A disease syndrome (termed resin disease) of Arceuthobiumamericanum on lodgepole pine is reported in Colorado, Idaho, Montana, Utah, and Wyoming. The disease effectively kills shoots from established dwarf mistletoe plants. In one area in Colorado, all of the dwarf mistletoe plants were diseased, and most of the aerial shoots were killed. Isolations from naturally infected resin disease cankers yielded 11 fungi; Alternariaalternata and Aureobasidiumpullulans were isolated most commonly. In inoculated dwarf mistletoe plants, Alternariaalternata was the most pathogenic, and it was followed closely by Aureobasidiumpullulans. Cladosporiumherbarum and Epicoccumnigrum may also play some role in disease development. Apparently, several weakly parasitic fungi can cause resin disease symptoms. A necrophylactic periderm was found in all naturally and artificially induced resin disease cankers examined and is presumably the mechanism by which dwarf mistletoe shoots are killed. The necrophylactic periderm isolates the shoots and cortical strands from the sinkers. Periderm formation apparently results from the invasion of the pine bark by the resin disease fungi.