Significance of host complexity and diversity for race-specific leaf-rust resistance in self-fertile synthetic rye populations

Leaf rust (Puccinia recondita Rob. ex. Des.) is the most frequently occurring leaf disease in German winter rye (Secale cereale L.). To test the usefulness of race‐specific resistance genes, the effects of increased host diversity and complexity by producing two‐ and four‐line synthetics from inbred lines carrying different resistance genes were analysed. Thirty‐three synthetics along with two full‐sib families and one hybrid variety were tested in 17 environments in Germany under natural infections. For comparison, the parent lines of the synthetics were evaluated in 11 environments. Only two synthetics and the full‐sib families were resistant across all environments. Observed resistance levels of the synthetics were highly correlated (r = 0.83, P = 0.01) with those predicted from the parental values. Host complexity had a minor effect in two‐line synthetics only. In conclusion, the effectiveness of race‐specific leaf‐rust resistances among environments, and increasing the host complexity and diversity does not lead to a higher resistance level than that expected from the resistances of the parents.