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GENETIC INTERDEPENDENCE OF HOST AND PARASITE IN EPIDEMICS *

In breeding for disease resistance in agricultural crops the traditional objective was to develop new cultivars that expressed high levels of resistance or near-immunity to the disease in question. Highly resistant cultivars, when introduced and widely grown, were responsible for large and often dramatic reductions in the reproductive potential of the parasite. But where the reduction had been effected through use of a cultivar that possessed a single new resistance gene ( R gene) the final outcome, in many instances, was complete or near-complete recovery of the level of parasite reproductivity that had caused the original disease problem. It is only recently that this final consequence of using single R genes has begun to receive the attention it deserves and that new approaches to disease control are being advocated. However, the genetic events that attend and make possible the eventual recovery of parasite virulence have not been intensively studied and are not well understood. Our objective in this paper is to present a tentative description of these genetic events.