Evolutionary Response of Barley Composite Cross II toRhynchosporium secalisAnalyzed by Pathogenic Complexity and by Gene-by-Race Relationships

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Abstract
Evolutionary changes in host-parasite interactions between barley composite cross II (CCII) and 61 races of Rhynchosporium secalis were analyzed in terms of pathogenic "complexity" and in terms of specific gene-by-race resistance-pathogenicity reactions. Races identified as complex by a differential set of host cultivars were, in general, able to infect more of the 28 parents of CCII than simple races and simple races were more often countered by the development of genetic resistance in the host population than were complex races. There were, however, many exceptions and it was concluded that pathogenic complexity provides a tenuous base for the analysis of host-pathogen interactions at the population level. Analysis of gene-by-race relationships showed that increases in the frequency of alleles for resistance occurred for each of the five genes known to govern disease reaction in CCII. Patterns of change in allelic frequencies indicated that selection favored alleles for resistance in seasons when scald disease was prevalent but that resistance alleles were detrimental to reproductive capacity in seasons that were unfavorable for scald. The results also suggested that many gene-by-race interactions, in addition to the five that were studied, affect evolutionary response in the CCII-R. secalis host-pathogen system. Studies required for a more comprehensive gene-by-race analysis of evolutionary change are discussed.