Differential diagnosis is a long name for a short habit: before choosing a cause, list the other causes that could give the same sign, then rule them out one at a time. In a mushroom room the causes fall into three bins.
Infectious: a fungus, bacterium or virus grows on the crop, spreads from a point, and needs a source such as casing, spawn, air or hands. Pest: an insect, mite or nematode feeds and leaves tunnels, adults or larvae a hand lens can find. Environmental: temperature, moisture, gas or compost chemistry is wrong; the sign is uniform or tied to a spot and disappears when corrected. The actions differ: hygiene, screens and traps, or a thermometer and a valve.