The invention is a new biological substance blocking the spread of pathogens, primarily in oyster mushrooms and champignons, by secreting an antibiotic and thus acting as an antagonist. The new biological substance is highly effective against the Pseudomonas tolaasii and Pseudomonas agarici pathogens, which are partly responsible for the formation of the rot described above. Bacterial blotch spread fast in mushroom cultivations therefore the crop loss reaches 50-60%. Up to now there are only anorganic copper solutions on the market but useable active pesticide agent against Pseudomonas spp. is not available. Applying this new biological substance reduces the loss therefore it makes a better yield and better quality in mushroom cultivations.