Soil Bacteriological Studies on the Control of the Phymatotrichum Root Rot of Cotton '

Search for methods of control or eUmination of the cotton root rot fungus (Phymatotrichum omnivorum (Shear) Dug.) in the alkahne soils of the Southwest has already continued for half a century. The literature involved has recently been reviewed by Rea (16)^ and by Streets (17). Nearly all of these discussions have been from the viewpoint of plant pathology or agronomy. Garrett (7) included this disease in his broad discussions of root-disease problems. Observations pointing to a soil microbiological factor began with PammePs (14) observation that the incidence of this disease was lessened by liberal use of stable manure. Actual predication of soil microorganisms as a factor in the conquest of the disease was offered by King and his colleagues (8, 9) in a series of papers upon the manure treatment of root rot plots in Arizona, beginning in 1923 and definitely proposing the microbiological interpretation of such control in 1934. Search for antagonistic organisms effective in the cotton field has thus far proved futile (Morrow et al., 12 and unpublished data), although such organisms are readily demonstrated in pure culture. Up to the present, no antibiotic agent has been able to destroy or render static a root rot fungus already established as a parasite in the cotton plant. The work discussed in this paper, therefore, deals with the relations of the soil micropopulation to the root rot fungus in the soil and in dead or decaying roots, not in its active phase as a parasite. The persistence of Phymatotrichum omnivorum on cotton roots under different soil or root conditions and the interpretation of the observed differences have remained subjects for speculation. King and Loomis (iO), McNamara and Hooton {11), and Ratliffe {15) have observed that the parasite is viable for considerable periods after its invasion of cotton roots. That its viability is decreased under certain conditions is also known. Rea {16) has stated that the older fungus strands on diseased and dying portions of cotton roots gradually exhaust the available food materials therein and die with the roots to which they are attached. Ezekiel {5) has noted decreased survival percentages of P. omnivorum on cotton roots following root cutting or girdling, as well as a decrease in alcohol-soluble solids contained in the roots. Such studies suggest that food exhaustion or changes in root composition become limiting factors for survival of the parasite. The role of microbial interrelationships in determining the survival of plant pathogens has been reviewed by Garrard and Lochhead {6). King, Hope, and Eaton {9) have discussed the importance of microbial factors in the manurial control of cotton root rot on irrigated land. Clark and Thom (4) concluded that the suggested microbiological