Penicillium is a common fungal contaminant encountered during the cultivation of *Pleurotus ostreatus*, also known as oyster mushrooms. While it typically exhibits only weak parasitic activity on the fruit bodies, it can still pose a threat under certain conditions. The asexual stage of *Penicillium* belongs to the subgenus *Penicillium* and *Permodium*.
(1) **Pathogen Characteristics**: The mycelium of *Penicillium* is colorless and septate. The vegetative structure appears white, with most hyphae penetrating into the substrate. Very few aerial hyphae are present, often forming a carpet-like or flocculent pattern. The conidiophores have a brush-like appearance at their tips. When spores accumulate in large quantities, they form a powdery layer that appears greenish, yellowish-green, or bluish-green.
(2) **Symptoms and Damage**: In the early stages of infection, white or yellow-white carpet-like colonies appear at the contaminated site. Within 1–2 days, these colonies turn into a light green or light blue powdery mold. The mold is usually white and expands slowly, often being covered by the mycelium of the oyster mushroom. *Penicillium* not only competes with *Pleurotus ostreatus* for nutrients and moisture but also blocks air exchange and secretes toxic substances, which inhibit or halt the growth of the mushroom mycelium at the infected area. If the growing medium becomes overly acidic or has low water content, *Penicillium* may infect the weakly developing fruit bodies, leading to yellowing or rotting symptoms.
(3) **Causes of Infection**: *Penicillium* spores germinate rapidly within 1–2 days at temperatures between 28–30°C. A low moisture content, an acidic environment, and weak *Pleurotus ostreatus* mycelium all favor the growth of *Penicillium*.
(4) **Control Measures**: Effective management includes following general integrated pest management practices for *Pleurotus ostreatus* cultivation. Early detection of contaminants is crucial to prevent hidden infections. During the bag cultivation phase, local contamination can be controlled by injecting 5% formaldehyde solution mixed with 1% clotriazol. Regular spraying of lime water or ash water (from grass or wood) on the culture bed helps promote healthy mycelial growth. In most cases, *Penicillium* does not cause serious problems and is either not present or causes only minor damage.
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