Pleurotus ostreatus rust, also known as dark brown or reddish spots on the caps and stems of oyster mushrooms, is a bacterial disease that commonly affects the fruiting bodies during their growth phase. The causative agent is *Pseudomonas* spp., a group of bacteria widely found in nature. At the initial stage, the lesions appear as small, light-colored spots and typically do not significantly impact the commercial value of the mushrooms. However, as the disease progresses, the spots increase in size and darken, often forming oval, spindle-shaped, or irregularly shaped patches. These lesions are frequently surrounded by a yellowish discoloration.
When the infection becomes severe, the entire mushroom bed can be affected, leading to yellowing of the fruiting bodies. Light-colored mushrooms may turn yellow, while darker ones may lose their color entirely. In mild cases, the disease might slow down fruiting body development but usually doesn’t pose a serious threat. However, if left unchecked, it can cause fruit body death, hinder the growth of oyster mushrooms, and create conditions favorable for secondary infections and pest infestations.
Mushrooms affected by rust spots often show shriveled stems, poor coloration, and reduced market value. Severely infected specimens may develop wrinkled, distorted gills, with incomplete spore release and residual spore dust trapped in the pleats. The sclerotium (the hardened fungal structure) may also become cracked and dehydrated due to the progression of lesions, ultimately rendering the mushrooms unsellable.
This disease thrives under high-temperature conditions, poor ventilation, excessive misting, and relative humidity levels above 95%. It spreads quickly and can cause significant damage if not managed properly.
To prevent and control Pleurotus ostreatus rust, it’s essential to improve air circulation and reduce humidity. Fungicides such as a 5% lime solution, a 1:600 dilution of bleach, or an antecutacin solution (500–800 times diluted) containing 120 units of oxytetracycline, streptomycin, gentamycin, or chloramphenicol per milliliter 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 bed cleaned, and after slight drying, a thin layer of lime powder or a small amount of bleaching powder can be spread to reduce pathogen load and prevent future outbreaks.
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