Inheritance of the stem rust resistance phenotype Pg-a in oats

Abstract

Summary Forty stem rust resistant oat cultivars and lines were selected from Australian oat breeding programs, Quaker Oat Nurseries and a Brazilian oat collection. Most, including the Pg-a reference stock Omega, were crossed with a susceptible line in order to study the inheritance of resistance. Omega was crossed with 15 resistant selections and a further 15 intercrosses were made among resistant selections. All selected lines possessed Pg-a which was inherited as a pair of recessive complementary genes. All crosses among resistant lines failed to segregate. In the majority of resistant/susceptible crosses, the F 2 ratio was 1 resistant : 2 intermediate : 13 susceptible, whereas in others it was 1 resistant : 15 susceptible. Progenies of plants with intermediate reactions always segregated. Although Kyto with Pg-12 was a parent of Omega, Pg-12 was not present in any resistant line and was not involved in the Pg-a response. The respective complementary genes were isolated in stem rust susceptible lines, which when intercrossed, segregated in F2 to produce resistant plants in the expected frequencies and with the typical Pg-a phenotype. The lack of effective genetic variability for stem rust resistance among important sources of germplasm used by Australian oat breeders is a major cause for concern.