Recently a serious outbreak of weed mould caused by a species of Hypocrea occurred in oyster mushroom (Pleurotus ostreatus) substrates in Korea. The disease was characterized by a rapid infestation of the oyster mushroom substrates by Hypocrea sp. and subsequent inhibition of fructification of the mushroom. In spite of it`s serious losses to the oyster mushroom industry in Korea, etiology and ecology of the disease have not been studied. Morphological characteristics of the fungus were examined and molecular characteristics of the fungus were compared with those of the green moulds (Trichoderma spp.) isolated from oyster mushroom bed. Stromata formed superficially on suface of the substrates were pulvinate to effuse or irreguler, initially white but becoming yellowish brown, measuring
. Perithecia were globose to subglobose, immersed in stroma,
{\times}167.3{\sim}231\;(Ave.204.1){\mu}m$})
in size. Asci were unitunicate, cylindrical, nonamyloid,
{\times}4.1{\sim}5.1\;(Ave.4.9){\mu}m$})
in size, 16 part-spored. Ascospores were bullet-shaped or somewhat oblong, hyaline, bicellular, roughened or warted,
{\times}3.6{\sim}5.5\;(Ave.4.7){\mu}m$})
in size. This fungus readily form the stroma on PDA. Mycelia on PDA nearly invisible and without cottony aerial mycelium. Optimum temperature for mycelial growth of this fungus was
on PDA and its growth rate was 15 mm per day. This species did not grow at below 10 and above
. Phialides in culture enlarged in the middle and aggregated to penicillate type. They were very variable, shorted ampulliform and occasionally curved when matured, but cylinderical when young, measuring
{\times}2.9{\sim}3.9\;(Ave.\;3.4){\mu}m$})
when matured and
{\times}2.8{\sim}4.9\;(Ave.\;3.5){\mu}m$})
when young. Phialosopres were ovoid to ellipsoid, smooth, measuring
{\times}2.6{\sim}3.3\;(Ave.\;2.9){\mu}m$})
. Nineteen isolates of Hypocrea sp. were analyzed on the basis of molecular characteristics and classified into phenotypic groups. On the basis of RAPD, URP-PCR, the fungus was confirm to monoclonal, and was classified as a different taxon from reported species of Hypocrea and Trichoderma and supposed to be a new species not previously reported in literature.
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