Test the cheapest and fastest first. A thermometer takes a minute, a hand lens five, a pH strip ten, a laboratory days. For brown-spotted caps: read temperature and RH now and check how long caps stayed wet; look for mites or larvae; only then treat blotch as the working diagnosis.
Causes chain. A room above 20 °C and 85 percent RH is an environmental fault that also invites blotch; low pH invites green mould; flies carry spores bag to bag. Correct the condition first, because it is under your control this hour, then contain the organism. If a chemical is considered at all, current label approval for mushrooms in India comes first. This site gives no doses.