Mycopathogens are serious threats to the crops in commercial mushroom cultivations. In contrast, little is yet known on their occurrence and behaviour in nature. Cobweb infections by a conidiogenous Cladobotryum-type fungus identified by morphology and ITS sequences as Hypomyces odoratus were observed in the year 2015 on primordia and young and mature fruiting bodies of Agaricus xanthodermus in the wild. Progress in development and morphologies of fruiting bodies were affected by the infections. Infested structures aged and decayed prematurely. The mycoparasites tended by mycelial growth from the surroundings to infect healthy fungal structures. They entered from the base of the stipes to grow upwards and eventually also onto lamellae and caps. Isolated H. odoratus strains from a diseased standing mushroom, from a decaying overturned mushroom stipe and from rotting plant material infected mushrooms of different species of the genus Agaricus while Pleurotus ostreatus fruiting bodies were largely resistant. Growing and grown A. xanthodermus and P. ostreatus mycelium showed degrees of resistance against the mycopathogen, in contrast to mycelium of Coprinopsis cinerea. Mycelial morphological characteristics (colonies, conidiophores and conidia, chlamydospores, microsclerotia, pulvinate stroma) and variations of five different H. odoratus isolates are presented. In pH-dependent manner, H. odoratus strains stained growth media by pigment production yellow (acidic pH range) or pinkish-red (neutral to slightly alkaline pH range).