Effect of two-component cultivar mixtures and yellow rust on yield and yield components of wheat

Four winter wheat (Triticum aestivum) cultivars and three two-component cultivar mixtures were planted in a replacement series both inoculated with or protected from yellow rust (Puccinia striiformis) in three environments. Each cultivar was susceptible to one or two of the rust races used. Mixtures yielded, on average, 7 and 4% more than their component pure stand means under inoculated and rust-free conditions, respectively. Though all yield components were affected by yellow rust, seed weight was the component that was most consistently influenced. The component genotypes within mixtures varied considerably with respect to yield, and the yield of the same component cultivar included in different mixtures sometimes differed significantly. The correlation between yellow rust severity/tiller and grain yield/tiller in mixture differed among cultivars and depended on their companion cultivar. Variance component analysis indicated that yellow rust was the most important experimental variable influencing grain yield. There was no relationship between yield of the cultivars in pure stands and their yields or competitive abilities in mixture. Disease did not change the competitive ranking of cultivars in mixture. Mixtures with complementary, negative, and overcompensatory interactions were identified. On average, mixtures showed no greater yield stability than did pure stands.