Anmin Wan

Wheat Stripe Rust Epidemic and Virulence of Puccinia striiformis f. sp. tritici in China in 2002

In China, wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most destructive diseases of wheat and can cause severe yield losses when susceptible cultivars are grown and weather conditions are favorable for the disease. Wheat stripe rust most frequently affects the winter wheat growing areas in Northwest, Southwest, and North China, and the spring wheat growing areas in Northwest China. In the 2001-2002 growing season, a widespread stripe rust epidemic affected about 6.6 million hectares of wheat in 11 provinces: Si-chuan, Chongqing, eastern Gansu, southern and western Shaanxi, southern and central Ningxia, Yunnan, Guizhou, Hubei, Henan, southern and central Hebei, and Shandong. The epidemic could be attributed to relatively warm weather from November 2001 to March 2002, high frequencies of stripe rust races CYR31 and CYR32, and widely grown susceptible cultivars. Race CYR31 was virulent on the Chinese differential cultivars Trigo Eureka, Fulhard, Lutescens 128, Mentana, Virgilio, Abbondanza, Early Premium, Funo, Danish 1, Fengchan 3, Lovrin 13, Shui-yuan 11, Lovrin 10, and Hybrid 46. Race CYR32 had all the virulence factors of CYR31, plus virulences on Chinese differential cultivars Jubilejina 2 and Kangyin 655, i.e., CYR32 was virulent on all differential cultivars, except Zhong 4. When tested on the world and European differential and some other resistant genotypes, CYR32 was virulent on Chinese 166 (Yr1), Heines VII (Yr2, Yr25, and YrHVII), Vilmorin 23 (Yr3a and Yr4a), Heines Kolben (Yr6 and YrHK), Lee (Yr7, Yr22, and Yr23), Clement (Yr9, Yr25, YrCle), VPM1 (Yr17), Selkirk (Yr27), Anza (YrA), Carstens V (YrCV1, YrCV2, and YrCV3), Gaby (YrG), Strubes Dickkopf (Yr25), and Suwon 92/Omar (YrSO). Resistance genes in Triticum spelta album (Yr5), Zhong 4, and Moro (Yr10 and YrMor) were effective against all races identified.

Virulence races of Puccinia striiformis f. sp. tritici in 2006 and 2007 and development of wheat stripe rust and distributions, dynamics, and evolutionary relationships of races from 2000 to 2007 in the United States

Abstract Stripe rust, caused by Puccinia striiformis f. sp. tritici (PST), has historically been the most frequently destructive disease of wheat (Triticum aestivum) in the western United States and has become a more frequent problem in the central and southeastern states since 2000. The race composition of PST has been determined every year from rust-infected leaf samples of wheat and grasses collected in the United States on a set of 20 differential wheat genotypes. In 2006, a total of 18 races were detected, of which five were detected for the first time. In 2007, a total of 30 races were detected, of which 11 were newly detected. Among the 16 new races detected in 2006 and 2007, PST-127 was the most important as it has the broadest virulence spectrum identified so far (virulent to all 20 differential genotypes except for ‘Moro’, AVS/6*Yr5 (Yr5), and ‘Tres’) and combined virulence factors to ‘Tyee’ (YrTye) and ‘Hyak’ (Yr17 and YrTye) and those common in the race group detected since 2000. The distribution, frequency changes, and evolutionary relationships for races detected from 2000 to 2007 were analyzed. Three major waves of race changes were identified during the eight-year period. From 2000 to 2002, the predominant races were PST-78 and PST-80, which were virulent on wheat genotypes ‘Lemhi’, ‘Heines VII’, ‘Lee’, ‘Fielder’, ‘Express’, AVS/6*Yr8, AVS/6*Yr9, ‘Clement’ and ‘Compair’. Race PST-80 is also virulent on ‘Produra’. From 2003 to 2006, the predominant race was PST-100, with the same virulence formula as PST-80 plus virulence on ‘Yamhill’ and ‘Stephens’. Starting in 2006, races with the same virulence formula of PST-100 plus virulence to Yr1 became predominant in California and races with the virulence of PST-100 plus virulence on Yr10 became predominant in the Pacific Northwest. During 2000 to 2007, races with more virulence factors became more predominant in the United States, indicating that races with increased virulence factors are at an advantage in the pathogen population over those with fewer virulence factors because they are able to infect more wheat cultivars.

