Botrydium granulatum

Botrydium granulatum is a species of yellow-green algae that frequently colonizes the surface of moist soils in greenhouses and nurseries. While not a classic fungal pathogen, it poses a significant risk to horticultural production by altering the surface soil environment, which can severely hinder the development of germinating seeds and young seedlings.

The organism consists of a visible vesicular body attached to an extensive rhizoidal system anchored in the soil. Under favorable conditions, it forms a dense, green granular layer on top of the substrate. This layer effectively acts as an impermeable seal, disrupting the necessary gas exchange between the soil and the atmosphere and potentially suffocating developing root systems.

This alga primarily affects vegetable and ornamental seedlings grown in high-humidity environments. Crops such as tomatoes, peppers, and various nursery-grown flowers are particularly susceptible. The presence of the algae can cause stunted growth, chlorosis, and increased seedling damping-off mortality due to the poor physical condition of the soil surface.

Development is driven by constant moisture, poor ventilation, and lack of direct sunlight. The algae thrive in compacted or poorly drained soils that remain waterlogged for extended periods. Once established, it can rapidly colonize large areas of nursery beds if the environmental conditions remain stable and conducive to its growth.

Control measures emphasize cultural practices aimed at drying the soil surface and improving air circulation. Proper management includes strictly regulating watering intervals to allow for surface drying, ensuring adequate spacing between containers to improve airflow, and increasing ventilation in greenhouse structures. Top-dressing with coarse sand can help prevent the accumulation of moisture on the soil surface, making it difficult for the algae to maintain its hold.