Euphytica 94: 279–286, 1997. 279 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Resistance to stripe rust in durum wheats, A-genome diploids, and their amphiploids H. Ma, R.P. Singh & A. Mujeeb-Kazi International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apdo. Postal 6-641, 06600 Mexico, D.F. Received 5 August 1996; accepted 29 November 1996 Keywords: Triticum turgidum, T. monococcum, T. boeoticum, T. urartu, wheat, Puccinia striiformis, stripe rust, rust resistance, amphiploid, suppressors Summary Stripe rust (caused by Puccinia striiformis Westend.) is a wheat disease of worldwide importance. Seedlings of 75 accessions of Triticum boeoticum, 12 of T. monococcum, 16 of T. urartu, 230 of durum wheat (T. turgidum L. var. durum), and 128 amphiploids (genome AAAABB) involving the crosses of the three diploid species (AA) with T. turgidum (AABB) were evaluated in the greenhouse for their reaction to P. striiformis race 14E14. Durum wheats and the amphiploids were also evaluated at two field locations in Mexico with the same race for their adult plant response. Resistant seedling reactions (infection types: 0-3 on a 0-9 scale) were seen for 10 (13%) accessions of T. boeticum, 19 (8%) accessions of T. turgidum and 32 (25%) amphiploids. The remaining accessions were either moderately resistant (ITs 4-6) or susceptible (ITs 7-9). The three amphiploids derived from the crosses of seedling resistant T. boeoticum and T. turgidum, were resistant as seedlings. Among the 51 amphiploids involving one resistant parent, 29 were resistant and the remaining 22 displayed intermediate to susceptible reactions. Suppressors for resistance were common in the A and AB genomes and suppression was resistance gene specific. Forty-five (20%) durums showed adequate field resistance (relative AUDPC 10% of the susceptible check ‘Morocco’). These included the 19 seedling resistant durums. Presence of genes involved in adult plant resistance was evident, because 26 of the remaining adult plant resistant durums had displayed intermediate-susceptible seedling reactions. Though the seedling reactions of the amphiploids varied from low to high, all involving the adult plant resistant durums possessed adequate field resistance. The resistant, newly produced, AAAABB amphiploids are useful genetic resources for stripe rust resistance which could be transferred to the cultivated T. turgidum. Introduction Stripe (or yellow) rust, caused by Puccinia striiformis Westend., is an important disease of wheat in cool and humid wheat-growing regions. Grain yield losses up to 75% were reported in severe epidemics (Roelfs, 1978). Host resistance is considered to be the most economical and environmently benign control method. A majority of the 23 characterized resistance genes to stripe rust (McIntosh et al., 1995) are no longer effective due to virulences in the pathogen. Our recent genetic studies (Ma et al., 1996, unpublished) have suggested that the current stripe rust resistance in some important high yielding and widely-adapted durum wheats (Triticum turgidum L.) developed at CIMMYT may be conferred by only 4 to 5 resistance genes. To sustain stripe rust resistance globally, the existing resistance gene pool should be widened and diversified. In the last decade, several Triticum species related to wheat have been explored for new resistance sources. Stripe rust resistance was found in T. tauschii (Ma et al., 1995a), T. monococcum (Mikhova, 1988),T. dic- occoides (Silfhout et al., 1989), and various Aegilops species (Damania & Pecetti, 1990). Some of these species (e.g., T. tauschii) carried resistance genes that were different from those reported earlier (Ma et al., 1995a). These alien resources could offer new sources of resistance genes for wheat breeding.