Light microscopic examination revealed that each branch terminated in a globose, multispored sporangium with a conspicuous columella. All four cultures produced abundant aerial sporangiophores that branched dichotomously. Cultures incubated at 25☌ covered 9-cm PDA plates within 2-to-3 days during and aerial mycelium reached the Petri dish lid. Four potato dextrose agar (PDA) pure cultures of the fungus obtained from mature, necrotic basidiocarps and primordia were accessioned into the ARS Culture Collection (NRRL, Peoria, IL), respectively, as NRRL 54814-5488-54819. Mushrooms colonized by the mold showed pitting, discoloration and necrosis. By third break 25% of the production surface at both farms was colonized by a fast growing mycelium that was initially white, subsequently yellow-to-golden brown, and finally grayish. This brown portabella strain is a hybrid between a commercial strain producing white basidiocarps and a brown wild type isolate of A. bisporus at two commercial mushroom farms in Chester County, PA. In August 2011 we first observed a mucoraceous mold colonizing primordia and mature basidiocarps of a relatively new brown portabella strain of A. They are the most important crop in Pennsylvania and an important high value crop in many other states. Technical Abstract: Agaricus bisporus (Lange) Imbach mushrooms are cultivated commercially under environmentally controlled conditions in several states within the US. First report of Syzygites megalocarpus (Mucorales) web mold on the commercial portabella button mushroom Agaricus bisporus in North America. Citation: Beyer, D.M., O Donnell, K., Paley, K., Wach, M.P.
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