RELATION OF STOMATAL MOVEMENT TO INFECTION 
BY CERCOSPORA BETICOLA 1 
By Venus W. Pool, Assistant Pathologist , and M. B. McKay, Scientific Assistant, 
Cotton and Truck Disease Investigations, Bureau of Plant Industry 
INTRODUCTION 
Leafspot infection of the sugar beet (Beta vulgaris L.) caused by Cer- 
cospora beticola Sacc. has been found to be closely related to if not directly 
controlled by stomatal movement in so far as the host is concerned. 
Penetration of the leaf by this parasite is effected, so far as known at 
present, only through open stomata. Consequently the factors favor¬ 
able to stomatal pore opening become of fundamental importance in the 
occurrence of the disease. 
The factors considered in this paper as most important in influencing 
stomatal movement are leaf maturity and certain environmental con¬ 
ditions. The term “leaf maturity” as employed in this paper is used 
to designate the condition of those leaves which have reached a maximum 
degree of physiological efficiency per unit area. Neither the size of the 
leaf nor its relative age in days can be taken as a reliable index to its 
degree of maturity. Under certain conditions young heart leaves of 
the sugar beet may be stimulated into physiological maturity before 
they have arrived at the average adult size, and such leaves will always 
remain small, while leaves which have attained average adult dimen¬ 
sions may still be physiologically immature. The varying degrees of 
leaf maturity have been found to be accurately indicated by the relative 
size and number of stomata per square millimeter of leaf surface, and these 
morphological factors have been observed to remain constant for a given 
maturity, even though the leaf size and position might indicate another 
stage of development. The stomata on leaves determined as mature by 
this method exhibited the greatest movement and responded most 
readily to changes in the environment. Light may be considered the 
essentially fundamental external factor affecting stomatal movement, 
although its influence may be greatly modified by different tempera¬ 
tures and relative humidities, the two factors that will be considered in 
detail in this paper. 
In addition to stomatal movement, infection is also influenced by the 
rapidity of growth of the conidial germ tube and the maturity of the 
leaves. Detailed field observations have shown that heart and extremely 
1 This study has been carried on in connection with a detailed investigation of the sugar-beet leafspot 
conducted by the United States Department of Agriculture in cooperation with a beet-sugar company at 
Rocky Ford, Colo., during 1912 and 1913. A continuation of the entire problem was made possible during 
the season of 1914 at Madison, Wis., through the kindness of Dr. L. R. Jones, of the University of Wisconsin. 
Journal of Agricultural Research, Vol. V, No. 22 
Dept, of Agriculture, Washington, D. C. Feb. 28, 1916 
cn G—74 
(ion) 
