Virulence Types of Magnaporthe oryzae to Hybrid Rice in Sichuan, China

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1 Rice Science, 2012, 19(4): Copyright 2012, China National Rice Research Institute Published by Elsevier BV. All rights reserved 1 Virulence Types of Magnaporthe oryzae to Hybrid Rice in Sichuan, China BAI Yu-lian 1, 2, ZHANG Xue-mei 1, 3, FENG Hui 1, 3, JI Hong-li 1, HUANG Yun 2, PENG Yun-liang 1, 3 ( 1 Key Laboratory of Integrated Pest Management on Crops in Southwest China, Ministry of Agriculture, Chengdu , China; 2 Sichuan Agricultural University, Ya an , China; 3 China National Rice Research Institute, Hangzhou , China) Abstract: A total of 638 isolates of rice blast (Magnaporthe oryzae) were isolated in from different rice varieties in different regions of Sichuan, China and inoculated onto seven rice varieties (Lijiangxintuanheigu, IR24, Minghui 63, Duohui 1, Chenghui 448, Neihui and RHR-1) to differentiate the virulence types of the fungus and trace the changes. The virulence to the seven varieties was respectively scored at 1, 2, 4, 8, 16, 32 and 64. The total scores of individual M. grisea isolates which were the sum of scores infecting differential varieties could, in turn, be used for the nomenclature of the virulence types due to their accordance to the special virulence patterns. The 638 tested isolates were then differentiated into 56 different virulence types. Type 15 virulent to Lijiangxintuanheigu, IR24 and Minghui 63, and Type 127 virulent to all of the seven varieties were the most dominant virulence types respectively with the occurrence frequencies of 15.99% and 15.83%. Type 19 and other seven virulence types were not monitored during Type 15 was the predominant virulence type in 2002, 2003, 2004 and 2007, whereas Type 127 had been the most dominant virulence type after 2005 except for the year 2007 when the province underwent severe drought. Five hundred and seven out of the 638 tested isolates were virulent to Minghui 63, and 89.58% of the 384 isolates virulent to either Duohui 1, Chenghui 448 or Neihui were virulent to Minghui 63, which indicated the impact of the extensive plantation of hybrid rice Minghui 63 as the restorer line on the virulence evolution of M. oryzae in Sichuan. The virulence pattern of the dominant virulence types suggested that the acquiring of virulence to all the major resistant restorer lines was the main routes of the evolution in virulence of M. oryzae to hybrid rice in Sichuan. The virulence frequencies of the 638 tested isolates to IR24, Minghui 63, Duohui 1, Chenghui 448, Neihui and RHR-1 were respectively 74.6%, 79.5%, 73.8%, 37.0%, 39.0% and 40.4%. The analysis for the sources of the different virulence type isolates indicated the pathogen on the newly released resistant varieties were stronger than conventional rice varieties which had become susceptible in the field since 1980s. Key words: Magnaporthe oryzae; virulence type; virulence frequency; hybrid rice Rice blast disease (Magnaporthe oryzae) is a worldwide devastating disease on rice (Ou, 1985; Shen et al, 1993). In China, rice blast disease occurs everywhere during rice cultivation, including Hainan, Xinjiang, Tibet, and Taiwan. The average disease acreage has been over hm 2. The disease causes a reduction in the yield averaged at 10% 20% in the whole epidemic regions while the severe loss in some fields could be as high as 80% 100%. The strategies to control the disease include the elimination of the inoculum sources such as disease straws and infested seeds, deployment of the resistant rice varieties as well as the application of fungicides (Sun et al, 1998). The most widely applied fungicides to control rice blast disease in China are tricyclazole and isoprothiolane. The former is systemic but only prevent the infection of the pathogen, while the latter is non-systemic and effective only at the sprayed sites. The frequent heavy rains at the heading stage of rice plants result in the failure of the chemical control and hence the severe Received: 2 January 2012; Accepted: 14 May 2012 Corresponding author: PENG Yun-liang (pengyunliang@yahoo.com.cn) loss in the yield of rice. The breeding and deployment of the resistant varieties have been the most efficient and safe methods for the disease control, but the breakdown of resistance in the varieties 3 8 years after their extensive plantation has been the bottleneck (Peng et al, 1998; Ebbole, 2007). Sichuan Province is on the east of Tibetan plateau and endures plenty of rainfall during the rice growing season, therefore, the blast disease has been the most important disease. Since late 1970s, hybrid rice had been extensively planted in Sichuan and the current planting acreage has reached hm 2. Three major epidemics of rice blast disease had been reported respectively due to the resistance breakdown in Shanyou 2 (Zhenshan 97A IR24) in , Minghui 63 in , Duohui 1 and Shuhui 527 which shared Tetep as the ancestor in (Peng et al, 1992, 1998; Ji et al, 2008). It is widely accepted that the breakdown of resistance is due to the accumulation of the virulent isolates (McDonald and Linde, 2002). Therefore, monitoring the changes in virulence of the pathogen population, frequently in the form of physiological races, is essential to the success of resistance breeding

