Assessment of adult plant resistance to stripe rust in wheat

English: The objective of this study was to develop and optimise methods to detect adult-plant resistance (APR) in wheat to stripe rust caused by Puccinia striiformis f.sp. tritici. A collection of spring wheats was tested at the seedling stage with different temperature and light regimes to test the hypothesis that environmental variation can induce APR expression in primary leaves. Treatments included pre-inoculation conditioning of seedlings at either 18° or 25°C, followed by post-inoculation temperature and light treatments. In some treatments intermittent low and high temperatures (18° or 25°C) were used as well as a treatment with continuous low temperature (10°C). The highest infection types were encountered with a pre-inoculation treatment of 18°C combined with a 24 h light cycle, and a post-inoculation treatment of 18°C combined with a 18 h light and 6h darkness cycle. Day length influenced infection types more than temperature. All treatments subjected to less than 14 h light per day showed a reduction in infection levels. Dark periods experienced prior to inoculation were not conducive to infection type development. Clear banding of infection zones and reduced sporulation occurred. Based on the lack of correlation between seedling infection types and adult responses observed in the field, no environment was conducive to expression of APR to stripe rust in seedlings. Rating of adult plants for stripe rust resistance has traditionally been problematic in terms of infection levels and repeatability. Furthermore, several attempts to investigate the reaction of normally grown adult plants in the glasshouse failed due to leaf damage. To improve these methods, an accelerated system of producing adult wheat plants in a controlled environment was tested. By manipulating plant density, pot size, light and temperature, wheat plants reached maturity quicker than normally grown adult plants and were referred to as mini-adult plants. In terms of leaf damage the mini-adults were better suited for flag (terminal) leaf evaluation. For optimum results, the mini-adult plants should be grown in a glasshouse rather than growth chamber and inoculated between heading and flowering. According to flag leaf infection type and whole plant reactions observed, the mini-adults provided an acceptable system for comparing adult plants to different isolates of P. striiformis. This system accurately detected APR in most winter wheats tested and was reliable for a collection of CIMMYT spring type wheats. Disease ratings of 98 CIMMYT lines tested with this accelerated system showed a correlation of 88% with field ratings. The mini-adult plant system was also tested with populations segregating for APR. Because of the likely application of genetic studies to resistance breeding, it is essential to correlate glasshouse tests with field assessments. The approach followed in the present study conformed to guidelines in terms of selecting cultivars with high levels of APR not previously analysed, as well as comparing glasshouse and field data. Segregation ratios indicated the presence of two resistance genes in the F3 of a Baviaans x Avocet S cross (χ²7:8:1 = 0.1029), but were inconclusive for a Sunmist x Avocet S cross where Mendelian ratios could not be confirmed. The accelerated screening system was less successful than the screening of pure lines. Based on the fact that the reaction of Avocet S was not completely susceptible in the glasshouse, and most likely influenced ratios, the lower success rate appeared to be a function of the parental genotypes rather than in the system in general. Compared to the normal glasshouse procedure, the improved APR system took significantly less time and physical resources to achieve ratings.