10 



James G. Horsfall 



more profound effects than if they appear late. In as much as this effect 

 is extremely difficult to evaluate, the calculation has not been altered 

 to account for it. 



In the course of the measurement of the diseased leaves, the question 

 arose as to the amount of affected tissue necessary to induce yellowing 

 and death. Obviously, the reactions in each spot affect the physiology 

 of the adjacent tissue, and hence the entire area need not actually be spotted 

 before the leaf dies. The most severely diseased leaf measured in pre- 

 paring table 1 had 19.82 per cent of its area visibly spotted; another leaf 

 had 17.28 per cent of its area spotted. To determine whether these leaves 

 approached the limit, a number of the most severely spotted unyellowed 

 leaves and severely spotted incipiently yellowed leaves were collected 

 from the same field and were measured as before. These data are 

 presented in table 2. 



TABLE 2. 



Percentage of Area of Red-Clover Leaves Spotted when Severely 

 Diseased by Macrosporium Sarcinaeforme 





The assumptions in making these measurements are (1) that the in- 

 cipiently yellowed leaves represent a more advanced stage of laminar 

 infection than do the unyellowed ones; (2) that therefore a larger per- 

 centage of their area is spotted; and (3) that a point midway between 

 these two extremes is the very largest percentage of area that could be 

 diseased without killing the leaf. A glance at table 2, however, shows 

 that these assumptions, at least in part, are false, because the unyellowed 

 leaves show more diseased area on the blades than do those just beginning 

 to turn yellow. The reason for the discrepancy fortunately is not far to 

 seek, for the incipiently yellowed leaves almost invariably have lesions 

 on their petioles. No such lesions, however, appear on leaf no. 1 in the 

 table of the unyellowed series or on leaf no. 5 of the yellowed series. These 

 petiolar lesions induce a general yellowed condition in the blades, causing 

 them to die promptly, whereas blade infection alone does not induce a 