Barberry as Alternate Host Is Important for Puccinia graminis f. sp. tritici But Not for Puccinia striiformis f. sp. tritici in the U.S. Pacific Northwest

Common barberry (Berberis vulgaris) is the alternate host of the wheat stem rust pathogen, Puccinia graminis f. sp. tritici, under natural conditions in the U.S. Pacific Northwest. Barberry was recently shown to be infected by basidiospores of the wheat stripe rust pathogen, Puccinia striiformis f. sp. tritici, under controlled conditions, but it is unclear if barberry plays any role in stripe rust epidemics under natural conditions. Aecial samples of Puccinia spp. collected from barberry plants in the Pacific Northwest from 2010 to 2013 were characterized to species by inoculation on wheat plants under controlled conditions and by molecular markers and sequences of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA. Inoculation of wheat plants with bulked aecia-bearing barberry samples resulted in most P. graminis f. sp. tritici uredia and some P. striiformis f. sp. tritici uredinia. Virulence tests demonstrated that the P. graminis f. sp. tritici isolates were sexually produced, whereas the P. striiformis f. sp. tritici isolates were clonal based on both virulence and simple sequence repeat marker tests, indicating urediniospores from wheat fields landing on barberry leaves as the possible source of P. striiformis f. sp. tritici inoculum. A method for simultaneously testing individual aecia for identifying of P. graminis f. sp. tritici and P. striiformis f. sp. tritici by pathogenicity and ITS markers. Using the method together with ITS sequencing, tested individual aecia were mostly P. graminis f. sp. tritici and occasionally some other formae speciales of P. graminis, but not P. striiformis. The results imply that barberry is essential for stem rust epidemics, but not for stripe rust under the natural conditions in the U.S. Pacific Northwest.

Comparative virulence phenotypes and molecular genotypes of Puccinia striiformis f. sp. tritici, the wheat stripe rust pathogen in China and the United States

Stripe rust (yellow rust) of wheat, caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases in both China and the United States. The Chinese and US populations of the stripe rust fungus were compared for their virulence phenotypes on wheat cultivars used to differentiate races of the pathogen in China and the US and molecular genotypes using simple sequence repeat (SSR) markers. From 86 Chinese isolates, 54 races were identified based on reactions on the 17 Chinese differentials and 52 races were identified based on the 20 US differentials. The selected 51 US isolates, representing 50 races based on the US differentials, were identified as 41 races using the Chinese differentials. A total of 132 virulence phenotypes were identified from the 137 isolates based on reactions on both Chinese and US differentials. None of the isolates from the two countries had identical virulence phenotypes on both sets of differentials. From the 137 isolates, SSR markers identified 102 genotypes, of which 71 from China and 31 from the US. The virulence data clustered the 137 isolates into 20 virulence groups (VGs) and the marker data clustered the isolates into seven molecular groups (MGs). Virulence and SSR data had a low (r = 0.34), but significant (P = 0.01) correlation. Principal component analyses using either the virulence data or the SSR data separated the isolates into three groups: group a consisting of only Chinese isolates, group b consisting of both Chinese and US isolates and group c consisting of mostly US isolates. A neighbour-joining tree generated using the molecular data suggested that the P. striiformis f. sp. tritici populations of China and the US in general evolved independently.

Secreted protein gene derived-single nucleotide polymorphisms (SP-SNPs) reveal population diversity and differentiation of Puccinia striiformis f. sp. tritici in the United States.

Single nucleotide polymorphism (SNP) is a powerful molecular marker technique that has been widely used in population genetics and molecular mapping studies for various organisms. However, the technique has not been used for studying Puccinia striiformis f. sp. tritici (Pst), the wheat stripe rust pathogen. In this study, we developed over a hundred secreted protein gene-derived SNP (SP-SNP) markers and used 92 markers to study the population structure of Pst. From 352 isolates collected in the United States, we identified 242 multi-locus genotypes. The SP-SNP genotypes had a moderate, but significant correlation with the virulence phenotype data. Clustering of the multi-locus genotypes was consistent by various analyses, revealing distinct genetic groups. Analysis of molecular variance detected significant differences between the eastern and western US Pst populations. High heterozygosity was found in the US population with significant differences identified among epidemiological regions. Analysis of population differentiation revealed that populations between the eastern and western US were highly differentiated while moderate differentiation was found in populations within the western or eastern US. Isolates from the western US were more diverse than isolates from the eastern US. The information is useful for guiding the disease management in different epidemiological regions.