2 BAI Yu-lian, et al. Virulence Types of Magnaporthe oryzae to Hybrid Rice in China 321 and deployment. In China, Tetep and other six varieties had been proposed in 1976 as the national differentiating host varieties for physiologic races of M. oryzae (All China Cooperation of Research on Physiological Races of Pyricularia Oryzae, 1980). The ambiguous genetic background, low differentiating ability and limited suitable areas had hindered their application, especially since late 1980s when they failed to detect the virulence changes causing the breakdown of resistance in Minghui 63 and other varieties (Peng et al, 1995; Ling et al, 2004; Ji et al, 2005). Ji et al (2005) detected new virulent isolates infecting several varieties of different resistance spectra. The authors then proposed seven varieties including Lijiangxintuanheigu as the complementary host varieties to differentiate the races of M. oryzae in Sichuan (Ji et al, 2008). In this study, these complementary host varieties were inoculated by 638 isolates collected during from different rice materials and regions in the province to further investigate their differentiating ability and the changes in the virulence of M. oryzae. Rice varieties MATERIALS AND METHODS Seeds of Lijiangxintuanheigu, IR24, Minghui 63, Duohui 1, Chenghui 448, Neihui and RHR-1 collected from breeders and seed companies were preserved in the laboratories at the Institute of Plant Protection, Sichuan Academy of Agricultural Sciences, China. M. oryzae isolates Specimens of ear blast were immersed in tap water for 2 3 h and rinsed for three times in sterilized distilled water before incubation at 27 ºC overnight in darkness for sporulation. Single spores were picked up by a fine needle under a microscope according to the methods of Chai and Jin (1991) and inoculated onto potato dextrose agar (PDA) plates scattered with pieces of sterilized filter paper. Seven days later, filter paper with mycelia and spores was transferred into paper bags and desiccated in a container with silica gel for the permanent preservation at -20 ºC. Totally 638 isolates derived from single spores of different host and geographic sources during were selected for the research. Seedling preparation Seeds of each variety were treated with 1.5% H 2 O 2 for 24 h and rinsed with water for three times before the incubation at 30 ºC for another 24 h. Twenty-four germinating seeds of each variety were sown in two rows in a 20 cm 30 cm 10 cm enamel tray filled with sterilized paddy soil. Kapastan (0.2%) was sprayed when coleoptiles emerged at cm. Trays of seedlings were covered with pored plastic membrane to keep the humidity. Ground water was pumped and sprayed onto the membrane from 8:00 am to 16:00 pm to keep the temperature under 28 ºC. Urine (200 ml, 0.2%) was poured into a tray at the 1.5-, 2.5- and 3.5-leaf stages. Imidacloprid (0.1%) was sprayed at the 2.5-leaf stage to prevent the pest insect. Seedlings with 30% 80% extended 4th leaf were ready for spray or in vitro inoculation. Inoculum preparation A piece of filter paper for each preserved isolate was transferred onto the plate of yeast starch agar (starch 10 g/l, yeast powder 2 g/l, agar 11 g/l) and incubated for 5 d at 26 ºC. Mycelia were then inoculated onto the plates of oat tomato agar (oat 40 g/l, tomato juice 150 ml/l and agar 20 g/l) for another 6 d. Sporulation was conducted under florescent light for 2 3 d after removing the aerial mycelia with a sterilized glass slide. Conidia were collected in 0.02% Tween-20 and centrifuged for 5 min at r/min before the adjustment of the suspension concentration up to spores/ml. Inoculation For spray inoculation, two trays of seedlings were sprayed with 80 ml spore suspension of a M. oryzae isolate and kept in darkness for 24 h at 26 ºC before incubation for another 6 d in a greenhouse at 26 ºC without direct sunshine. After disease investigation, the rice varieties showed no susceptible symptoms were in vitro inoculated by the same isolates. For in vitro inoculation, leaf segments at the length of 5 6 cm were cut cm from the tip of partly extended 4th leaf. Five segments were then spread on the wet filter paper in a Petri dish and covered with strips of wet filter paper at both ends. Segments in two Petri dishes were first sprayed by 0.02% Tween-20 and then 50 µl of the spore suspension of an isolate were inoculated in three drops onto each segment by a 200 µl pipette. The Petri dishes were put in an enamel tray with shallow water, kept in a wet plastic bag and incubated the same way as the sprayed seedlings. The in vitro inoculation was repeated twice if the varieties still showed no susceptible symptoms. Disease investigation and data analysis The disease investigation was conducted according

