Genetic diversity and complexity of host resistance and pathogen virulence in the hybrid rye/leaf rust pathosystem

Leaf rust is the most frequently occurring leaf disease in German winter rye. We wanted to test the effectiveness of race-specific resistance genes and to analyse the genetic composition of the leafrust population. A high diversity and complexity of pathotypes was found through three years by analysis of about 800 isolates with a differential set of 17 genotypes in the primary-leaf stage. In field experiments, we grew 30 synthetic populations segregating for one to four resistance sources along with their parental inbred lines in 17 environments in Germany under heavy natural infection. The different levels of host complexity and diversity were evaluated for their influence on the degree and variation of resistance that might be achievable in heterogeneous double-cross hybrids. Only two out of 30 synthetics and two Russian full-sib families were resistant across all environments, the remainder synthetics displayed various levels of resistance. High genotype x environment interaction was found caused by highly differing virulence patterns of the local rust populations as also demonstrated by Simpson indices around 0.9. Virulence complexity ranged from 3 to 15 across years with a median of 9. Observed resistance levels of the synthetics moderately corresponded to those predicted from the parental values, i.e. no advantage of host diversity occurred. In conclusion, highly effective race-specific resistances or new quantitative resistances are needed and should be combined to achieve a satisfactory level of leaf rust resistance in hybrid rye.