Family Chenopodiaceae 127 



and older leaves. As the inner foliage advances in 

 age, it becomes infected in turn. As serious as the 

 disease may appear, it never kills the plant. The 

 result, however, is noticeable on the roots, which are 

 undersized and elongated instead of round. Leaf 

 spot generally appears during a moist spell followed 

 by a period of dry weather. The disease increases 

 in severity as the plants are weakened by heat and 

 drought. 



The Organism. The fungus, Cercospora beticola, 

 like most fungi, is composed of a vegetative part of 

 mycelium and of spores. The latter are microscopic 

 in size, somewhat needleshaped, and divided by 

 means of a cross wall into two to seven cells (fig. 

 20 h). Each of these cells may germinate by send- 

 ing out a threadlike tube, which penetrates the leaves 

 through the stomata. The spores are borne on a 

 cluster of stalks or conidiophores, at the base of which 

 is formed a small stroma. The temperature and re- 

 lative humidity of the air influences the production 

 and infection of conidia. According to Pool and 

 McKay 1 a temperature of 80 or 90 degrees F. with a 

 minimum of not less than 60 degrees at night is most 

 favorable to the production of conidia. They are, 

 however, checked By a temperature of 100 degrees or 

 higher, or of 50 to 80 degrees F. Conidia are gener- 

 ally formed on the lower surface of the leaves, no 

 doubt because these are subject to a higher humidity. 



Control. For practical purposes leaf spot may 



1 Pool Venus, and McKay, M. B., U. S. Dept. Agr. Journ. Agr. 

 Research, 7 : 21-60, 1916. 



