| Powdery mildew, caused by the biotrophic fungus Blumeria graminis f. sp. tritici(Bgt), is a major constraint to wheat(Triticum aestivum L.) production throughout the world. Powdery mildew(Pm) resistance genes can be overcome by virulent Bgt, which often has diversified virulence structures and is prone to mutation. However, some Pm genes that have lost their resitance in most wheat producing areas of China are still applied by breeders. This reduced the resistance to powdery mildew in newly developed cultivars. Therefore, it is important to monitor Bgt isolates and explore new Pm genes in controlling powdery mildew and developing resistant cultivars. In this study, virulence frequencies of 60 Bgt isolates that were collected from the main wheat production regions of China were determined using a set of wheat cultivars or lines with 21 known Pm genes. Allelism test was performed to disclose the relationship between genes PmLX66, PmW14, and Pm2, which had been located on the same locus of wheat chromosome 5DS. The current status of resistance against powdery mildew in a series of commercial wheat cultivars was examined and certain Pm genes were detected using gene-linked molecular markers. In addition, genetic analysis was performed to understand the gene for resistance to powdery mildew in the wheat line H962 R. The fullowing conclusions can be drawn based on the results from this study: 1. The virulence frequencies of the 60 isolates tested ranged from 0.10 to 0.84 with a mean of 0.51±0.14. One isolate was virulent on genes Pm1 c and Pm21, and 3 isolates were virulent on Pm24. The number of isolates that were virulent on Pm2, Pm5 e, Pm13 and Pm48 were 13, 8, 9 and 6 isolates, respectively. Therefore, these genes were effective against the Bgt isolates originating from the wheat producing regions in China. However, the virulence frequency of this set of Bgt isolates on Pm1 a and Pm33 were 0.43 and 0.47, respectively. More than half of the Bgt isolates were virulent on genes Pm3, Pm3 b, Pm3 c, Pm3 g, Pm4 a, Pm4 b, Pm5 a, Pm6, Pm7, Pm8, Pm17 and Pm19, indicating that these genes are not useful in breeding for resistance to powdery mildew. 2. Liangxing 66 and Wennong 14 are commercial winter wheat cultivars in China. The resistance genes PmLX66 and PmW14 in these cultivars have been located on chromosome 5DS and shared the same location and linked markers SCAR203 and Xcfd81 with Pm2. Allelism tests were performed to determine the relationship between PmLX66, PmW14, and Pm2. All progeny from the crosses Liangxing 66 × Ulka/8*Chancellor(= Ulka/8*Cc), Wennong 14 × Ulka/8*Chancellor, and Liangxing 66 × Wennong 14 were resistant when tested with Bgt isolate E20, indicating that PmLX66 and PmW14 are allelic to Pm2 and to each other. Liangxing 66 was resistant to 76.7% of the 60 Bgt isolates from northern China, a slightly smaller proportion than Ulka/8*Cc(78.3%). Liangxing 66 is differed in their reactions to 9 and 11 Bgt isolates with Wennong 14 and Ulka/8*Cc, respectively, Wennong 14 is differed in 12 Bgt isolates with Ulka/8*Cc. Based on these findings, PmLX66 and PmW14 are new alleles at the Pm2 locus. 3. Using 8 Bgt isolates, the reactions to powdery mildew in 119 commercial wheat cultivars were determined. Fourty-eight cultivars(40.3%) were susceptible to all the isolates tested. Only Jinhe 9123 was resistant to all the isolates. Eight cultivars(6.7%) were resistant to 4-7 Bgt isolates. Sixty-two(52.1%) cultivars were resistant to 1-3 Bgt isolates. Among this set of commercial cultivars, the best one was Jinhe 9123, followed by Jimai 22, Liangxing 99, Yangmai 13, Shannong 20, Bainong 160, Xinmai 21, Yujiao 5, and Xiaoyan 22, which were resistant 4-6 Bgt isolates. 4. Wheat line H962 R displayed a broad spectrum of resistance, and was effective agaist 52 Bgt isolates out of 57 examined(91.2%). The genetic analysis for the resistance to powdery mildew was carried using the cross between a pair of sib lines H962 R and H962 S that exhibited different reactions to powdery mildew. A population consisting of 343 F2:3 lines from the cross H962 R × H962 S showed a segregation ratio that fits to a 1:2:1 ratio for resistance: segregation: susceptibility, indicating that the resistance in H962 R is controlled by a single gene. This gene was temporarily designated PmH962 and mapped on wheat chromosome 7B by gene postulation analysis and 90 K iSelect gene chip technique. The reaction pattern when tested with 57 Bgt isolates in H962 R was most similar as Fuzhuang 30 carrying Pm5 e. Molecular marker XTC17, linked to Pm5 e, was used in screening 343 F2 plants derived from H962 R × H962 S. Twenty-nine recombinant idividuals were identified, resulting in genetic distance of 4.2 cM between PmH962 and XTC17. Therefore, PmH962 was located on wheat chromosome 7BL near locus Pm5 e. Comparison of reactions to different Bgt isolates indicated that H962 R showed different infection types from Fuzhuang 30(Pm5e), Xiaobaidong(mlxbd) and Honquanmang(PmH) in reactions to 3, 27 and 1 Bgt isolates, respectively. However, Pm5 e, PmH and mlxbd are all recessive genes. Based on the result of molecular detection and spectrum of resistance, PmH962 is most likely a new gene or a gene tightly linked to Pm5 e. |
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