80 James G. Horsfall 



DISEASES OF GRASSES 

 HETEROSPORIUM LEAF SPOT OF TIMOTHY 



Suscepts 



Timothy alone has been found to be a suscept of the Heterosporium leaf 

 spot. Although a similar disease has been observed on red top, it probably 

 is not identical. Gregory (1919:578) states that orchard grass is never 

 infected when inoculated in parallel with timothy. The writer has obtained 

 infection only on timothy in the greenhouse. Inoculations upon Kentucky 

 blue-grass, red top, and orchard grass have uniformly failed of positive 

 results, although timothy in the same series of experiments was always 

 infected. 



The disease 



The name Heterosporium leaf spot is used in this paper to distinguish 

 this from other leaf-spot diseases on timothy. Gregory (1919:576) first 

 applied this name. 



History and range 



This disease apparently was first described in a short paper by Gregory 

 in 1919. He states that he found timothy plants affected by this disease 

 in many parts of New York, and he is of the opinion that it probably 

 occurs wherever timothy is grown (1919:576). The only foreign report of 

 the disease since its discovery is a doubtful case mentioned by J<£rstad 

 (1924:17). 



Economic importance 



Gregory (1919:576) believes that the actual damage is negligible, but 

 states that this is the most common disease of timothy in New York. 

 The writer has observed several cases in which the disease was so severe 

 that 100 per cent of the plants in the field were more or less spotted. 

 Although the loss from leaf spots is hard to estimate, the damage probably 

 is not negligible. 



Symptomatology 



Once the identity of the spots, which occur only on leaves, has been 

 recognized, they are readily distinguished thereafter. The spots are small, 

 oval in shape, with the long axis parallel with the veins. They vary from 

 0.5 to 1 millimeter in width and from 1 to 4 millimeters in length; occasion- 

 ally they are larger. The center of the spot is characteristically light mauve 



