Pleurotus ostreatus rust, also known as dark brown or rust-colored spots on the caps and stems of oyster mushrooms, is a bacterial disease commonly observed during the fruiting stage of the mushroom's growth. The causative agent is typically *Pseudomonas* species, which are widely present in the environment. At the initial stages, the lesions appear as small, light-colored spots that usually do not significantly impact the commercial value of the product. However, as the disease progresses, the affected areas expand rapidly, turning darker and forming oval, spindle-shaped, or irregularly shaped patches. These lesions often have a rust-like yellow discoloration surrounding them.
When the infection becomes severe, the entire fruiting body may turn yellow, transforming light-colored mushrooms into yellow ones and dark-colored ones into dull, discolored specimens. In mild cases, the disease might slightly hinder the growth of the mushroom but does not pose a major threat. However, as the damage worsens, it can lead to the death of the fruiting bodies and interfere with the overall development of the oyster mushrooms. It may also create favorable conditions for secondary infections and pest infestations.
Mushrooms affected by rust spots tend to have shriveled stems, poor coloration, and reduced marketability. Severely infected mushrooms may develop distorted gill structures, with incomplete spore release and residual spores trapped within the gill folds. Additionally, the sclerotium (a hardened fungal structure) may become wrinkled and cracked due to the depression of the lesions, further diminishing the product’s quality.
This disease typically occurs under high-temperature conditions, poor ventilation, excessive misting, and humidity levels above 95%. Due to its rapid spread and significant impact, early intervention is crucial.
To manage the disease, improving air circulation and reducing humidity is essential. Fungicides such as 5% lime solution, a 1:600 dilution of bleach, or antecutacin at 500–800 times concentration (containing 120 units of oxytetracycline, streptomycin, gentamycin, or chloramphenicol per mL) can be applied. A mixture of 3 parts bleach and 1 part soda ash, or a 0.5% solution of this combination, is also effective. If the mushroom bed is heavily infected, all affected fruiting bodies should be removed immediately. The yellowing and aging mycelium should be scraped off, the surface cleaned, and after slight drying, a thin layer of lime powder or a small amount of bleaching powder can be applied to reduce pathogen load and prevent future outbreaks.
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