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2001 STEEP III Progress ReportRESEARCH PROJECT TITLE: Improved Methods for Evaluating Resistance To Cephalosporium Stripe of Wheat INVESTIGATORS:
Chris Mundt, Tom Wolpert, and Lynda Ciuffetti, Botany and Plant Pathology,
OSU PROJECT OBJECTIVES: KEY WORDS: Cephalosporium stripe, conservation tillage, fungal toxins, molecular mapping STATEMENT OF PROBLEM: Cephalosporium stripe has become a limiting factor for many Pacific Northwest wheat growers in erosion-prone areas, especially when early planting and/or trashy fallow are practiced. Burning or plowing stubble and delayed seeding can provide substantial control of Cephalosporium stripe. However, these cultural control methods conflict strongly with attempts to control soil erosion. Though no soft white winter wheat cultivar shows complete resistance to Cephalosporium stripe, there is considerable variation in the degree of resistance among cultivars. Further, higher levels of resistance may be available in exotic germplasm. However, identifying resistance in breeding programs remains problematic. Expression of resistance is incomplete and environmentally dependent. In addition, the disease tends to be aggregated within fields, thus requiring large plots to make useful comparisons, which is not possible in early generations of cultivar development. ZONE OF INTEREST: low and intermediate rainfall ABSTRACT OF RESEARCH FINDINGS: During this funding year, we developed base-line data from the field regarding Cephalosporium resistance of advanced breeding lines with favorable agronomic and quality characteristics, and evaluation of potential resistance sources from outside of Oregon. We identified variation among advanced lines that will be useful in identifying appropriate cultivars for areas where Cephalosporium stripe is a problem. We identified a few lines of French soft red winter wheat derived from the cultivar 'Rossini' that have outstanding yield potential and a level of resistance to Cephalosporium stripe similar to that of PNW club wheats. Populations are being developed to study the inheritance of toxin insensitivity in genotypes adapted to Oregon growing conditions, and will be assayed with the toxic fraction and in field studies as soon as sufficient seed of stable lines are available. We obtained the first season of field resistance data for 84 progeny of the International Triticae Mapping Initiative (ITMI) population for comparison with results of the lab-based toxin assay obtained in the first year of the project. Heritability of whitehead percentage was 88%. Regression analysis of DNA-based markers suggested the presence of determinants of resistance in chromosomes 2D, 4A, 6A, and 6D. These four regions accounted for 41% of the phenotypic variance. Associations among field ratings, toxin sensitivity, and DNA-based markers are currently being evaluated. RESULTS AND INTERPRETATION: Objective 3. Genetics of Resistance and Screening - As indicated in last year's STEEP III Progress Report, we found that sensitivity of wheat genotypes to the toxic fraction falls into three categories, with common wheats being the most sensitive, club wheats the least sensitive, and durum wheats intermediate. Sensitivity of these groups to the toxic fraction correlated very well with their disease reaction in the field. These results have been published in the journal Phytopathology (see Rahman et al., 2001 below). Progress on this objective during the past year was on two fronts. First, we have developed base-line data from the field regarding Cephalosporium resistance of advanced breeding lines with favorable agronomic and quality characteristics, and evaluation of potential resistance sources from outside of Oregon. Artificially inoculated plots at Pendleton provided whitehead data that were very repeatable between replicates. We identified variation among advanced lines that will be useful in identifying appropriate cultivars for areas where Cephalosporium stripe is a problem. Potential sources of resistance from the Mid-West were disappointing. However, we identified a few lines of French soft red winter wheat derived from the cultivar 'Rossini' that have outstanding yield potential and a level of resistance to Cephalosporium stripe similar to that of PNW club wheats. We are currently determining whether these lines also demonstrate insensitivity to the toxic fraction in our detached leaf assay. Populations are being developed to study the inheritance of toxin insensitivity in genotypes adapted to Oregon growing conditions. Three crosses of moderately resistant x susceptible cultivars have been made: Coda x Stephens, Madsen x Stephens, and Rossini x Stephens. Progeny from these crosses are being increased through single seed descent in the greenhouse, and will be assayed with the toxic fraction and in field studies as soon as sufficient seed of stable lines is available. 4. Mapping population - As described in last year's STEEP III Progress Report, 112 recombinant inbred lines from the International Triticae Mapping Initiative (ITMI) population were evaluated for sensitivity to the toxic fraction. Frequency distribution of the wilting data suggested a continuous distribution skewed towards insensitivity. Differences among the progeny were highly significant (P # 0.0001), and heritability of toxin sensitivity was 88%. During the past year, we have focussed on obtaining field data for comparison with results of the lab-based toxin assay obtained in the first year of the project. This was complicated by the fact that the mapping population is in a spring background, whereas Cephalosporium stripe is mainly a disease of winter wheat. A spring 2000 planting was inoculated, but did not express disease symptoms. We then planted and inoculated the mapping population at Pendleton in fall 2000, in the hope that a mild winter would allow survival and field evaluation of these spring genotypes. This turned out be highly successful. We were able to evaluate 84 mapping population progeny in a randomized complete block design with two replicates. Plots were scored for stand density, overall stand health, and percent whiteheads in spring 2001. Heritability of toxin insensitivity was 88%. DNA-based marker genotypes and percent whitehead scores were subjected to regression analysis to identify marker-trait associations. Results suggested the presence of determinants of resistance in chromosomes 2D, 4A, 6A, and 6D. Three of these regions were from the resistant parent and one from the susceptible parent. The proportion of the phenotypic variance accounted for by significant regions on chromosomes 2D, 4A, 6A, and 6D were 15, 12, 11, and 8%, respectively. These four regions accounted for 41% of the phenotypic variance. The region on chromosome 4A is particularly interesting since it has also been associated with resistance to chlorosis induction by Pyrenophora tritici-repentis, causal agent of tan spot of wheat. The same progeny have been planted and inoculated at Pendleton again in mid-September 2001. Associations among field ratings, toxin sensitivity, and DNA-based markers are currently being evaluated. Summary - The ultimate goal of the project is to produce wheat cultivars with resistance to Cephalosporium stripe combined with favorable quality, yield, and other important agronomic characteristics. Such cultivars will greatly increase ability of wheat growers to successfully implement conservation tillage practices in erosion-prone areas. In the short-term, we will identify and exploit useable levels of resistance in adapted germplasm. In the longer term, we hope to develop gene introgressions with potentially very high levels of resistance from synthetic populations via molecular markers. INTERACTION WITH OTHER SCIENTISTS CONDUCTING RELATED ACTIVITY: Information regarding the biology and control of Cephalosporium stripe is exchanged with other pathologists in the Pacific Northwest, primarily Tim Murray (WSU) and Dick Smiley (OSU). In addition, contact is maintained with wheat breeding programs in Idaho, Oregon, and Washington to identify promising lines for evaluation, and to provide breeding programs with information regarding reactions of wheat cultivars and lines to Cephalosporium stripe. PUBLICATIONS AND
PRESENTATIONS: "Identifying Resistance to Cephalosporium Stripe of wheat", Moro Station Field Day, June 2001. Rahman, M., Mundt, C.C., Wolpert, T.J., and Riera-Lizarazu, O. 2001. Sensitivity of wheat genotypes to a toxic fraction produced by Cephalosporium gramineum and correlation with disease susceptibility. Phytopathology 91:702-707. |
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