Nature of mixed infection type 2(5) observed in rye (<em>Secale</em> <em>cereale</em> L.) plants carrying the <em>Pr1</em> leaf-rust resistance gene

Abstract

When challenged with a complex leaf-rust inoculum, rye plants carrying the dominant leaf-rust resistance gene Pr1 predominantly react with a mixed infection type, IT 2(5). The present study investigates the nature of this mixed infection type. The results demonstrate, that IT 2(5) is not caused by partial dominance of Pr1 nor by the genetic background. Rather, IT 2(5) reflects the occurrence of Pr1-virulent leaf-rust races among the inoculum in low frequency. A simple approach was followed to estimate the frequency of Pr1-virulent races in a leaf-rust population. This approach included comparative detached-leaf testing of Pr1 carriers and Pr1-deficient genotypes and relating the numbers of pustules observed on both genotypes. For the present case we estimated a frequency of 3.19% of Pr1-virulent leaf-rust races among a local leaf-rust population sampled in the field. This estimate was verified in a validation experiment.