Phormidium autumnale

Phormidium autumnale is a filamentous cyanobacterium that often presents as a problematic biological growth in greenhouse environments and nursery operations. While it is not a traditional fungal pathogen, its proliferation on the surface of soil, peat, or hydroponic substrates is classified as a significant agronomic issue that compromises plant health.

This organism primarily affects greenhouse-grown seedlings, transplants, and potted ornamental plants. It thrives on moist substrates where fertilizers are readily available, forming dense, dark-colored mats that cover the surface of the growth medium, effectively insulating the soil from the atmosphere.

Symptoms of infestation include the appearance of a dark green, olive, or blackish slimy layer on the growing medium. As the colony matures, it forms a hard, leathery crust that cracks upon drying. This layer is highly detrimental as it creates a physical barrier that restricts water penetration and impedes essential gas exchange for the plant's roots.

The conditions favoring the development of Phormidium autumnale include persistent high humidity, poor aeration of the substrate, and the presence of abundant light combined with nutrient-rich irrigation water. Incomplete drainage or over-irrigation in greenhouse trays significantly accelerates the spread of these cyanobacterial mats.

Mitigation and control strategies focus on environmental modification rather than harsh chemical intervention. Effective practices include:

• Increasing airflow and optimizing humidity levels in the greenhouse.
• Maintaining proper drainage to prevent standing water on the substrate surface.
• Applying a thin layer of sand or grit to the soil surface to disrupt light penetration.
• Routine shallow cultivation (scratching) of the top layer to prevent mat formation.