Genetic Relationships of Crown Rust Resistance, Grain Yield, Test Weight, and Seed Weight in Oat

mained effective in North America longer than 5 yr after the resistance genes were released in pure-line Integrating selection for agronomic performance and quantitative cultivars (Holland, 1997). Monogenic resistances can be resistance to crown rust, caused by Puccinia coronata Corda var. avenae W.P. Fraser & Ledingham, in oat (Avena sativa L.) requires overcome rapidly by new races of the pathogen that an understanding of their genetic relationships. This study was con- emerge because of the selection pressure exerted by ducted to investigate the genetic relationships of crown rust resistance, large areas of uniformly resistant hosts (Harder and grain yield, test weight, and seed weight under both inoculated and Haber, 1992; Kolmer, 1997). Evolution of new races in fungicide-treated conditions. A Design II mating was performed be- crown rust populations can occur via accumulation of tween 10 oat lines with putative partial resistance to crown rust and mutations in asexual populations or by sexual recombinine lines with superior grain yield and grain quality potential. Proge- nation on the alternate host, buckthorn (Rhamnus canies from this mating were evaluated in both crown rust-inoculated thartica L.), which occurs naturally in North America and fungicide-treated plots in four Iowa environments to estimate (Chong and Kolmer, 1993; Dinoor et al., 1988). genetic effects and phenotypic correlations between crown rust resisMethods proposed to improve the durability of crown tance and grain yield, seed weight, and test weight under either infection or fungicide-treated conditions. Lines from a random-mated pop- rust resistance in oat include gene pyramiding, gene ulation derived from the same parents were evaluated in three Iowa deployment, multiline breeding, and selection for partial environments to estimate heritabilities of, and genetic correlations resistance. Gene deployment, wherein breeders in difbetween, these traits. Resistance to crown rust, as measured by area ferent regions agree to release cultivars with different under the disease progress curve (AUDPC), was highly heritable sets of resistance genes, should exert disruptive, rather (H 0.89 on an entry-mean basis), and was favorably correlated with than directional, selection pressure on the pathogen grain yield, seed weight, and test weight measured in crown rust- population (Frey et al., 1973). Multilines are expected inoculated plots. AUDPC was unfavorably correlated or uncorrelated to exert stabilizing, rather than directional, selection on with grain yield, test weight, and seed weight measured in fungicidethe pathogen (Frey, 1982). Both gene deployment and treated plots. To improve simultaneously crown rust resistance, grain multiline breeding strategies depend upon the availabilyield, and seed weight under both lower and higher levels of crown rust infection, an optimum selection index can be developed with the ity of large numbers of effective resistance genes, which genetic parameters estimated in this study. are not currently available for crown rust resistance in oat. Compared with resistance from a single major gene, gene pyramiding may enhance the durability of resistance because it should be more difficult for virulence