8 James G. Horsfall 



(4) by increasing the percentage of leaves that shatter during harvest. 

 The operation of the second and third factors mentioned results in sub- 

 sequently diminished growth and a lowered quant ty of sugars and other 

 carbohydrates available for food in the hay. Stem and root diseases 

 cause more profound injuries than do leaf -spot diseases, because they 

 attack the supporting and conduct on systems of the plants, resulting in 

 loss of vitality, in wilting, and in stunting. 



Amount of loss 



Method of determining relative importance 



The acquisition of an index of infection for each disease places the 

 investigator in a position to determine the relative importance of the 

 diseases. 



Method of determining absolute loss 



The index of infection was used as a basis for calculating the absolute 

 loss caused by these leaf -spot diseases. Methods involving the measuring 

 or weighing of plants from representative fields of each infection group 

 were abandoned because variations in soil, stand, age of stand, age of 

 plants, varieties, air and water drainage, as well as inaccuracy of balances 

 with pans large enough to hold a plant, all function jointly in making 

 such methods valueless. 



It thus was essential to make all measurements in the same field and, 

 if possible, on the same plants. The possibility of leaf measurements pre- 

 sented itself in this connection. A field of red clover in which about 45 

 per cent of the leaves were diseased, and was therefore moderately infected 

 with Macrosporium sarcinaeforme, was chosen for these leaf measure- 

 ments. The disease in this field seemed as nearly ideal as possible for 

 study because (1) the clover was relatively free of other pathogenes; 

 (2) the area of the leaf spots, which were practically circular, was easy 

 to determine by averaging two diameters; and (3) the clover, which was 

 growing rapidly, gave a fair amount of foliar material on each plant with 

 which to work. 



Plants picked at random five paces apart were brought to the laboratory 

 and were placed in water. The number of dead leaves was noted, the 

 area of each living leaf was found with a planimeter, and the spots on the 

 leaf were classified according to size in thirteen groups ranging from 1/50 

 up to 13/50 inch in diameter. From these data the total leaf area, the 

 total spotted area, and the percentage of spotted leaf area for each plant 

 could be calculated. 

 The figures thus obtained are set forth in table 1. 



