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This study explores various potential substitutes for gypsum in the production of compost for white button mushrooms (Agaricus bisporus). During compost preparation, calcium sulfate (CaSO4) was replaced with calcium carbonate (CaCO3), ammonium sulfate ((NH4)2SO4), and monocalcium phosphate (Ca(H2PO4)2). Complete replacement of gypsum with calcium carbonate led to a significant pH increase during the second phase of composting, adversely affecting mushroom mycelium growth. Compost parameters were observed to be similar in scenarios where calcium sulfate was supplemented with calcium carbonate in 8:2 and 6:4 ratios, both with and without the presence of ammonium sulfate, and in 3:1 and 1:1 mixtures of calcium sulfate and monocalcium phosphate, when compared to traditional gypsum-based processes. All experimental compost mixtures yielded comparable mushroom crops in cultivation trials. Notably, the 8:2 mixture of calcium sulfate and calcium carbonate demonstrated superior performance in cultivation trials relative to the 6:4 mixture. However, supplementing these mixtures with ammonium sulfate resulted in similar crop yields. Monocalcium phosphate also emerged as a promising partial gypsum substitute, showing comparable crop production in both 3:1 and 1:1 ratios to the technological optimum. The exploration of alternative calcium sources like calcium carbonate and monocalcium phosphate reflects the adaptability of the industry in response to resource availability challenges. The potential use of byproducts like ammonium sulfate from the composting process itself offers a cost-effective and environmentally friendly approach to compost formulation, underscoring its worldwide relevance.

White button mushroom—Agaricus bisporus (J.E.Lange) Imbach—is among the most popular cultivated mushrooms worldwide. The most serious challenge in industrial mushroom production is the green mold disease caused by Trichoderma species. Our aim was to isolate and examine bacterial strains from mushroom casing material for their potential use as biocontrol agents. Twenty-seven bacterial strains were isolated and tested against mold pathogens of white button mushroom. The Bacillus velezensis strain SZMC 25431 was selected for further examination and tested under simulated Agaricus cultivation conditions against T. aggressivum SZMC 23834 in a 1200-L Fitotron SGC120 standard plant growth chamber. Our results showed that the bacterial treatment was effective against the pathogen in all cases, but the best results were achieved at an application concentration of 105 cells mL−1. Industrial-scale experiments were also carried out in Agaricus growing houses with a bearing surface of 480 m2: the bacterial suspension was mixed in water tanks applied for daily irrigation. The results suggest that the bacterial treatment may even increase the crop yield of A. bisporus. Based on our results, we concluded that the selected B. velezensis strain may potentially be used for biological and integrated treatment in Agaricus cultivation.