40 James G. Horsfall 



It appears also that the disease is more severe under cool conditions. 

 It is most destructive in New York on the first cutting, but this may be 

 due in part to the more humid conditions prevailing during that stage of 

 growth. Another bit of evidence in favor of the cool- weather hypothesis 

 is that none of the Southern States except Virginia and Tennessee have 

 reported the disease. These two States, especially in the mountainous 

 sections, are cooler than the other Southern States. Jones (1919:18) 

 found that spores are produced more abundantly below 18° C. than above 

 that point. If this condition holds for the field, then the disease should 

 be expected to occur more commonly in the Northern States and even 

 there to be worse in the early months of the growing season. 



Control 



The control of this important disease is an enticing problem which is 

 still to be solved. 



Miss Massee (1914:66-67) in England advocates disinfestation of seed 

 with hydrogen peroxide, since she maintains that the fungus is carried on 

 trash with the seed and even on the seed itself. Jones (1919:35) was 

 unable to prevent the introduction of the fungus into fields by seed 

 treatment. 



In view of the evidence above as to the suscept specificity of the Pseudo- 

 poziza patho«enes, Miss Massee's (1914) suggestion to clean up weed 

 species of Trifolium and Medicago is of little value. 



Sulfur dust for the control of this disease has been tried by the writer 

 both in the field and in the greenhouse. In field experiments in 1926 

 eight contiguous plots, each 1 rod by 3 rods in size, separated by alleys 

 1 yard wide, were dusted checkerboard fashion with Kolodust, making 

 four dusted and four undusted plots. The dusting was begun on May 22 

 and was continued at approximately weekly intervals for six weeks. 

 Pseudopeziza leaf spot was the most serious disease on the clover in the 

 plots, so that weight yields may be taken as a fair index of the control of 

 the disease. At harvest time the green weight of clover from each plot 

 was determined to the nearest one- fourth pound. The average of the 

 dusted plots was 331.3 pounds and that of the undusted plots was 315.8 

 pounds, a difference of 15.5±7.5 pounds as calculated from pairing the 

 contiguous plots. It is obvious that the difference is not significantly 

 great, and, even if it were, it is too small to be of economic importance. 



This conclusion was checked by counting all the clover leaves on three 

 scattered areas 1 foot square in each plot. The leaves were divided into 

 four groups: namely, dead, containing more than 10 spots on each leaf, 

 containing less than 10 spots on each leaf, and disease-free. More than 

 10,000 leaves were examined. 



The data thus obtained showed that the dusted plots had 5.3 per cent 



