Abstract
Oak dieback is one of the most important diseases that presently affects the Kermanshah oak forests (West Iran). During the period from 2013 to 2015, oak trees exhibiting branch dieback were sampled, and fungal colonies resembling those of the Paecilomyces sp. were obtained from diseased tissues. Based on morphology, physiology, and phylogeny of the internal transcribed spacer rDNA, all isolates were identified as Paecilomyces formosus. Pathogenicity tests in vivo were made on 2-year-old seedlings, potted Q. brantii plants, and excised branch sections under controlled conditions. Inoculated seedlings produced characteristic symptoms, and subsequent pathogen recovery confirmed pathogenicity. We determined these to be thermophilic fungi, which may be related to warming and drought. This is the first report of Paecilomyces formosus as the causal agent of Q. brantii dieback in Iran and the world.