Abstract:Wetwood of poplars was distributed very widely and caused a serious damage in China.A fungus was frequently isolated from wetwood of stems and roots,which were collected from more than 20 Populus species in Hubei,Shandong,Hebei,Liaoning Provinces and Beijing City in 1995 and 1996.According to its morphological and cultural characteristics,the fungus was identified as Fusarium proliferatum(Matsushima)Nirenberg.The results of the laboratory and the field tests showed that F.proliferatum was a powerful pathogenic fungus,and an important pathogen of poplar wetwood disease.The observation with scanning electron microscopy indicated that membranes of vessel-to-ray pits were destroyed by F.proliferatum.The functions of bacteria and physiological factors in the course of wetwood development were discussed.