The Multi-Adversity Resistance (MAR) genetic improvement program has made significant progress in producing cotton cultivars with resistance to diseases and insects. Phymatotrichum rot is one disease to which resistance is being gained. One hypothesis for the program is that root microflora influences host resistance. Model studies suggest that exudate properties in MAR cottons have an influence in controlling quality and quantity of microorganisms in the rhizosphere. Resistance to Phymatotrichum root rot appears to be influenced by such a mechanism. Experiments were conducted to determine what effect microflora from MAR cottons may have on resistance to root rot.

To test the hypothesis, bacteria and fungi which were predominant in the rhizosphere of seedlings of a MAR cultivar, Tamcot CAMD-E, were isolated and used. Applications were made to plants grown in experiments conducted in fields where Phymatotrichum was known to occur naturally. Four bacterial isolates (WA-E, SW-E, RW-E, and T-E; unidentified), and three fungal isolates (two being Fusarium oxysporum, and one being F. sciani) were used. The microorganisms were applied singularly as treatments and in bacterial-fungal mixtures at concentrations of 109 bacteria/ml and 106 propagul es/ml to two cotton cultivars (Tamcot CAMD-E and SP21S). Applications were by a root-drench technique to the base of the cotton plant, effectively drenching the upper root sections below the soil surface. Treatment inoculations were applied prior to expected natural occurrence of field symptoms (50-70 days after planting), followed by an additional treatment two weeks later. Number of dead plants were recorded weekly and percentages calculated at the end of the season. Plots were harvested for yield. Tests, randomized block design with six replications, were conducted at two locations over a two-year period (1980 and 1981).

The results suggest that microorganisms from Tamcot CAMD-E applied as treatments can influence the incidence of cotton root rot, and may be involved in host resistance. The treatments that were most consistent over locations and years in reducing root rot were isolates WA-E and a mixture of the Fusarium species. Although not significant, effective trends of disease reduction were present. Other treatments showed similar trends, but were less consistent. In cultivar-treatment interactions, the CAMD-E isolates were effective in yield improvement on the CAMD-E cultivar, but no so effective on SP21S, eventhough percent reductions of infected plants were obtained on both cultivars. The results suggest that key beneficial microorganisms are cultivar dependent. The microorganisms responsible in demonstrating a simulated resistance to disease are believed to be a component of the rhizosphere that predominate because of the quality of exudates which are under genetic, control. This study adds greater insight towards understanding the role of microorganisms and the host in reducing disease severity.