COTTON ROOTROT SPOTS 



By C. S. SCOFIELD, Agriculturist in Charge, Western Irrigation Agriculture, Bureau of 

 Plant Industry, United States Department of Agriculture 



INTRODUCTION 



The disease of cotton commonly known as rootrot, which occurs in 

 certain sections of Texas, New Mexico, and Arizona, usually appears in 

 cotton fields during the latter part of the growing season. The affected 

 plants wilt down rapidly and within a few days become dry and turn 

 brown. 



It is generally believed that this disease is due to a soil-inhabiting 

 fungus known as Ozonium omnivorum,^ which invades the root system 

 of the host plant and by breaking down the root tissue cuts off the water 

 supply and causes death. The same fungus is believed to attack many 

 species of plants other than cotton, though the grasses appear to be 

 immune. 



One of the peculiarities of the rootrot disease as it occurs in cotton 

 fields is that it usually appears in certain well-defined areas or spots 

 •mthin the limits of which nearly every cotton plant is killed. With the 

 advance of the season, these spots of dead cotton gradually increase in 

 size, the disease apparently spreading from plant to plant. Occasionally 

 a plant remains alive within the infected area, but upon examination it 

 is found that the lower roots are dead and that continued growth is sup- 

 ported by one or more lateral roots that branch out close to the surface 

 of the soil. 



The well-defined areal occurrence of the disease and the completeness 

 with which it kills all the plants within the area naturally led to the 

 impression that its destructiveness must be due to some purely local soil 

 condition. Furthermore, it has been thought that the disease reap- 

 pears from year to year in the same spots. 



FIELD OBSERVATIONS 2 



Rootrot has been prevalent in the vicinity of San Antonio, Tex., for 

 many years, and an opportunity has been afforded to observe its be- 

 havior at a field station located about 5 miles south of the city of San 

 Antonio, where an extensive series of crop rotations have been con- 

 ducted since 1909. The disease was so serious on the rotation plots of 

 cotton in 191 6 that it seemed advisable to survey each plot and locate 

 definitely the infected areas, with a view to determining the rate of 



' More recently named Phymatolrichum omnivorum (Shear) Duggar. (Duggar, B. M. the TBXAS root- 

 rot FUNGUS AND ITS coNiDiAL STAGE. In Ann. Mo. Bot. Card., v. 3, No. i, p. 22. 1916.) 



* The author is indebted to Mr. C. R. Letteer, Superintendent, and Mr. A. A. Bryan, Assistant, at the 

 San Antonio Field Station for cooperation in making the observations here reported. 



Journal of Agricultural Research, Vol. XVIII, No. 6 



Washington, D C. Dec. is, 1919 



SI Key No. G-i8a 



(305) 



