ABSTRACT
In searching the literature for publications on cotton rhizosphere and rhizoplane microorganisms, to give an historical perspective to this talk, it very quickly became apparent that our predecessors were not totally absorbed with the kinds and numbers of microbes that reside on the cotton root or in the soil immediately adjacent to it. In fact, it was not until just prior to WW II that information on this subject began to appear in the literature. This was in connection with attempts to control Phymatatrichum root rot of cotton. In 1941 Mitchell et al.,(7) studied the influence of the cotton root microflora on the survival of Phymatotrichum omnivorum, and the extent to which the microflora on the surface was influenced by plant injuries inflicted by early plowing or girdling. They found that, under field conditions, cotton roots injured during late summer or early fall showed pronounced increases in micropopulations associated with root surfaces, and that these levels were inimical to the growth of the parasite. They also found that the microbial populations were much higher on younger plant roots than on those from older plants. In 1946, Eaton and Rigler (2) studied the influence of carbohydrate levels and root surface microfloras on Phymatotrichum root rot in cotton. They found that by manipulating the fruit load and/or leaf surface on developing cotton plants, they could influence the carbohydrate content of the root system, and both qualitatively and quantitatively alter the microflora on the root surface. Increased root carbohydrate content resulted in decreased numbers of Bacillus spp. and total bacteria, but increased the numbers of fluorescent pseudomonads on the root surface. These changes also resulted in decreased susceptibility to cotton root rot. They also determined that observed resistance of cotton seedlings and corn to root rot was due to root surface microbes in the case of corn, but that cotton seedling resistance was internal in nature and independent of the root surface microflora.
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