Discolouration due to senescence differs from blotch-related discolouration, which might indicate differential mechanisms being operational. In samples infected with bacteria or with a partially purified toxin extract, a higher degradation of total tyrosinase than in senescening mushrooms was found. Simultaneously, the active tyrosinase was increasing resulting in an increase in percentage of active tyrosinase. Phenolic substrates of the active tyrosinase were being oxidized, proportionally to the damage detectable on the mushroom cap. γ-Glutaminyl-3,4-dihydroxybenzene was degraded first, followed by γ-glutaminyl-4-hydroxybenzene and later tyrosine. The amount of melanin that was synthesized was larger than the sum of oxidation of the phenols measured. Principal Component Analysis explained 84% of the variance in symptoms, and it demonstrated the phenol oxidation and active tyrosinase level as the most important parameters for the browning induced by bacteria or a tolaasin preparation treatment.