Gene Action and Inheritance of Resistance to Rhizoctonia solani and Pythium ultimum in Cotton Seedlings (May, 1986) Maxwell Amlai Todi Poswal, B.Sc. (Agric.) Ahmadu Bello University, Zaria, Nigeria M.Sc. (Crop Protection) Ahmadu Bello University, Zaria, Nigeria. Chairman of Advisory Committee: Dr. K.M. El-Zik Six cotton cultivars, three multi-adversity resistant (MAR) and three non-MAR, and their 15 Fi and 15 F2 progenies obtained from a diallel set of crosses (no reciprocals) were used to determine the genetic system controlling traits associated with resistance to R. solani and P. ultimum. Experiments were conducted for two years in a field naturally infested with both pathogens, and in temperature controlled water bath tanks, using soil artificially inoculated with each of R. solani and P. ultimum separately at two inoculum densities. Genetic information based on the diallel analyses of Hayman, Jinks and Griffing for the data collected in the field were inconclusive. However, measures of gene action and interaction, and the inheritance of resistance to the two pathogens were obtained from the temperature tank experiments. Independent action of non-allelic genes (no epistasis) was the only genetic assumption of the diallel analysis that was not valid for all traits for both pathogens. Dominance effects (H1 and H2) were the most predominant in the F1 and F2 diallel analyses for most traits associated with resistance to both R. solani and p. ultimum. The additive effects (D) were smaller in magnitude than the dominance effects. Average degree of dominance ranged from partial to overdominance with different responses to both pathogens. The analysis of F2 data detected a significant epistatic effects for all traits and for both pathogens. Heritabilities in the narrow sense were low, ranging from 0.0% to 20.7% for R. solani. For P. ultimum, heritabilities ranged from 0.0% to 10.2%. General combining ability effects were important for the expression of resistance to R. solani and specific combining ability effects for P. ultimum. The results indicated that resistance in cotton seedlings to both R. solani and P. ultimum was polygenically inherited and conditioned by a complex of minor genes. Extension of the genetic analysis to the F2 provided more reliable information on the genetic system. The most important trait associated with final stand establishment of cotton seedlings in the presence of R. solani was post-emergence damping-off, explaining 92% of the variability in final stand. Pre-emergence damping-off due to P. ultimum, explained 62% of the variability in final stand.