ARS | Publication request: First report of Syzygites megalocarpus (Mucorales) web mold on the commercial portabella button mushroom Agaricus bisporus in North America

Submitted to: Plant Disease
Publication Type: Research Notes
Publication Acceptance Date: September 4, 2012
Publication Date: January 15, 2013
Citation: Beyer, D.M., O Donnell, K., Paley, K., Wach, M.P. 2013. First report of Syzygites megalocarpus (Mucorales) web mold on the commercial portabella button mushroom Agaricus bisporus in North America. Plant Disease. 97(1):142.1-142.1.

Technical Abstract: Agaricus bisporus (Lange) Imbach mushrooms are cultivated commercially under environmentally controlled conditions in several states within the US. They are the most important crop in Pennsylvania and an important high value crop in many other states. In August 2011 we first observed a mucoraceous mold colonizing primordia and mature basidiocarps of a relatively new brown portabella strain of A. bisporus at two commercial mushroom farms in Chester County, PA. This brown portabella strain is a hybrid between a commercial strain producing white basidiocarps and a brown wild type isolate of A. bisporus. 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. Mushrooms colonized by the mold showed pitting, discoloration and necrosis. 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 54814-54815 and 54818-54819. Cultures incubated at 25°C covered 9-cm PDA plates within 2-to-3 days during and aerial mycelium reached the Petri dish lid. All four cultures produced abundant aerial sporangiophores that branched dichotomously. Light microscopic examination revealed that each branch terminated in a globose, multispored sporangium with a conspicuous columella. In addition, individual cultures of NRRL 54818 and 54819 produced large (175-250 x 200-250 µm) barrel-shaped, dark brown to black zygosporangia between opposed suspensors, indicating these isolates were homothallic. Results of the phenotypic analysis indicated the fungus was Syzygites megalocarpus (3), a member of the Mucorales reported to colonize diverse, mostly fleshy basidiomycetes (2), including cultivated matsutake (Tricholoma matsutake) in Korea (1). Molecular phylogenetic confirmation of the morphological identification was obtained by PCR amplifying and sequencing domains D1 and D2 at the 5’ end of the nuclear ribosomal large subunit (LSU rDNA). A search of the NCBI nucleotide database, using a partial LSU rDNA sequence from NRRL 54814 as the BLAST query, revealed that it shared 99.5% identity with AF157216.1, a reference isolate of S. megalocarpus NRRL 6288 (4).