Inheritance of resistance to race 330 of Plasmopara halstedii in three sunflower (Helianthus annuus L.) lines

Abstract

Sun¯ower lines RHA-274, HA-61 and RHA-325 were studied for their resistance to race 330 of downy mildew (Plasmopara halstedii). The same inbred line, with normal (HA-89) or sterile cytoplasm (cmsHA-89) was used in all the crosses as susceptible parent, and, in each cross, only one genotype of the resistant parent was studied. The resistant-to-susceptible ratios obtained in the BC1 and F2 progenies from the crosses of the lines RHA-274 and HA-61 to cmsHA-89 and HA-89, respectively, suggested that, in each resistant line, two dominant genes are responsible for resistance to this downy mildew race. One of the genes (A) is epistatic to the other (B), and the recessive allele b in homozygosity is also epistatic to aa, with plants carrying aabb genotypes being resistant. Resistance to race 330 seemed to be controlled by two complementary genes in the sun¯ower inbred line RHA-325, the dominant allele of one of them being present in cmsHA-89. In the genotypes HA-89 or cmsHA-89, the existence of genes that modify the expected segregations following the crosses with resistant parents is proposed. It is concluded that, although major genes have been described as responsible for monogenic resistance to downy mildew, other types of regulation of this character, such as complementarity and epistatic relationships, do occur.