Nostoc

Nostoc is a genus of cyanobacteria that often presents as a problematic "disease" in agricultural settings, particularly in greenhouse nurseries and high-humidity environments. Although not a fungal pathogen, its rapid proliferation on the surface of soil or potting media mimics the appearance of jelly-like fungal colonies, causing significant physiological stress to cultivated plants.

These organisms are photosynthetic prokaryotes that form macroscopic colonies. When hydrated, they expand into gelatinous masses, ranging in color from deep green to blackish-brown. In dry conditions, they appear as inconspicuous, crusty films. This unique biological trait allows them to survive adverse conditions and quickly recover when moisture returns.

Nostoc typically affects nursery seedlings, greenhouse vegetables, and ornamental container plants. The primary damage is indirect; the dense, mucilaginous mats form an impermeable barrier on the soil surface, inhibiting gaseous exchange (oxygen) to the roots. This leads to root suffocation, stunted plant growth, and susceptibility to opportunistic soil-borne pathogens.

The development of Nostoc is driven by specific environmental factors: chronic overwatering, poor greenhouse ventilation, high humidity, and the accumulation of excess fertilizers, particularly phosphorus, on the soil surface. Because these organisms are highly adaptable, they can easily spread via irrigation water, equipment, or even footwear, colonizing any damp, nutrient-rich surface.

Effective control requires a multi-faceted agronomic approach:

• Implement a strict irrigation schedule to allow the surface of the soil to dry between waterings.
• Improve air circulation and ventilation within the greenhouse.
• Use coarse grit or gravel as top-dressing to inhibit light penetration and organism growth.
• Apply copper-based algicides or fungicides if infestations become severe to suppress colonization.