Inheritance of durable adult plant resistance to stripe rust (Puccinia striiformis f. sp tritici) in 'Otane' wheat (Triticum aestivum)

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is an important disease of wheat (Triticum aestivum) in New Zealand and throughout the world. The wheat cultivar 'Otane' carries durable resistance to stripe rust that has remained effective in New Zealand since the cultivar's release in 1984. To determine the mode of inheritance of this durable adult plant resistance to stripe rust, 'Otane' was crossed with the stripe rust susceptible 'Tiritea', and 140 F-1-derived double haploid (DH) lines were evaluated in glasshouse and field experiments for their reaction to the stripe rust pathogen. Transgressive segregation occurred, indicating that both cultivars possess resistance genes. Genetic gain as a percentage of the moderately resistant parent for the most resistant 5% of the DH lines was statistically significant (83% and 68% in the glasshouse and field experiments, respectively). The distribution of DH lines assessed through infection types (IT) in the glasshouse supported a digenic ratio, whereby resistance genes from both parental cultivars act additively to produce resistant DH lines. Moderately resistant DH lines were produced when only the gene from 'Otane' was present, and the absence of the 'Otane' gene produced susceptible DH lines. The distribution of reaction of DH lines in the field fitted a trigenic ratio. This model proposed that resistant DH lines were produced when at least two genes from both parents interact; moderately resistant DH lines were produced when either two genes (one modifier) or one gene from 'Otane' were present, and susceptible DH lines were produced when either resistance genes from both parents or the major gone from 'Otane' were absent. Segregation of final disease severity measured in the field also supported the segregation of three genes in the DH population. The demonstrated durability of stripe rust resistance in 'Otane' in New Zealand, which is influenced by environment, may be the result of a combination of resistance alleles at two loci at least.