36. 
Effectiveness of a resistance gene from Gumei 2 against leaf and neck blast
under field conditions in China
1. Wu’, J. ZHUANG1, Y. Fan1, D. L?, H. LEUNG3 and K. Zheng1
1) China National Rice Research Institute, Hangzhou 310006, China
2) Institute of Biotechnology, Zhejiang University, Hangzhou 310029, China
3) 
Entomology and Plant Pathology Division, International Rice Research Institute, P.O. Box 3127, 1271 
Makati City, Philippines

 
     Rice blast is commonly observed at both vegetative (leaves) and reproductive (necks of panicles) stages. The economic loss due to neck blast is particularly severe since a single neck infection can result in total loss of the panicle. Although many genes conferring leaf blast resistance have been reported (McCouch eta!. 1994; Iwata 1996), relatively little is known about the genetic basis of neck blast resistance. Using recombinant inbred lines (RILs) derived from the cross Thong 156/Gumei 2, we previously mapped two genes conditioning resistance to blast isolate 92-183 onto chromosomes 6 and 12, respectively (Zhuang et a!. 1998). The resistance gene on chromosome 6 was from Gumei 2, a durable blast resistance variety, and conferred resistance at both vegetative and reproductive stages. The other resistance gene on chromosome 12 was from Zhong 156 which conferred only leaf blast resistance. As part of our continuing effort to understand neck blast resistance, we evaluated the effectiveness of the resistance gene from Gumei 2 under field conditions in Fujian province in China.
     Field performance of 304 RILs, their parents and local susceptible controls (Yuanfengzao at Tonglu and Luhongzao 1 at Jiangle) were tested at two blast hot-sports in Tonglu county, Thejiang province and Jiangle county, Fujian province, China in 1998. The experiments were carried out using a randomize complete block design with two replications and 20 plants per line for each replication. The susceptible controls were planted in the intervals between each RIL, and the plots were surrounded by the control as border rows as well.
    Leaf blast was recorded twice with the first time at the maximum tillering stage and the second time one week after. Neck blast was recorded 4 weeks after heading. The disease evaluation used by Chinese National Coordinating Research Team on Rice Blast (CNCRTRB, 1980) was adopted in this study. There was not sufficient disease observed in Tonglu in 1998. Thus only data from Jiangle, Fujian was collected.
    In Fujian, Zhong 156 was susceptible and Gumei 2 was resistant at both growth stages. A total of 239 lines had sufficient plants for evaluation of blast resistance. For neck blast, 133 out of 239 lines were highly resistant, 101 lines were highly susceptible and 5 lines were moderately susceptible. Based on the genotypes of DNA markers, 98 out of the 133 highly resistant lines contained Gumei 2 alleles at both marker loci flanking the resistance gene on chromosome 6. Only 8 out of 133 resistant lines had the Zhong 156 alleles at both flanking loci. The remaining 20 lines possessed the Gumei 2 and Thong 156 alleles at either flanking loci (Fig. 1). On the other hand, 81 of the 101 highly susceptible lines had Zhong 156 alleles at both flanking marker loci, 12 had Zhong 156 and Gumei 2 alleles at either flanking marker loci, and only 7 had Gumei 2 alleles at both flanking marker loci (Fig. 1). The phenotypic and molecular data together suggest that the resistance gene from Gumei 2 is responsible for resistance to the field isolates present at the hot spot in Fujian province.
     Field evaluation of the 239 lines for leaf blast showed that 172 RILs were resistant (including 134 highly resistant and 38 moderate resistant lines). Sixty-seven of the 239 lines showed moderate susceptibility and all these lines were highly susceptible at reproductive stage. Among the 134 highly resistant line, 131 lines were also resistant to isolate 92-183 which is one of the predominant isolates in Zhejiang. However, 41 out of the 67 moderately susceptible lines were resistant to 92-183. The leaf blast resistance observed in the field was not associated with DNA markers flanking the resistance gene based on inoculation with 92-138. Thus, the data suggest that the natural pathogen population in Fujian has a broader spectrum of virulence than 92-183 from Thejiang.
    In summary, the resistance gene from Gumei 2 provides resistance to both neck and leaf blast under field condition. In contrast, the resistance gene from Zhong 156 is less effective against the field isolates at the Fujian site. Neck blast resistance is difficult to evaluate due to the need from growing the plants to maturity. The association between DNA markers and neck blast resistance in the field suggests the possibility of using marker- aided selection to accelerate the improvement of neck blast resistance.
 
References
McCouch, S.R., R.J. Nelson, J. Tohme and R.S. Zeigler, 1994. Mapping of blast resistance genes in rice. In Rice blast disease, R.S. Zeigler, S.A. Leong and P.S. Teng (eds.), IRRI and CAB International.
Iwata, N., 1996. Report of the committee on gene symbolization, nomenclature and linkage group. RGN 13:
12-18.
Zhuang, J.Y., J.L. Wu, R.Y. Chai, Y.Y. Fan, M.Z. jin, H. Leung and K.L. Zheng, 1998. Rapid identification of resistance gene analogs linked to Iwo blast resistance genes in rice. RGN 15: 125-127.
CNCRTRB, 1980. Evaluation of rice varietal resistance to blast (Pyricularia oryzae Cay.). Chinese Journal of Agriculture 4:44-52. (in Chinese)