1. Brown Rot in Phoenix Oyster and Oyster Mushrooms is caused by Pseudomonas tolaasii bacteria. The symptom shows up as light yellow dots on mushroom caps, which change to brown and spread all over. There will be a yellow or red-brown eruption on the stalk skin, which will go down when water is poured on the spot. This disease makes the mushroom smaller than the usual size, the caps light brown and easily bruised and therefore not desirable in the market.