A Study of Meadow-Crop Diseases in New York 7 



In illustrating the method of assembling data, suppose a single suscept, 

 red clover, be used. The data for each disease are arranged in a fraction, 

 the " index of infection." The total number of fields of red clover observed 

 is the denominator, which is the same for all diseases of this suscept. 

 Each of the five groups of infection is represented by a bracket of figures in 

 the numerator, the figures showing the percentage of cases in which the 

 amount of infection has been placed in that group. For example, 

 Pseudopeziza leaf spot has been found to be represented by the fraction 



(3) (24) (42) (13) (18) 



— — From this fraction, it is readily concluded that 



119 



red clover was observed in 119 fields; the disease was adjudged heavy in 

 3 per cent of the cases, moderate in 24 per cent, light in 42 per cent, a 

 trace in 13 per cent, and 18 per cent of the fields showed none of it. If, 

 therefore, the numerator be large toward the left side, the disease is severe 

 in the State; but if large toward the right side, the disease is relatively 

 unimportant. Unfortunately, the fractions may not necessarily be 

 directly comparable. Anthracnose is a case in point. It has the following 



fraction: l^__i_: . . Hence it may be said to be not so serious 



119 

 as the Pseudopeziza leaf spot. Such a conclusion probably is fallacious 

 in as much as the anthracnose organism attacks stems and tends to stunt 

 or kill parts above the lesion, whereas Pseudopeziza causes only local injury 

 to the leaves. 



It became more and more evident as the work progressed that the 

 selection of the grouping method was a happy one, for in all fields in the 

 same group approximately the same ratio appeared to exist between the 

 percentage of healthy leaves, the percentage of leaves in various stages of 

 disease, and the percentage of dead leaves. If, for example, 60 per cent of 

 the leaves were infected, then about 15 per cent of these would be dead, 

 and the others would lie along an infection gradient from severely infected 

 to healthy. If such a relation held true, then a method of measuring 

 the loss, say in a field rated as moderately infected, should be applicable 

 to the other groups as well. But to find such a method — - that was more 

 difficult. 



economic importance 



Nature of loss 



Leaf-spot diseases of meadow crops induce loss (1) by lowering the 

 quality or palatability of the affected tissues, (2) by reducing the carbo- 

 hydrate-producing area, (3) by causing defoliation during growth, and 



