COTTON ROOTROT IN ARIZONA 1 
By C. Ji King 
Assistant Agronomist, Crop Acclimatization and Adaptation Investigations , Bureau of 
Plant Industry, United States Department of Agriculture 
It has been generally accepted that the fungus, Ozonium omnivorum , 
described by Pammel 2 in 1889, is the cause of the disease known as 
Texas rootrot, but definite proof has been lacking* The records avail¬ 
able show that but little has been added to the knowledge of the life 
history of the fungus since it was first studied by Pammel, 2 a notable 
exception being the work of Duggar 3 in 1916, who described a conidial 
stage from material which he found in a cotton field near Paris, Tex., in 
1915. However, Duggar’s classification, based on the character of this 
fruiting form, does not appear to have been adopted by other pathol¬ 
ogists. 
The conditions at Sacaton, Ariz., during the seasons of 1921 and 1922 
permitted unusually satisfactory observations upon the intimate rela¬ 
tionship existing between the disease and a fruiting form of the fungus, 
which is evidently identical with that described by Duggar. In the 
Salt River Valley, Ariz., during the seasons 1917 to 1919 the fruiting 
fungus had been observed on several occasions, sometimes in abundance 
following a rainy period and often as isolated spots where a depression 
or crack in the soil afforded favorable conditions; but at no time was 
the material so abundant and so definitely associated with the presence 
of the disease as at Sacaton during the past two seasons. 
In comparing the manner of spread of the disease in various crops, it 
was noted that in alfalfa fields its behavior was very suggestive of fairy 
rings. The perfectly formed circles, consisting of an outer ring of 
recently dead plants, an inner ring, or “bare zone,” where only the old 
stubble of dead plants remains, and a central zone occupied by reestab¬ 
lished plants arising from fragments of crowns or roots not fully de¬ 
stroyed, establish the fact that the disease spreads from a center in ever- 
widening circles and that having passed a given place leaves this spot 
free from the disease until reinfected. In badly infected fields where 
the disease is of long standing the crossing and recrossing of the widening 
rings may completely obscure the fairy-ring effect. 
The resemblance to fairy rings is made still more striking by the 
abundant crops of fruiting bodies, which in Arizona appear on the 
periphery of the circles shortly after the occurrence of rainy weather. 
The newly formed fruiting bodies appear as feltlike mats on the surface 
of the ground, or in cracks or depressions, and are rarely found more 
than 6 or 8 inches from the outer circle of recently wilted plants. At 
times they have been so abundant as to cover over 300 square feet of 
soil surface in a 2 2-acre alfalfa field where 3 or 4 acres of the plants had 
died. 
1 Accepted for publication Dec. 16,1922. 
\ Pammel. L. H. cotton root-rot. In Tex. Agr. Exp. Sta. 2d Ann. Rpt. 1889, p. 61-86, 3 pi. 1890. 
■ Duggar, B. M. the Texas root-rot fungus and its conidial stage. In Mo. Bot. Gard. Bui., v. 
3, p. u-23, 5 fig. 1916. Bibliography, p. 23. 
Journal of Agricultural Research, Vol XXIII, No 7 
Washington, D. C. Feb. i 7f 1923 
^ Key No. G-28X 
(525) 
