Between 2008 and 2011, outbreaks of cobweb were observed in commercial white button mushroom (Agaricus bisporus) crops in Castilla-La Mancha (Spain). In the last 3 years, the presence of the disease has notably increased resulting in serious economic losses. Based on morphological and genetic analyses, the casual agent of cobweb was identified as Cladobotryum mycophilum. A. bisporus mushroom crops were surveyed over a 2-year period to estimate the incidence of cobweb. The presence of the disease was detected in 32% of the mushroom crops observed, being of particular concern in autumn (44% of crops infected) and winter (37%). As regards the casing material, the percentage of crops affected by cobweb was 34% in crops using mineral casing and 29% in those cased with a peat-based casing, with no statistical relationship between the casing and the presence of cobweb. Two cropping trials inoculated with C. mycophilum were set up to evaluate the pathogenicity of the causal agent of cobweb in three peat-based casings (C1, C2 and C3). The effect of cobweb on mushroom productivity was evaluated by comparing mushroom production and the cobweb patches detected in the casing soil. The decrease in total yield of mushrooms attributed to cobweb reached 12.9% with C2, and the crop area colonised by cobweb reached a final percentage of 36% with C3.