Mushrooms are grown in specially prepared compost which favors the growth of mushroom mycelium. In the same way a plant growing out of place is referred to as a weed, unwanted mold (also referred to as fungi), growing out of place are often referred to as weed molds.
Some people refer to these weed mods as indicator molds because, when other molds grow it is an indicator that some part of the substrate preparation process has not been carried out appropriately.
Weed or indicator molds do not directly ‘attack’ the mushroom but they compete for the nutrients and some appear to be antagonistic to mushroom mycelium.
Unlike many weed molds Pythium is antagonistic and can inhibit or prevent the growth of mushroom mycelium.
Weed molds may be defined as molds that grow in competition or in association with mushroom mycelium. These fungi compete for nutrients and may have a negative influence on the growth and nutrient uptake by Agaricus Bisporus.