Hybridization between Puccinia graminis tritici and Puccinia graminis secalis and its evolutionary implications

The F2 progenies of four crosses, including one reciprocal, between Puccinia graminis tritici and P. graminis secalis included only cultures that were less virulent on wheat and rye than the parent that attacked one or other of these hosts. They closely resembled the F1 culture from which they were derived. A number of F2 cultures had low virulence on both wheat and rye. It is hypothesized that the hybrids resemble a more primitive form of Puccinia graminis and that evolution in the stem rusts of cereals is progressing from low virulence and wide host range to high virulence and reduced host range.