Relation of Postpenetration Events in Idaed 59 Wheat Seedlings of Low Receptivity to Infection byPuccinia graminisf. sp.tritici

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Abstract
Seedlings of T. aestivum ''Idaed 59'' and the differential line W2691SrTt-1 with the specific resistance governed by SrTt-1 had .apprx. 35% less receptivity to infection by the putatively virulent race 15-TLM of P. graminis f. sp. tritici than did seedlings of the wheat cv. Baart. In all 3 wheats a small percentage of penetrants ceased development after formation of substomatal vesicles, primary infection hyphae, primary haustorial mother cells, or the secondary hyphal branches. Penetrants that failed to develop into colonies when primary haustorial mother cells were attached to necrosed host cells also were present in the 3 wheats, but the probability of this event was significantly greater in ''Idaed 59'' and W2691SrTt-1 than in ''Baart''. The greater frequency of this event probably accounted for most of the reduction in uredial number on the seedling leaves of ''Idaed 59'' and W2691SrTt-1. In ''Baart'', there were no colonies with 1 or more secondary haustorial mother cells attached to necrosed host cells, but in ''Idaed 59'' and W2691SrTt-1 the frequency of such colonies increased with time to comprise almost all colonies at 160 h after inoculation. Apparently most colonies with necrosed host cells produced uredia of a high infection type on the seedling leaves, but growth ceased in a few with many necrosed host cells to produce a fleck infection type. The maximum linear growth of secondary hyphae was similar in colonies with and without necrosed host cells in ''Idaed 59'' and W2691SrTt-1 and was significantly less than in ''Baart''. A significant lag in the formation of primary infection hyphae and primary haustorial mother cells also occurred in the wheats with low receptivity. Thus, the dominant gene associated with SrTt-1 in Idaed 59'' that conditions low receptivity to infection of adult plants by races virulent for SrTt-1 also conditions a resistance in seedlings with a mixture of reactions similar to the mesothetic infection type.