CONTEOL OF THE SUGAR-BEET LEAF-SPOT. 15 



during the preceding year and where the tops were allowed to remain 

 upon the ground, the first infection in the new crop appeared upon 

 the leaves which rested upon the ground or were more or less sub- 

 merged and held to the ground through irrigation. In contrast to 

 this, a field of beets which followed alfalfa for the first time showed 

 a period of infection about three weeks later than the preceding. 

 Moreover, the first symptoms of the disease were upon the leaves 

 which were well exposed to the air. Tliis may be explained in the 

 following way: When the new beet leaves attain sufficient size the 

 temperature and humidity being favorable, the fungous organism 

 in the old leaves, which lie immediately beneath the new growth, 

 becomes active and infects the new leaves near the ground. Now, 

 if there happens to be no material affording such source of infection, 

 as in the case of the second field mentioned, where sugar beets fol- 

 lowed alfalfa, infection becomes impossible excej^t from living spores 

 of the leaf-spot fungus carried in the air. Such infection occurs only 

 after the spores formed upon the lowest leaves find their way into 

 the air. They then fall upon the higher and more exposed leaves 

 and produce infection here rather than upon the leaves near the 

 ground, which are more or less covered. Thus, the second infection 

 coming later and appearing on a different part of the plant points to 

 the fact that if the first infection did not occur other infections would 

 be impossible. 



CAUSES FOR THE SPREAD OF INFECTION. 



It has been proved to be a fact that the leaf spots increase only 

 slightly in number upon the old leaves wliich have passed the period 

 of their greatest activity and are beginning to yellow. Numerous 

 spores, however, which are produced in these spots infect the sur- 

 rounding green leaves, so that the source of infection is continued 

 even after the old leaf is dead. When the condensed moisture upon 

 the leaves dries, the spores of Cercospora are borne by the wind or 

 simply float through the air to other leaves, or they may be carried 

 from one plant to another by insects, by man, or upon various kinds 

 of cultural implements. The irrigation water has been proved to be 

 one way in which the organism may be carried from one place to 

 another in the same field, or through the -drainage water to other 

 fields. 



POSSIBLE CONTROL BY REMOVAL OF BEET TOPS. 



Since the beet tops and crowns are the great source of infection and 

 afford a means of carrying the disease over from one season to the 

 next, it is evident that care should be taken to remove these from the 

 fields as thoroughly as possible. In order to do this, the tops should 



[Cir. 121] 



