Competitive ability of races of Puccinia graminis f. sp. tritici on Three barley cultivars and a susceptible wheat cultivar

Races QCC (virulent to the resistance gene Rpgl in barley) and QFC and TPM (moderately avirulent to Rpgl) of Puccinia graminis f. sp. tritici were mixed and cultured in two experiments for four and six uredinial generations, respectively, on adult plants of the barley cultivars Tupper and Robust (each with resistance gene Rpgl), Harrington (rpgl), and the stem rust susceptible wheat cultivar Little Club. When mixed in equal proportions, race QCC comprised over 88% of the populations selected on 'Tupper' and 'Robust' after one uredinial generation. By the fourth generation, QCC comprised 90% of the population from 'Harrington' and 80% from 'Little Club'. The competitive advantage of race QCC over races QFC and TPM also was observed when the proportion of race QCC was reduced in the original inoculum mixture. In fitness tests, all populations selected from the sixth uredinial generation showed significant increases in infection frequency (number of uredinia per square centimeter of stem tissue) and urediniospore production (milligrams of urediniospores per square centimeter of stem tissue) as compared with the original inoculum mixture. The changes in infection frequency and urediniospore production of the selection populations, however, was related to the frequency of race QCC in a given population. Selection pressure exerted by widely grown barley cultivars with gene Rpgl and a higher competitive ability of QCC relative to other common races on susceptible barley and wheat would explain the widespread distribution and rapid increase of QCC in the northern Great Plains of the United States and Canada.