COMPETITIVE ABILITY OF RACES OF PUCCINIA-GRAMINIS F-SP TRITICI ON 3 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. tritic i were mixed and cultured in two experiments for four and six uredinia l generations, respectively, on adult plants of the barley cultivars T upper and Robust (each with resistance gene Rpgl), Harrington (rpgl), and the stem rust susceptible wheat cultivar Little Club. When mixed i n equal proportions, race QCC comprised over 88% of the populations se lected on 'Tupper' and 'Robust' after one uredinial generation. By the fourth generation, QCC comprised 90% of the population from 'Harringt on' and 80% from 'Little Club'. The competitive advantage of race QCC over races QFC and TPM also was observed when the proportion of race Q CC was reduced in the original inoculum mixture. In fitness tests, all populations selected from the sixth uredinial generation showed signi ficant increases in infection frequency (number of uredinia per square centimeter of stem tissue) and urediniospore production (milligrams o f urediniospores per square centimeter of stem tissue) as compared wit h the original inoculum mixture. The changes in infection frequency an d 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 s usceptible barley and wheat would explain the widespread distribution and rapid increase of QCC in the northern Great Plains of the United S tates and Canada.