The Effect of Stand Density on the Development of Puccinia graminis f. sp. tritici in Barley.

The progress of stem rust epidemics caused by race Pgt-QCCJ of Puccinia graminis f. sp. tritici was examined in field plots of the six-rowed barley cvs. Robust and Steptoe at four stand densities, and in short rows and hill plots. Cv. Robust carries Rpg1, which confers resistance to most races of P. graminis f. sp. tritici, other than Pgt-QCCJ, while cv. Steptoe lacks any reported genes for resistance to P. graminis f. sp. tritici. The stand-density treatments established were approximately double, the equivalent of, half of, and one quarter of the planting rate recommended for commercial barley. Stem rust caused by race Pgt-QCCJ spread more rapidly in cv. Steptoe than in cv. Robust, irrespective of the stand-density treatment. The effect of stand density on rust severity was, however, greater than the effect of cultivar. Rust severity was 18 to 36% greater in sparse stands than in dense stands of both cultivars. Reductions in kernel size were most severe in sparse stands where rust development was greatest. Knowledge that reduced stand densities may promote the development of stem rust in barley may be useful in refining procedures for screening breeding material for resistance to stem rust, and in allaying concern over the high levels of stem rust occasionally reported in lightly seeded seed increases and in commercial fields where sparse stands are encountered.