With the exception of the Mlo gene, most R genes are rapidly overcome when deployed in the field. Wild populations of barley with ancestral populations of mildew. Enormous potential of pathogen for population growth and spread through asexual propagules. R-genes are highly effective, heavy selection pressure. Also easy to score.
Sexual cycle exists allowing recombination in pathogen.
RFLP map exists for pathogen. Genetic interaction between host and pathogen is well-characterized. Mutation in E. graminis perhaps due to transposon activity. This element is also found in other pathotypes of the E. graminis pathogen, suggesting it is ancient.