Identification of stripe rust resistance in wheat relatives and landraces

Alexander Loladze

Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Eriks, is a major disease of wheat (Triticum aestivum L. em Thell.) in regions with a temperate climate. Two types of resistance have been identified, seedling and adult-plant resistance. Seedling, or all-stage, resistance is race specific and is expressed in all stages of plant development. One type of adult plant resistance is high-temperature adult-plant (HTAP) resistance. HTAP resistance is non-race specific and is expressed in adult stages of plant development at higher temperatures and is durable. Selection pressure on the pathogen enhances the prevalence of new virulent races and, as a result, all-stage resistance genes are frequently defeated over time. New sources of durable resistance are needed to protect improved cultivars from this disease. The first objective of this research was to develop a new evaluation technique for all-stage resistance on detached wheat seedling leaves. Detached leaf assays are more efficient then intact seedling assays since the need for greenhouse and dew chamber space is eliminated. A detached leaf assay has not been developed for evaluation of stripe rust resistance due to the long latent period of the pathogen, which requires detached wheat leaves to survive on artificial media for extended periods. The goal of this experiment was to create such an assay. The second objective of this research was to evaluate resistance to stripe rust among 164 accessions from nine species of the genus Triticum collected from Georgia, which is considered a center of origin of stripe rust. The germplasm was obtained from U.S. National Small Grain Germplasm Collection. We expected to identify resistant host genotypes from this region that had co-evolved with the pathogen. Field trials for stripe rust evaluation were conducted in 2005 at four locations in Washington, under natural infections. Accessions identified as resistant in the field were tested for all-stage resistance to five races in the seedling stage using the detached leaf assay. The optimum artificial media for prolonging the senescence of the detached leaves for 21-25 days consisted of 0.5% water-agar with 10mg/l kinetin, which was used as a plant senescence retardant, and with pH adjusted to 7. The pathogen produced uredia on the detached leaves 12-15 days after inoculation. Disease ratings from the detached leaf assay corresponded to those from the intact seedling assay. Results of the germplasm evaluation indicated that seventy-four of the164 accessions tested were resistant to stripe rust in the adult stage in the field. Fifty-nine of those had adult plant resistance, since they were susceptible in the seedling stage but resistant in the field. Fifteen accessions had all-stage resistance, since they were resistant in both adult and seedling stages. The adult plant resistance from eight accessions of the species T. dicoccum and four accessions of T. timopheevii was similar to HTAP resistance, since infection types (IT), which rate disease reaction of host genotypes on a 0 (no infection) to 9 (completely susceptible) scale, reached 8 prior to the flowering stage and decreased to 2 after flowering. In conclusion, the new detached leaf assay for evaluation of all-stage resistance to wheat stripe rust eliminates the need for greenhouse and dew chamber access, and provides reliable results. The majority of the resistant genotypes from the Georgian accessions had adult plant resistance, which may be novel sources of stripe rust resistance for cultivar improvement efforts.

Identification of stripe rust resistance in wheat relatives and landraces

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