A Study of Meadow-Crop Diseases in New York 11 



uniform yellowing but rather the yellowing spreads outward from the 

 infection foci. A uniform yellowing therefore is an unsafe criterion for 

 determining death of leaves from leaf spots. Since, however, no leaf of 

 the 106 measured shows more than 20 per cent of its area involved, the 

 conclusion is that when more infections occur, the leaf dies and fails to 

 appear on the data sheet. 



This conclusion is pertinent to the discussion regarding the appropriate- 

 ness of the grouping method for determining the degree of severity of the 

 disease, because it gives a reason for the apparent correlation between 

 the number of dead leaves and the number of living leaves on each plant. 

 Since a leaf can withstand only a certain number of infections before it 

 dies, the number of dead leaves should vary directly with the degeree of 

 severity of the disease. A corollary of this conclusion is that the greatest 

 loss arises from leaves killed by the fungus and not from spots in living 

 tissue, even though these spots appear exceedingly numerous to the 

 observer. 



Now that each percentage of diseased leaves has been shown to cause 

 0.25 per cent actual loss, the reduction in yield caused by clover leaf spots 

 in New York can be calculated from the index of infection. Of course, 

 the basic assumption in this calculation is that each organism kills the leaf 

 when approximately 20 per cent or less of its area is involved. This is 

 true within the limits of error of this method of calculation. 



The index of infection, the percentage loss, and the loss in tons that 

 have been figured for each leaf-spot disease of clover and alfalfa are 

 arranged in descending order of importance and presented in table 3. 

 The calculation of the percentage-loss column may be illustrated by 

 Macrosporium leaf spot of red clover. The index of infection is 



_ _ 1. The essential calculation is to find the percentage 



119 

 of diseased leaves, because the loss induced by each per cent is already 

 known. The average percentage of diseased leaves in each group, as 

 defined on page 6, is as follows: heavy, 82 per cent; moderate, 45 per 

 cent; light, 12 per cent; trace, essentially per cent; and none, per cent. 

 If, therefore, the percentage of fields falling into each group be multiplied 

 by the average percentage of diseased leaves in that group, the product 

 will be the percentage of diseased leaves in that group for the State, and 

 the sum of these products will be the percentage of diseased leaves in the 

 entire State (with respect to the disease in question). Thus (0 X 82) 

 + (3 x 45) + (34 x 12) + (25 X 0) + (38 x 0) = 5.43 per cent of the red-clover 

 leaves in the State diseased with Macrosporium leaf spot. Since each 

 per cent of diseased leaves means an actual loss of 0.25 per cent, this 

 disease diminishes the crop by 1.36 per cent. 



