HABITS OF THE COTTON ROOTROT FUNGUS 1 
By C. J. King 
Assistant Agronomist, Office of Crop Acclimatization and Adaptation Investigations, 
Bureau of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
In recent years much study has been given to a disease of cotton and 
alfalfa commonly known as rootrot, which occurs chiefly in certain 
sections of Texas, New Mexico, and Arizona. It is generally believed 
that this disease is caused by a soil-inhabiting fungus which attacks the 
roots of the host plants and by invading and breaking down the tissues 
of the roots, cuts off the water supply and causes death. This fungus 
was described by Shear (8) 2 as a facultative parasite with sterile myce¬ 
lium, and was named by him Ozonium omnivorum 3 
While the occurrence of a disease called rootrot had been observed for 
over 20 years in the Salt and Gila River Valleys of Arizona it was not 
until recent years, when a large percentage of the cultivated areas in 
these valleys was planted to cotton, that the disease received much 
notice or caused much alarm. This probably is owing to the fact that 
the disease is never so striking to the eye when occurring in alfalfa fields 
as it is when present in cotton fields. In alfalfa fields the dead areas 
often are invaded quickly by grasses and weeds of different kinds which, 
being green in color, conceal the fact that the alfalfa plants have been 
killed. There is a tendency also for a considerable number of alfalfa 
plants in the dead areas to recover by sending out new lateral roots 
from small portions of the old roots which are not killed back completely 
but remain alive just below the crown. Often there are sufficient 
numbers of these reestablished plants to prevent the recently infected 
plants from receiving notice. In cotton fields a striking contrast is 
noticeable between the black and brown streaks and patches of dead 
plants and the green areas of healthy plants. 
It appears from the literature that practically all of the study that has 
been given to this disease by different investigators has been made with 
cotton as the host plant. While some descriptions have mentioned the 
fact that the disease spreads in a more or less centrifugal manner, 
others (4) have stated that “the dead patches have no definite bound¬ 
aries, but extend in all directions through the field.” While studying 
the disease in its relation to soil conditions in the Salt and Gila River 
Valleys during the seasons of 1918 to 1922, inclusive, the writer was 
struck by the fact that frequently in alfalfa fields almost perfect circles 
were formed by the dead and dying plants as the disease spread outward 
in all directions from a central plant which was the first to show injury. 
1 Accepted for publication Aug. n, 1923. 
2 Reference is made by number (italic) to “ literature cited,” p. 418. 
* Duggar (/), after finding in 191s what he believed to be the conidial stage of the fungus, tentatively 
assigned the organism to a new genus, Phymatotricum. 
Journal of Agricultural Research, 
Washington, D. C. 
ahf 
(405) 
Vol. XXVI, No. 9 
Dec. 1, 1923 
Key No. G-340 
