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

Forty stem rust resistant oat cultivars and lines were selected from Austra lian oat breeding programs, Quaker Oat Nurseries and a Brazilian oat collec tion. Most, including the Pg-a reference stock Omega, were crossed with a s usceptible line in order to study the inheritance of resistance. Omega was crossed with 15 resistant selections and a further 15 intercrosses were mad e among resistant selections. All selected lines possessed Pg-a which was i nherited as a pair of recessive complementary genes. All crosses among resi stant lines failed to segregate. In the majority of resistant/susceptible c rosses, the F-2 ratio was 1 resistant : 2 intermediate : 13 susceptible, wh ereas in others it was 1 resistant : 15 susceptible. Progenies of plants wi th 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 i nvolved in the Pg-a response. The respective complementary genes were isola ted in stem rust susceptible lines, which when intercrossed, segregated in F-2 to produce resistant plants in the expected frequencies and with the ty pical Pg-a phenotype. The lack of effective genetic variability for stem ru st resistance among important sources of germplasm used by Australian oat b reeders is a major cause for concern.