James G. Horsfall 



Figure 1. 



PERCENTAGE OF CROP LAND IN NEW YORK DEVOTED TO TIMOTHY AND CLOVER 

 MIXTURE 



a minimum expenditure of time in each field. It is essentially the same 

 as that used by Kock (1927) in determining the susceptibility of apple 

 varieties to powder}' mildew. 



The following classification of the amount of disease present was used: 

 heavy, moderate, light, a trace, and no infection. The limits in the 

 percentage of diseased leaves or stems for each group, depending upon 

 the nature of the malady, were arbitrarily established as follows: heavy, 

 from 65 to 100 per cent; moderate, from 25 to 65 per cent; light, from a 

 trace to 25 per cent; trace, only a few scattered cases in a field. At first 

 these limits were founded upon actual counts of each disease in each field, 

 but after the groups had been used a short time, ability to estimate the 

 percentages rather accurately was attained. The limits of each group are 

 widely separated, to be sure, but since a great man}' factors, such as 

 local rainfall, air and soil drainage, density of stand, variety, and soil 

 types, jointly determined the amount of infection, the method probably is 

 as useful as that of making several counts of each disease in each field. 



