Allelic Relationships among Genes for Resistance to Sunflower Rust

Sunflower rust, incited by Puccinia helianthi Schwein., is a major disease in sunflower ( Helianthus annuus L.) growing areas of the world. Resistance to the four prevalent races is lacking in most of the commercial hybrids. Six accessions (PI 413023, PI 413037, PI 413048, PI 413118, PI 413171, and PI 413175) with a high frequency of rust‐resistant plants were identified. This research examined the allelic relationships among resistance genes present in these wild accessions and the inbred line HAR4. Plants in each PI that were resistant to all four races were crossed, then backcrossed, to the susceptible inbred line HA89; BC 1 F 1 progenies resistant to all four races were identified, and each progeny was heterozygous for the four dominant genes conferring resistance to the four respective races. The F 1 's of HA89/HAR4 also had four independent dominant genes in heterozygous condition for resistance to the four races. The F 1 's of HA89/HAR4 were then intercrossed with the BC 1 F 1 rust‐resistant plants derived from the six accessions. Resistant genotypes from the intercrosses were testcrossed with HA89. The resultant progenies indicated that the gene for resistance to each race of rust in five of six wild accessions was different from those of HAR4, whereas genes for resistance in PI 413118 were the same as those in HAR4. The resistant BC 1 F 1 's of PI 413023, PI 413118, PI 413171, and PI 413175 were intercrossed. Testcross progeny evaluations revealed that each accession had rust‐resistance genes unique to specific races. The apparent genetic diversity of these rust‐resistant accessions could be exploited to diversify the protection against the four prevailing North American sunflower rust races.