Cytological studies on the infection of barley cultivars with known resistance genotypes by Rhynchosporium secalis

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Abstract
Seven races of Rhynchosporium secalis were screened for their virulence on a variety of barely cultivars. Four races were identified as virulent on cultivar Atlas 46 (resistance loci Rrs1 and Rrs2) but virulent on the near-isogenic cultivar Atlas (Rrs2). For one of these races, US238.1, the fungal infection cycle was followed on the susceptible cultivar by means of light and scanning electron microscopy. From a comparative analysis of fungal development on the susceptible and resistant cultivars, two lines of plant defense emerged: (i) inhibition of spore germination on the leaf surface and (ii) prevention of the establishment of the subcuticular stroma. Investigations of the development of race US238.1 on different barley cultivars with and without Rrs1 and on F1 individuals from different crosses excluded involvement of Rrs1 in the inhibition of spore germination. Possible pathogenicity mechanisms are discussed.