomatal gas exchange and reduces the intensity of light reaching the epidermis. Additionally, the presence of the biofilm keeps plant tissue constantly damp, creating an ideal environment for secondary bacterial and fungal pathogens to invade the plant.

• Maintain optimal greenhouse humidity through mechanical ventilation.
• Implement strict irrigation schedules to prevent standing water.
• Apply specialized sanitization protocols for greenhouse surfaces and tools.
• Regulate nutrient concentrations in fertilizers to limit algal nutrition.