Six cotton cultivars,three multi-adversity resistant (MAR) and three non-MAR, and their 15 F₁ and 15 F₂ progenies obtained from a diallel set of crosses (no reciprocals) were used to investigate the genetic system controlling traits associated with resistance to Rhizoctonia solani Kuhn and Pythium ultimum Trow. Experiments were conducted in temperature controlled water bath tanks, using soil artificially inoculated with either R. solani or P. ultimum at two inoculum densities. Traits measured included rate of seedling emergence (coefficient of velocity of emergence), emergence, pre- and post-emergence damping-off, final stand, and hypocotyl and root damage. Measures of gene action and interaction, and the inheritance of resistance to the two pathogens, were obtained based on the diallel analyses of Hayman, Jinks, and Griffing.

Independent action of non-allelic genes (no epistasis) was the only genetic assumption of the diallel analyses that was not valid for all traits for both pathogens. Dominance effects (H(1) and H(2)) accounted for most of the observed variability in the F₁ and F₂ diallel analyses for most traits associated with resistance to both R. solani and P. ultimum. The additive (D) and epistatic effects were smaller in magnitude than the dominance effects. Average degree of dominance ranged from partial to over dominance with different responses to each pathogen.

Heritabilities in the narrow sense were low, ranging from 0.1% to 20.7% for R. solani and from 0.1% to 10.2% for P. ultimum. General combining ability effects were important for thee expression of resistance to R. solani and specific combining ability effects for minimum.

Results indicate that resistance in cotton seedlings to both R. solani and P. ultimum is polygenical inherited and conditioned by complex of minor gene's. Extension of the genetic analysis to the F₂ provided additional information on the genetic system. Direct selection for the desired expression of traits conditioned by genes with such actions would be difficult. Therefore, a phenotypic recurrent selection procedure would enhance the accumulation of favorable genes. In addition, an adequate selection pressure for both pathogens would help in the identification of resistant seedlings.