Physarum

Physarum is a genus of slime molds (myxomycetes) that occasionally manifest on the surface of agricultural crops. While they are not typical plant pathogens that invade living tissues, they act as surface contaminants that can smother plants, leading to mechanical stress, reduction in photosynthesis, and an increased risk of secondary infections.

The causative agents are unicellular organisms that form a plasmodium—a multinucleate mass of protoplasm capable of moving across surfaces. This vegetative structure feeds on bacteria, fungal spores, and organic matter present on the plant surface, rather than feeding directly on the plant's cells, yet its presence remains an agronomic concern.

Commonly affected plants include greenhouse vegetables, nursery seedlings, and low-growing crops where the foliage comes into contact with moist soil. The physical presence of the slimy substance creates a micro-climate of excessive humidity beneath it, which often leads to the development of necrotic spots and soft rot caused by opportunistic bacteria.

The development of Physarum is strictly dictated by environmental conditions, particularly moisture levels.

• High relative humidity in greenhouse structures.
• Waterlogged soils or inadequate drainage systems.
• Excessive organic debris on the soil surface providing a food source.
• Lack of airflow between plants in dense stands.

Management and prevention focus on eliminating the conditions that favor slime mold growth. Agronomists recommend improving soil drainage, increasing ventilation in greenhouses, and maintaining a clean planting environment by removing rotting debris. By lowering the surface moisture and improving air circulation, the habitat for the plasmodium is effectively destroyed, preventing further spread.