Flat-top agaricus

Flat-top agaricus (Agaricus praeclaresquamosus) is a toxic mushroom species that frequently acts as a competitive weed in mushroom cultivation facilities. Its presence is highly undesirable because it competes with commercial Agaricus species for nutrients and space within the growing medium.

This fungus behaves as an aggressive saprotroph, utilizing the organic matter in compost beds. In professional mushroom farming, its development is considered a biological hazard, as it can colonize growing blocks rapidly, leading to significant yield losses and potential contamination of harvested batches with toxic fruit bodies.

Key symptoms include the rapid appearance of fruit bodies with a distinct grayish-scaly cap. When the tissue is bruised or cut, the flesh turns a characteristic bright yellow. A strong, unpleasant chemical or medicinal smell, which becomes even more pronounced when cooked, serves as a clear warning sign for farmers to reject the contaminated produce.

Optimal conditions for the spread of this species include warm temperatures (18–25°C) and high moisture levels within the growing houses. The pathogen is typically introduced via contaminated casing soil or through spores transferred by tools, clothing, and air circulation systems if the hygiene protocols are not strictly followed during the composting and filling stages.

Effective prevention focuses on rigorous quality control of the composting process and casing material. Pasteurization must be sufficient to eliminate spores. If identified, manual removal of the fungus before the cap opens is crucial to preventing spore dispersal, coupled with strict sanitation of the area and the implementation of a rigorous biosecurity protocol to minimize further contamination.