The appearance and spread of new Pst races are common consequences of the widespread use of single resistance genes in one or more widely grown cultivars, with epidemics occurring some time later. Based on the geographical situation in China, epidemiology of stripe rust can be divided into three major zones, namely autumn sources of inocula, spring sources of inocula, and the spring epidemic areas. About 67 stripe rust resistance genes (Yr1 – Yr67) and some temporarily designated genes have been catalogued in cultivated wheat varieties. Many of the genes have unique linked markers that enable their transfer by marker assisted selection (MAS). We recommend firstly that wheat breeders, rust geneticists and pathologists work in together in evaluating the effectiveness of resistance in multi-pathotype seedling tests in the greenhouse and in field trials at hot-spot locations to identify the genes conferring stable resistance across environments; and secondly to apportion the available resistance genes to the different epidemiological regions. We expect that such regional diversity of resistance genes will provide strong barriers to seasonal spread between regions.
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A Pst pathotype group named V26, virulent to wheat lines possessing Yr26 (=Yr24) has become the third most frequent group in China after races CYR32 and CYR33. Twenty four near-isogenic lines (NILs) and 19 Chinese differentials were used to identify the avirulence/virulence spectra of 36 Yr26-virulent isolates from four provinces (Qinghai, Gansu, Sichuan and Ningxia). Eight races were identified when tested on the NIL set, and 7 races were identified on the Chinese set. There was no relationship with province of origin. Three races identified on the NILs occurred at relatively high frequencies (23, 3, and 3 isolates). Virulence differences existed for Yr1, Yr4, Yr6, Yr9, Yr17, Yr25, Yr32, YrSp, and YrTr1. Among the 7 races identified on the Chinese differentials, one (CYR32 + Yr26 virulence) was represented by 13 isolates and another (CYR33 + Yr26 virulence) included 15 isolates. Among the entire group there were virulence differences on Trigo-Eureka (Yr6+), Lovrin 13 (Yr9+), Kangyin 655, Fengchan 3 (Yr1+), Lovrin 10 (Yr9+), and Hybrid 46 (Yr4+). All isolates were avirulent on Zhong 4 and T. spelta. Using 18 polymorphic simple sequence repeat (SSR) markers, we identified 35 genotypes clustered into two molecular groups (MGs) at a similarity coefficient level of 0.70. SSR analysis also indicated a high level of recombination within the V26 group. The considerable diversity indicates a threat not only to cultivars carrying Yr26, but also to other currently resistant materials.
Pathogen migration is a source of new pathogens and pathotypes to a particular region. Studies on origin and migration of pathogens were difficult in the past, but DNA sequence data and new analytical approaches now enable us to analyze population genetic structures from which we can determine possible migration routes. The Himalayan and neighboring regions (Gansu, China) were earlier suggested as putative centers of origin for Puccinia striiformis f. sp. tritici (Pst) and sampling locations for Chinese isolates were strongly focused in South Gansu. Previous field surveys indicated that Yunnan might be the primary source of the Gansu population in China. Using samples collected in 2008 and 2011 we compared the Pst population structures of Yunnan and south Gansu for SNPs in housekeeping genes Cdc2, Ef-1? and Mapk1, and the trajectories of upper air flow during wheat growing seasons from 2005 to 2013. The ancestral haplotype was detected in the Yunnan population, which had higher a mutation rate than the Gansu population, but the latter contained more recombination events. Both populations were highly diverse. There was massive air flow between Yunnan and Gansu with trajectories being mainly from Yunnan to Gansu. We propose a putative dispersal route of Pst from Yunnan to South Gansu, making Yunnan the center of origin for Pst in China, whereas Gansu is a secondary center of origin. It is proposed that Pst migrates from South Asia to Yunnan under the influence of westerly weather patterns, and subsequently spreads to other parts in China. DNA sequence comparisons should be undertaken to compare Pst populations of South Asia and China in order to confirm these hypotheses.
China is the largest stripe rust epidemic area in the world. Central Shaanxi, as an important stripe rust overwintering zone for the disease serves as a “bridge” for the pathogen, where early sown wheat infected during the previous autumn provides inoculum for spring epidemics in more eastern regions. Studies of resistance and Yr-gene distribution among local candidate cultivars provide valuable insights into the influence of host genotype on selection of the rust pathogen population. A total of 183 local advanced lines from 2009 to 2011 were tested for seedling resistance with 12 Pst races in the greenhouse, and with mixed races at Tianshui in Gansu province. Gene postulations were based on the seedling response data and molecular markers. Four (2.2%) entries were resistant at all growth stages; 15 (8.2%) were resistant as adult plants; 164 (89.6 %) were susceptible to one or more races at the seedling or adult stages; and 40 were resistant to the currently prevalent races CYR32 and/or CYR33, but susceptible to at least one of the potentially important races Su11-4, Su11-5 and Su11-7, V26/CM42 and V26/Gui22. All entries showed seedling stage susceptibility at Tianshui. Postulated genes included Yr7, Yr9, Yr10, Yr17, Yr18, and Yr24/Yr26. Yr5, Yr15 and Yr61, currently effective against all Chinese races, were not present. Although advanced wheat lines bred in Shaanxi may be diverse our results show that most of them are highly susceptible to one or more prevalent or low frequency races in Shaanxi or adjacent Gansu. This situation indicates that Shaanxi farmers should be using partial adult plant resistances to reduce inoculum levels and hence reduce the amount of primary inoculum spread to more easterly wheat growing areas.