Shiitake growers in Virginia are experiencing considerable diminution of the fruiting life of oak logs, due primarily to Ascomycetous, competing, wood-rotting, contaminating "weed fungi" that either invade the logs after trees are felled or are present in tree tissues before felling. We surveyed several shiitake farms, and, although the fungal flora differed among them, the predominant colonizing fungi were identified as Graphostroma platystoma (also identified as Diatrype stygma), Eutypa spinosa and Slereum aff. complicatum. Other less frequently observed fungi were Stereum ostrea, Schizophyllum commune, Hymenochaete sp., Poria sp., Hypoxylon atropunctatum and Hypoxylon punctulatum. Site and means of invasion (colonization) of these "weed fungi" were studied by monitoring endophytic fungal populations in stressed and non-stressed oak trees in the Jefferson National Forest Montgomery County, Virginia. Twenty-six samples from Poverty Creek (non-stressed) and 23 from Brush Mountain (stressed) were studied. Fifty percent of the samples from the non-stressed were sterile, 23% yielded Paecilomyces variota., and 27% were colonized by other fungi. On the other hand, 100% of the samples from stressed trees (Brush Mountain) were colonized by fungi; 74% yielded P. variotii and 26% by other fungi. Thirty samples from a pin oak (Quercus paluslris) plot at Virginia Tech (non-stressed) were 74% sterile, 16% colonized by an unidentified yeast, and 10% colonized by other unknown fungi.