THE CONTROL OF THE SUGAR-REET LEAF-SPOT.^ 



By V. W. Pool and M. R. McKay, Scientific Assistants, Cotton and Truck Disease and 



Sugar-Plant Investigations. 



INTRODUCTION. 



A detailed investigation of the leaf-spot of the sugar beet is at 

 present bemg carried on in cooperation with the American Beet- 

 Sugar Co., at Rocky Ford, Colo. Although the study is as yet 

 incomplete, a brief account of the results thus far obtained and 

 interpreted may prove helpful to those interested in sugar-beet 



problems. 



THE CAUSE OF THE LEAF-SPOT. 



The leaf -spot of the sugar beet is caused by a parasitic fungus. 

 In its development this fungus, Cercospora heticola Sacc, produces 

 fruit bodies or spores. Wlien these spores fall upon the surface of a 

 beet leaf, the temperature and moisture conditions being favorable, 

 they germmate and push through the breathmg pores mto the 

 Ulterior of the leaf. After an entrance has thus been gamed the 

 fungus continues to grow, kUls the adjacent leaf portions, breaks out 

 through the dead area or ''spot," and here produces more spores, 

 which in turn cause infections at other pomts on the leaf surface, 

 and thus the life cycle of the fungus is continued. The masses of 

 spores on the diseased areas give to the leaf-spot the grayish color 

 which is so readUy recognized durmg July, August, and September. 

 The spot is usually surrounded by a red ring, which represents the 

 attempt of the beet plant to isolate the invading organism and pre- 

 vent its further spread through the leaf. The individual spots do 

 not increase to any great size, but new spots are contmually bemg 

 formed, and when the number reaches approximately a thousand 

 the entire leaf is killed and the beet plant is forced to produce more 

 leaves in order to keep normal the manufacture of the sugar in the' 

 plant. If too many leaves are killed the beet plant can only main- 

 tain its normal sugar production by temporarily reducmg the amount 

 in the root. Accordingly, sugar from the root is transferred and 

 used in the development of new leaf tissue. Since this sometimes 

 requires from 3 to 5 per cent of the sugar content of the root, the 

 final total is correspondingly reduced. 



1 Issued Apr. 12, 1913. 

 [Cir. 121] • 13 



