Genetic basis for and inheritance of resistance to Race 1 and Race 2 of Kabatiella caulivora in Trifolium subterraneum ssp. subterraneum and ssp. yanninicum

A study of a partial diallel involving crosses of seven cultivars of subterranean clover (Trifolium subterraneum ssp. subterraneum and ssp. yanninicum) was undertaken to determine the genetic basis of resistance to both Race 1 and Race 2 of Kabatiella caulivora at seedling and adult plant stages. Relationships between variance (V-r) and covariance (W-r) revealed that adult plant resistance to Race 1 and both seedling and adult plant resistance to Race 2 were controlled by simple additive and dominant gene effects, whilst seedling resistance to Race 1 was controlled by epistatic gene effects. Three of the four most resistant parents (cultivars Daliak, Denmark and Meteora) showed a dominant gene effect for adult plant resistance to Race 1. However, the other resistant parent, cv. Goulburn, and the most susceptible parent, cv. Woogenellup, both were controlled by recessive gene effects (i.e, gene effects conferred by recessive genes). Cultivar Denmark again showed dominant gene effects in relation to resistance to Race 2 at both seedling and adult plant stages, whilst recessive gene effects were evident in susceptible adult plants of cv. Daliak in response to Race 2. Estimates of genetic variance components confirmed that dominance variance (H-1 and H-2) was much greater than additive variance (D) and there were more dominant alleles (i.e., more alleles that had dominant effects) than recessive alleles (i.e., more alleles that had recessive effects) present across all the cultivars. An examination of segregation ratios in three F-2 populations revealed that inheritance of adult plant resistance to Race 1 was controlled by a dominant gene in cv. Daliak and cv. Meteora, and by two dominant genes in cv. Denmark. The additional resistance gene present in cv. Denmark may well explain its apparent stable resistance to both races in the field. This study provides an insight into the genetic basis for and inheritance of resistance to this disease that will assist breeding programs aimed at improving resistance.