Abstract: In the region of East China, Marssonina leaf blight of poplars is caused by Marssonina populi (Lib.) Magn. The blight induces high mortality of poplar seedlings and premature defoliation of plantation and shade trees. According to the observations carried out the causal fungus on different host poplars can bo divided into two specialized forms. For the first form, its conidia germinates one germtubc well in tap water but seldom germinates in distilled water; the germination temperature ranges from 12° to 28°C; no water-soluble inhibitor of germination is to be detected on the surfaces of fleshly collected spores; Populus adenopoda, P. tomentosa and the hybrids of the poplars of Section Leuce are subject to be infected in nature and in artificial inoculation of detached leaves in vitro. This form is named Marssonina populi f. sp. monogermtubi. The name of the other form is marssonina populi f. sp. multigermtubi, the conidia of which germinates 1-5, mostly 2-3 germtubes in both tap and distilled water; the germination temperature ranges from 16° to 32°C; a water-soluble germination-inhibiting substance is to be detected, so only after washing will spores have a high potential for germination. The main natural hosts of this form include many species of Section Aigeiros and their hybrids, and hybrids of Section Aigeiros with Section Tacamahaca, but infection has never been found on species of Section Taca-mahaca such as P. cathayana, P. maximowiczii and P. yunanensis. By artificial inoculation of detached leaves, almost all the poplars examined including some species of Section Leuce and Section Tacamahaca are susceptible to infection.