Stripe rust management in spring and winter wheat by varietal resistance and fungicide application

Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici Eriks., is prevalent throughout western Canada in some years. As a result, there is a need to study stripe rust control for winter wheat grown in western Canada. The objective of the first thesis project was to evaluate the effects of fall fungicide application (metconazole and pyraclostrobin) on stripe rust and leaf spot severity, as well as yield and quality of winter wheat. The effects of fungicide application in the fall, spring, or both fall and spring (dual) on four winter wheat cultivars varying in disease resistance: ‘AC Bellatrix,’ ‘Moats,’ ‘Radiant,’ and ‘CDC Osprey’ were evaluated at four sites: Saskatoon and Indian Head in SK and Lethbridge and Lacombe in AB. Stripe rust severity on susceptible cultivars, ‘AC Bellatrix’ and ‘CDC Osprey,’ and leaf spot severity on these cultivars and ‘Radiant,’ were reduced by a single spring or dual fungicide applications (fall and spring) but not by fall application when over-all disease severity was high. Single spring and dual fungicide applications on stripe rust susceptible cultivars maintained yield potential compared to the unsprayed treatment, which was reduced by nearly 30%; grain quality was also maintained by the same treatments. The stripe rust resistant cultivar ‘Moats’ was not affected by fungicide application at any timing. Fall fungicide application had no effect on disease severity, yield or grain quality during the study. The dual fungicide application did not offer additional benefits. Race-specific stripe rust resistance genes (Yr genes) are widely deployed in wheat cultivars to control stripe rust. However, race-specific resistance break-down by the pathogen occurs frequently. The residual effects, the expression of partial, non race-specific resistance to virulent pathogen races by defeated race-specific disease resistance genes, is a potential source of durable stripe rust resistance. In the second project, the residual effects of race-specific Yr genes in wheat were evaluated. The parental near isogenic lines (NILs) with defeated single Yr genes (Yr10, Yr26, and Yr32) in the ‘Avocet’ background, F5 NILs with two Yr genes in the combinations of Yr26/Yr10, Yr32/Yr10 and Yr32/Yr26 and the susceptible cultivar ‘Avocet-S’ were inoculated with each Pst isolate or isolate mixtures (W020, W049, T034/W052) virulent to all three Yr genes in growth chambers. Infection type (IT), infection area (IA) and latent period (LP) were assessed and recorded. An increased number of Yr genes was correlated with reduced IT and IA and longer LP. The same NILs were evaluated in stripe rust field nurseries at Saskatoon, SK and Lethbridge, AB in 2018. There was an interaction between genotypes and isolates, and correlations between increasing numbers of Yr genes and all parameters. When inoculated with T034/W052, Yr32/Yr10 and Yr32/Yr26 wheat genotypes had lower IT and IA and longer LP than ‘Avocet-S’. With two other isolates, Yr32/Yr10 and Yr32/Yr26 wheat genotypes tended to have lower IA and longer LP than ‘Avocet-S’ but not always. The Yr26/Yr10 wheat genotype often showed no residual effect. The results from field disease nurseries were inconclusive due to the avirulent nature of the Pst population to Yr10 and Yr26 wheat genotypes. The results indicated that some defeated Yr genes have residual effects in certain combinations and further study is required for a better understanding of the mechanism of residual effects