Virulence and Molecular Characterization of Experimental Isolates of the Stripe Rust Pathogen (Puccinia striiformis) Indicate Somatic Recombination

Puccinia striiformis causes stripe rust on wheat, barley, and grasses. Natural population studies have indicated that somatic recombination plays a possible role in P. striiformis variation. To determine whether somatic recombination can occur, susceptible wheat or barley plants were inoculated with mixed urediniospores of paired isolates of P. striiformis. Progeny isolates were selected by passing through a series of inoculations of wheat or barley genotypes. Potential recombinant isolates were compared with the parental isolates on the set of 18 wheat or 12 barley genotypes that are used to differentiate races of P. striiformis f. sp. tritici (the wheat stripe rust pathogen) and P. striiformis f. sp. hordei (the barley stripe rust pathogen), respectively, for virulence changes. They were also tested with 51 simple-sequence repeat and 90 single-nucleotide polymorphism markers for genotype changes. From 68 possible recombinant isolates obtained from nine combinations of isolates based on virulence tests, 66 were proven to be recombinant isolates by molecular markers. Various types of recombinants were determined, including lost virulence from both virulent parental isolates, gained virulence from both avirulent isolates, combined virulences from both parents, and inherited virulence from one parent and avirulence from another. Marker data indicate that most of the recombinants were produced through chromosome reassortment and crossover after the hybridization of two parental isolates. The results demonstrate that somatic recombination is a mechanism by which new variants can be generated in P. striiformis.

Variability of the Stripe Rust Pathogen

The stripe rust pathogen, Puccinia striiformis, is highly variable. The variation has been studied for about 100 years based on virulence, for about 30 years based on molecular markers, and for about 10 years based on genome sequencing and functional genomics. Virulence characterization using host differential genotypes has identified a large number of races or pathotypes that overcome race-specific resistance genes in cultivars of wheat and barley crops. Such information is essential for developing cultivars with effective resistance to the disease. Molecular characterization of the pathogen using various marker techniques has generated data that are useful in determination of population structure, identification of pathogen introduction and dissemination and understanding of mechanisms of variation. Molecular characterization has conducted to support findings from virulence tests and/or obtain information that is unable to produce through virulence characterization. The high variability of the stripe rust pathogen is due to its high reproductivity, capability of long-distance dissemination, and ability for adapting to various host species and environments. Among the genetic and evolutionary mechanisms for producing variation, mutation is most important in generating new races and genotypes that are under selection of host species, host cultivars and environments. The pathogen also produces new races and genotypes through somatic recombination. Sexual recombination has been recently demonstrated mainly under controlled conditions in generating variation in races and phenotypes. Natural reproduction on alternate hosts, mainly Berberis spp., has been so far found in China, but its role in generating aeciospores to start stripe rust epidemics on cereal crops and creating new races to attack crop cultivars is still unclear in this country, while ruled out in some other countries. Compared to other fungal pathogens, research on genome sequencing and functional genomics has started relatively late, but recent studies have generated huge data that have helped in understanding the pathogen variation and created useful resources for developing new tools for further studying and monitoring the pathogen variation.

High-temperature adult-plant resistance to stripe rust in facultative winter wheat

Abstract. Stripe (yellow) rust, caused by Puccinia striiformis Westend. f. sp. tritici Erikss., is one of the most damaging diseases in wheat and is especially damaging for winter and facultative wheat. The objective of this study was to understand stripe rust resistance in 100 wheat and facultative wheat entries from the International Winter Wheat Improvement Program by conducting experiments in a greenhouse and in four field environments in Washington State, USA, and by genotyping molecular markers linked to Yr genes. Percentages of entries resistant to the rust races at the seedling stage were: PST-17, 44%; PST-37, 32%; PST-43, 45%; PST-45, 49%; PST-116, 18%; PST-100, 17%; and PST-127, 8%. Molecular markers were positive for genes Yr9, Yr17, and Yr18 and negative for Yr5, Yr10, and Yr15. Yr18 was present in 44 entries (44%). By using the highly virulent races PST-127 and PST-100 under controlled conditions, 16 entries were shown to have high-temperature adult-plant (HTAP) resistance and resistant–moderately resistant field reactions at all four field sites. Resistant entries, especially those with HTAP resistance, were also identified in the field experiments.