"QUANTITATIVE INDOOR ASSAY" (QIASSAY), A TOOL TO IDENTIFY, SELECT AND DEVELOP CEREAL GERMPLASM THAT PERFORMS WELL UNDER BARLEY YELLOW DWARF (BYD) DISEASE PRESSURE AND OTHER ENVIRONMENTAL STRESSES. S HABER1 and A COMEAU2 1Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg MB R3T 2M9, Canada; 2Soil and Crops Research and Development Centre, Agriculture and Agri-Food Canada, Ste.Foy, Qu‚bec, QC G1V 2J3, Canada Background and objectives BYD shares the characteristic with other cereal diseases such as wheat streak mosaic and leaf rust of crop loss severity being highly correlated with earliness of seedling infection. However, neither the degree of apparent severity of early BYD symptoms nor their extent and/or speed of onset in small-scale indoor tests have been particularly helpful in ident ifying germplasm that yields well under disease pressure in the field. The BYD-host interaction appears to be a complex one, in which BYDV infection of seedlings and subsequent disease development may differentially affect several parameters of growth and development. These in turn may vary with host genotype and/or the nature of additional interactions with environ mental factors [1]. Any evaluation system using small-scale tests would need to be based on assays that achieved low variation in the parameters they assessed. Materials and Methods QIAssay "simplifies and exaggerates" the effects of host-BYD interactions. Variability in seed, soil, and lighting and temperature conditions is mini mized by conducting the seedling-to-heading phase of experiments in growth cabinets. Long inoculation access periods (10 d) at cool temperatures (10 C) reduce plant-to-plant variability in the early infection progress. Multiple regression analyzes such growth parameters as heading and matur ity dates, and height as independent variables which exhibit little inter- plant variability within a treatment. To reduce variation in the dependent variable, seed yield per plant, secondary tillers are trimmed and only the first 2 emerged heads are assessed. Results and Conclusions The QIAssay protocol's uniformity of seedling infection and subsequent disease development allows quantitative assessments in tests of models predicting field performance from results of small-scale indoor tests. The models developed from QIAssay tests are validated by the results of in dependently conducted large-scale field trials, and by the repro-ducibil ity of observed quantitative effects of BYD on plant growth and develop ment parameters. For the tested spring wheat lines, the most critical QIAssay parameters for predicting field performance under BYD pressure are: length of delay of heading, reduction in volume of the main head, and reduction of flag leaf area, but virus titres assayed by ELISA are not useful in predicting performance under pressure from seedling infection with BYDV. Results from multi-year field trials also suggest that BYD-tolerant germ plasm, even in the absence of strong BYD pressure, may cope better with environmental stresses and provide superior long-term yield stability. Improved methods for identifying and exploiting BYD-tolerant germplasm may therefore yield benefits beyond simply reduced losses from disease. References 1. Haber S, 1995. in D'Arcy, CJ and Burnett, PA (eds.) Barley Yellow Dwarf: 40 Years of Progress. APS Press, St. Paul, U.S.A., pp.145-167.