Genetic control of resistance to Kabatiella caulivora in Trifolium subterraneum subspecies yanninicum I. Diallel analysis of variation in disease development parameters

Abstract

A l0 x 10 complete diallel cross of Trifolium subterraneum subspecies yanninicum genotypes was evaluated for resistance to Kabatiella cauliuoua as F2 swards. Analysis of variance of the diallel cross data indicated that variation in 10 October disease score, total disease development (TDD), rate of disease development (Rate) and time to 50% of maximum disease rating (Delay) were heritable. Coefficients of regression of Wr on Vr were significantly greater than zero but not significantly different from a value of 1 for all four disease parameters. Although this evidence is not conclusive, it does suggest that gene interactions are relatively unimportant. Most non-additive gene action could be attributed to dominance effects and, in terms of both 10 October disease score and total disease development (TDD), a high proportion of alleles conferring resistance were dominant. No clear relationship between phenotype and relative proportions of dominant and recessive alleles was evident for either Rate or Delay. Estimates of dominance variance components were, with the exception of Delay, substantially greater than the additive genetic component. Values of F were positive for all parameters and, except for Delay, values of v(H1/D) were greater than 1, which indicated that dominant alleles are more frequent than recessive alleles among the parents and that dominance is complete at those loci exhibiting dominance. Heritabilities (broad and narrow sense) were substantially greater for TDD and 10 October disease score than for Rate and Delay. The implications of these results are discussed in relation to the development of efficient breeding strategies.