Rust

To inform breeders and growers of important changes in virulence and to facilitate development and deployment of resistant cultivars, isolates of wheat rust fungi are routinely evaluated on seedlings of a set of differential wheat lines containing different resistant genes. However, the methods used to evaluate and report virulence changes in most regions of the world seem inadequate for accomplishing these goals and could be improved by adherence to three principles. Firstly, for each region, the resistance genes in the set of differentials should match the effective genes in contemporary cultivars and breeding lines. Most differential sets contain several resistance genes that have been ineffective for decades and do not contain genes found in cultivars and breeding lines. Given the importance of genes for race-specific adult-plant resistance, these should be included in differential sets. Secondly, intermediate reactions on differential lines that had been highly resistant are important warnings of gradual increases in virulence. Naming races requires isolates to be either virulent or avirulent on each line in a fixed set of differentials and is a hindrance to identifying gradual changes in virulence on currently effective genes. Utilizing virulence formulae with a designation for intermediate virulence (e.g. parentheses around the gene or differential) seems to be a simple solution for both documenting partial virulence and for easily changing differentials to match genes in cultivars and breeding lines. Thirdly, the method for evaluating virulence against a particular differential should predict the result of that host-pathogen interaction in the field. Growth stage and environmental conditions are important for expression of some resistance genes, and all currently effective genes are not likely to be expressed under the same conditions. Following these principles will make virulence surveys more predictive of important changes in the field and thereby contribute to more effective management of rust diseases.

An important component of the management of wheat stem rust is an understanding of the population diversity of the pathogen, Puccinia graminis f. sp. tritici (Pgt). The discovery of “Ug99” resulted in renewed efforts on pathogen surveys, sample collections and pathotyping of Pgt, with a primary focus on Africa. In the last few years these efforts have been expanded to include other targeted regions, however a global effort is needed. The aims of the “Global Pgt Initiative” is: to capture and maintain living cultures that collectively reflect the entire global diversity of Pgt in the years 2014 - 2016; pathotype and genotype this collection; develop DNAbased diagnostic tools that will be able to rapidly detect shifts in Pgt populations, and provide an early warning system of the vulnerability of wheat to new virulent strains; and provide a genetic baseline for comparison of Pgt populations over time, both forward and backwards. This initiative will provide the wheat rust community with a geographically distributed, well characterized, living culture collection that represents the global diversity of Pgt; a global open access knowledge bank on Pgt pathotypes and genotypes; and advanced molecular diagnostic tools for rapid detection and tracking of Pgt populations. The Global Pgt Initiative represents the most comprehensive effort to capture and characterize the global diversity of Pgt and provide a unique resource to the global wheat rust community.

Targeted breeding to develop high yielding wheat germplasm resistant to Ug99 and other rusts initiated at CIMMYT in 2006. Ug99 resistant materials, especially those with adult plant resistance (APR), were used in crossing. F3 and F4 populations from simple, BC1 and top crosses were grown for two generations under high rust pressures at Njoro, Kenya in a Mexico-Kenya shuttle breeding scheme. Parallel populations were also grown in Mexico for comparison. Approximately 5,000 advanced lines were tested for grain yield performance at Ciudad Obregon, Mexico in 2009/10 season, and phenotyped for resistance to Ug99 and other rusts. The 728 retained lines were evaluated for grain yield performance in five environments during the 2010/11 season in Mexico. About 68% of the 728 lines had nearimmune (16.5% entries) to adequate APR to Ug99. An additional 13.6% lines carried one of the six (Sr25, Sr26, SrTmp, SrHuw234, SrSha7, and an unidentified gene) race-specific resistance genes often in combination with APR gene Sr2. About 80% entries were highly resistant to yellow rust in Kenya and Mexico, and 90% entries to leaf rust in Mexico. Yield distribution of lines derived from Mexico-Kenya shuttle breeding was similar to lines selected only in Mexico. Sufficient lines with >5% superior yields than the Mexican checks varieties in 2 years testing were identified. Our results indicate that targeted crossing and shuttle breeding are powerful tools for a simultaneous improvement of grain yield potential and resistance to rusts.

In China, wheat is grown on approximately 24 million hectares with an annual yield of 100 million tonnes. Stem rust, caused by Puccinia graminis f. sp. tritici, is a threat mainly to spring wheat in northeastern China. Leaf rust, caused by P. triticina, occurs on crops in the late growth stages in the Yellow-Huai-Hai River regions. Stripe rust, caused by P. striiformis f. sp. tritici (Pst), is destructive in all winter wheat regions and is considered the most important disease of wheat in China. During the last 20 years, widespread stripe rust epidemics occurred in 2002, 2003, and 2009, and localized epidemics occurred in many other years. In recent years, major yield losses were prevented by widespread and timely applications of fungicides based on accurate monitoring and prediction of disease epidemics. A total of 68 Pst races or pathotypes have been identified using a set of 19 differential wheat genotypes. At present, races CYR32 and CYR33 virulent to resistance genes Yr9, Yr3b, Yr4b, YrSu and some other resistance genes are predominant. Moreover, these races are virulent on many cultivars grown in recent years. Of 501 recent cultivars and breeding lines 71.9% were susceptible, 7.0% had effective all-stage resistance, mostly Yr26 (= Yr24), and 21.2% had adult-plant resistance. Several resistance genes, including Yr5, Yr10, Yr15, Yr24/Yr26, YrZH84 and some unnamed genes, are still effective against the current Pst population. All have been widely used in breeding programs. Lines with one or more of Yr1, Yr2, Yr3, Yr4, Yr6, Yr7, Yr8, Yr9 and other unnamed resistance genes are susceptible to currently predominant races. Durable adult plant resistance sources are being increasingly used as parents in breeding programs. Progress has been made in genomics and population genetics of Pst, molecular mapping of resistance genes, and cytological and molecular mechanisms of the host-pathogen interactions involved in stripe rust.