162 BOTANICAL GAZETTE [mance 
least may weaken it to such an extent that it is not able to with- 
stand the adverse factors in its environment. In a nursery of 
young elm trees these leaf spots may do much more damage than 
when they occur on older trees. 
Most important American leaf spot 
DISTRIBUTION AND HISTORY 
Chief among the fungi causing leaf spots of the elm in this 
country is Gnomonia ulmea (Schw.) Thiim. This disease, known 
as the elm leaf spot or elm leaf scab, occurs most commonly on 
Fic. 1.—Distribution of Gnomonia ulmea in United States 
U. americana, and is found in greater or less degree throughout 
the entire range of its host. The writer has examined exsiccati 
specimens of it which were collected in New York, Massachusetts, 
Vermont, Maine, New Hampshire, Rhode Island, Connecticut, 
Pennsylvania, Ohio, Michigan, Indiana, Illinois, Wisconsin, North 
Dakota, Iowa, Nebraska, South Dakota, Missouri, Kentucky, 
Tennessee, North Carolina, Georgia, South Carolina, and Texas, as 
well as several from Canada. Text fig. 1 gives a better idea of its 
wide distribution than does this list of states. It is more than 
probable that it occurs also in the remainder of the states east of 
