STUDIES ON THE ORIGIN, SPREAD, AND EVOLUTION OF AN IMPORTANT GROUP OFPUCCINIA-RECONDITA F-SP TRITICI PATHOTYPES IN AUSTRALASIA

Wheat brown rust pathotype (pt) 104-2,3,(6),(7),11 was first detected in Australasia in Victoria during 1984. Although it appeared similar t o a pre-existing pathotype, 104-2,3,6,(7), detailed greenhouse tests r evealed nine pathogenic differences between the two rusts. Six differe nces involved contrasting virulence/avirulence for the resistance gene s/specificities Lr12, Lr27 + Lr31 and Lr16, and three uncharacterised genes, present in the wheat cultivars Gaza and Harrier, and in tritica le cultivar Lasko. Differences in partial virulence between the pathot ypes were found for the genes Lr2a, Lr13 and Lr26. A comparison of the phenotypes for 13 isozyme systems in the two pathotypes revealed two differences, including a Pgm2 allele in pt 104-2,3,(6),(7),11 not foun d in other contemporary Australasian Puccinia recondita f. sp. tritici pathotypes. On the basis of these differences, it was concluded that pt 104-2,3,(6),(7),11 was introduced into the Australasian region befo re or during 1984. Seven variants of pt 104-2,3,(6),(7),11, that diffe red by single virulences, were detected during 1984-1992. Pt 104-2,3,( 6),(7),11 and a derivative pathotype with virulence for Lr20 underwent rapid increases in frequency, largely displacing pathotypes which pre dominated before 1984. Although first detected in eastern Australia, b oth pathotypes spread to New Zealand, and the derivative pathotype app eared in Western Australia. The rapid spread and increase of these pat hotypes could not be explained by host selection. Pt 104-2,3,(6),(7),1 1 and derivatives may therefore be more aggressive than other contempo rary Australasian pathotypes.