Abstract: Melampsora rostrupii Wagner severely infected the seedlings and young trees of Populus tomentosa L,which caused a sharp decreasing in the growth-rate both in length and in width.Generally,the mycelia of the pathogen overwintered in the winter buds, or in the cambium region of the current twigs occasionally.Although the teliospores were existed,yet since no alternate host had been found,that they were of little importance in the occurrence of the disease. Uredospores were produced abundantly on young leaves,leafscars,or infected stems, beginning from the end of March.Such initial infection would soon become an out- break of the disease from May,until the end of September. Uredospores germinated quickly on the back of leaves after inoculated at 19.8— 23.6℃ in 1—2 days at a relative humidity of 100 per cent.However,none or very few spores germinated on the upper surface of the leaf.The development of the epidemic required a monthly mean temperature 18.2—26.8℃,and it would be of little influence when a relative humidity was below 48.1%.Thus,temperature was the main environ- mental factor which influenced the development of this epidemic. Uredinia hypophyllous,scattered or somewhat confluent,roundish,small,1 mm in width;naked when young,somewhat pulverlent,orange-yellow,light-yellow on the oppo- site sides of the pustules,and amphigenous on infected leaves. Uredospores globose,sub-globose or oval,15—22.5×17.5—27.5μ,wall colorless,3μ thick,evenly verrucose with fine papillae;paraphyses capitate or clavate,smooth,50— 75μ long,head 15—23μ broad,wall 3—6μ thick. Telia hypophyllous,sometimes amphigenous,scattered,around the old uredinia,ir- regular roundish,small,1/2—1 mm width,subepidermal,slightly elevated,greyish-brown; teliospous prismatic,1-celled,flat or round at the head,broader in top,7.5—10× 37.5—50μ,light brown,smooth,wall 1—2μ thick. Based on these characters,this fungus is considered to be Melampsora rostrupii Wagner,and it is believed to be the first collection in this country.