THE LIFE HISTORY OF THE TEXAS ROOT ROT FUNGUS, 
OZONIUM OMNIVORUM SHEAR 1 
By C. L. Shear 
Senior Pathologist, Bureau of Plant Industry, United States Department of 
Agriculture 
The fungus now generally accepted 
as the cause of root rot of cotton and 
a large number of other cultivated and 
wild plants in Texas and other parts of 
the Southwest as far as southern Cali¬ 
fornia was first described by Pammel 2 
and tentatively referred to Ozonium 
auricomum Link. This fungus seems 
to be a native of the southwestern 
United States and is most common in 
the so-called “black waxy soils” of 
Texas; but it seems to be becoming 
more widespread and more serious in 
southwestern Arizona in recent years 
with the increased growing of alfalfa 
and cotton there. 
Investigations of this fungus made 
by the writer in the period 1902 to 1907 
and the comparison of it with Link’s 
type of O. auricomum in Berlin indi¬ 
cated that the organism was quite 
different from Link’s species, and as 
no description which seemed to apph T 
to it could be found in the literature 
it was described and named Ozonium 
omnivorum Shear. 3 
At that time the fungus was known 
only as a sterile mycelium. Circum¬ 
stantial evidence, however, based upon 
the occurrence of a conidial fungus pro¬ 
ducing spores in great abundance on 
the surface of the soil in portions of 
cotton fields where the plants had been 
killed by the fungus, suggested that 
this might be a sporogenous form of the 
Ozonium. 
Duggar, 4 in 1916, reported the pres¬ 
ence of conidia-bearing hyphae on the 
characteristic mycelium of the Ozonium 
found on the roots of affected plants, 
and also the identity in artificial 
culture of the mycelium obtained from 
diseased roots with that obtained from 
cultures of these conidia. 
The conidial condition of the fungus 
(fig. 1) studied by Duggar was referred 
to the genus Phymatotrichum of the 
Hyphomycetes and the new combina¬ 
tion Phymatotrichum omnivorum (Shear) 
Duggar proposed. 
Taubenhaus and Killough 5 state 
that the senior author has confirmed 
Duggar’s work by producing conidia of 
the Ozonium in pure culture on 
sterilized soil in the laboratory. Only 
one out of six cultures, however, 
produced conidia. As this spore 
form belongs to the so-called imperfect 
fungi which are believed to be merely 
conidial conditions of the higher 
forms, it was natural to suppose that a 
perfect stage of the fungus might 
still occur, though perhaps rarely. 
In hope of determining the complete 
life history of the fungus, numerous 
cultures were made on different media 
and kept under various conditions for 
long periods, but they always remained 
sterile. Careful observation and search 
in diseased areas on cotton roots and 
stems which had been killed by the 
fungjus were made at different seasons, 
especially in the winter, spring, and 
early summer. Numerous fungi were, 
of course, found but none could be 
proved to belong to the root-rot fungus. 
In August, 1903, however, the writer 
found what he believes to be the 
perfect stage of this Ozonium, although 
the evidence at present is circum¬ 
stantial. In a cotton field near Paris, 
Tex., there was a so-called “dead spot” 
of cotton in which the plants had all 
been killed by this fungus, and im¬ 
mediately adjoining this spot was a 
hedge of osage orange ( Maclura auran- 
tiaca Nutt.). In the margin of this 
diseased area, not far from the dead 
and dying cotton plants,' was a small 
sprout of the osage orange about 1 foot 
high which showed the characteristic 
wilt produced by the Ozonium. An 
1 Received for publication June 27, 1924; issued May, 1925. 
2 Pammel, L. H. cotton root-rot. Tex. Agr. Exp. Sta. Ann. Rpt. (1889) 2 : 73, pi. 1-3. 1890. 
3 Shear, C. L. new species of fungi. Bui. Torrey Bot. Club 34 : 305. 1907. 
4 Duggar, B. M. the texas root rot fungus and its conidial stage. Ann. Mo. Bot. Gard. 3 :11-23 
illus. 1916. 
5 Taubenhaus, J. J., and Killough, D. T. texas root rot of cotton and methods of its control, 
Tex. Agr. Exp. Sta. Bui. 307, p. 5, illus. 1923. 
Journal of Agricultural Research 
Washington, D. C. 
( 475 ) 
Vol. XXX, No 5 
Mar., 1925. 
Key No. G-462 
