The aim of the research was (i) to evaluate white and cream strains ofA. bisporusyields when challenged or not with the pathogenL. fungicola, (ii) to identify the more aggressive pathogen isolates, and (iii) to develop a diagrammatic scale of spot symptoms on infected mushrooms. The experiment was carried out using two strains ofA. bisporus(white and cream strains, ABI 19/01 and PB 19/01, respectively) and four isolates ofL. fungicola(LF 19/01, FL 19/02, LF 19/03, and LF 19/04). TheA. bisporuswhite strain (ABI 19/01) reached a higher yield of healthy mushrooms compared to the cream strain (PB 19/01) with values of 23.8 and 14.1%, respectively. The LF 19/03 pathogen was most aggressive, reducing the yield of healthy mushrooms by up to 64% for the PB 1/01 strain, and 49.6% for the ABI 19/01 strain. Additionally, for the same isolate, larger mushroom areas were affected, while also displaying symptoms precociously during the second flush. Finally, using the set of visually displayed symptoms assessed in this study, we were able to construct a diagrammatic scale to assist commercial mushroom growers for managing diseased crops.