3 322 Rice Science, Vol. 19, No. 4, 2012 to the grading system of leaf blast disease of the International Rice Research Institute (IRRI), the Philippines. Symptoms of 0 2 grades were resistant (R) and those of 3 9 grades were susceptible (S). The isolates failed to cause susceptible symptoms after three inoculations were considered to be avirulent to a variety. Virulent frequency (%) = 100 (number of virulent isolates/number of inoculated isolates). Occurrence frequency (%) = 100 (number of isolates of a virulent type/number of inoculated isolates). RESULTS Virulence types and their nomenclature The virulence to Lijiangxintuanheigu, IR24, Minghui 63, Duohui 1, Chenghui 448, Neihui and RHR-1 was respectively scored at 1, 2, 4, 8, 16, 32 and 64. The sum of the virulence scores of each isolate was in accordance with its distinct spectra and could be used for the nomenclature of its virulence type. For example, the isolates in virulence Type 127 were virulent to all of the seven differential host varieties with the virulence score = = 127 (Fig. 1). Virulence composition of M. oryzae in Sichuan Virulence types of M. oryzae in Sichuan from 2002 to 2009 After the inoculation of 638 isolates of M. oryzae collected from Sichuan Province during , Lijiangxintuanheigu was susceptible to all of the isolates and only 4.36% of the isolates were virulent to Lijiangxintuanheigu. Totally 56 virulence types were detected out of the 64 virulence types which could be differentiated by differential host varieties other than Lijiangxintuanheigu. Types 19, 27, 41, 89, 91, 97, 105 and 113, which were virulent to either Chenghui 448, Neihui or RHR-1, but not to IR24 and Minghui 63, failed to be detected. The occurrence frequencies of 24 virulence types were higher than 0.50%. The predominant virulence type was Type 15 with the occurrence frequency of 15.99%. The other dominant virulence types were Types 127, 1 and 47, respectively, with the frequencies of 15.83%, 8.62% and 5.64% (Fig. 2). Among the 638 tested isolates, 507 were virulent to Minghui 63. Out of the 384 virulent isolates to either Duohui 1, Chenghui 448 or Neihui which were sequentially released as the restorer lines of commercial hybrid rice after Minghui 63 became susceptible, 89.58% were virulent to Minghui 63. In addition, 69.53% of the virulent isolates to any Fig. 1. Infection of seven differentiation host rice varieties by isolates in virulence Type 127 of Magnaporthe oryzae. 1, Lijiangxintuanheigu; 2, IR24; 3, Minghui 63; 4, Duohui 1; 5, Chenghui 448; 6, Neihui 99-14; 7, RHR-1. restorer lines posterior to Duohui 1 were virulent to IR24, Minghui 63 and Duohui 1. Annual fluctuation in composition of virulence types The numbers of tested isolates and the detected virulence types during are listed in Table 1. The increased number of the tested isolates consequently resulted in the increase in the number of the detected virulence types. The frequency of Type 1 decreased during (Fig. 3) except 2007 when there was a severe drought summer and the majority of the samples were collected from conventional varieties (data not shown here). The virulence types only infecting Lijiangxintuanheigu, IR24 or Minghui 63 (Type 3, 5 or 7) kept at low frequencies during the period. Type 15 infecting Lijiangxintuanheigu, IR24, Minghui 63 and Duohui 1 was the predominant virulence type before 2004 and in Type 127 infecting all of the varieties had been the predominant virulence type since 2005 except The frequencies of the virulence types only infecting Chenghui 448 or Neihui had been always lower than 5.0%. Table 1. Numbers of tested isolates and detected virulence types of Magnaporthe oryzae in Sichuan, China during Year No. of isolates No. of virulence types

4 BAI Yu-lian, et al. Virulence Types of Magnaporthe oryzae to Hybrid Rice in China 323 Fig. 2. Frequencies of detected virulence types of Magnaporthe oryzae in Sichuan Province, China. Fig. 3. Annual changes in the frequencies of the major virulence types of Magnaporthe oryzae in Sichuan Province, China. Virulent frequencies of M. oryzae from 2002 to 2009 The virulent frequencies of the 638 isolates to IR24, Minghui 63, Duohui 1, Chenghui 448, Neihui 99-14, and RHR-1 were respectively 74.45%, 73.82%, 36.99%, 39.03% and 40.44%. The annual changes in the virulent frequencies of M. oryzae to the differential host varieties are shown in Fig. 4. The virulent frequencies to IR24, Minghui 63 and Duohui 1 had been higher than 66.07%, 74.80% and 64.17%, respectively. Significant increase in virulent frequencies to Chenghui 448 and Neihui as well as that to the once resistance donor RHR-1 had been experienced in M. oryzae in Sichuan during 2002 to Virulence to hybrid rice of M. oryzae from different origins The virulence types of M. oryzae from different host and geographic origins in 2008 were analyzed (Table 2). Out of the 123 isolates collected from 24 counties, Types 63 and 127 were respectively detected in 7 and 12 counties. Type 1 was detected in five counties. Out of the 23 isolates in Type 127, nine were from Pujiang and neighboring Ya an. Twenty isolates in Type 127 were from hybrid rice and only three from glutinous landraces. All of the three isolates from once resistant DYou 177 were in Type 127 whereas only four out of the 28 isolates from the highly susceptible Xinongyou 30 belonged to Type 127. Five out of the six isolates in virulence Type 63 were of hybrid rice origins. Among the eight isolates in Type 1, seven were collected from conventional varieties. The weaker virulence of M. oryzae from conventional rice was also observed in 2009 (Table 3). Type 63 and Type 127 were respectively detected in 1 and 15 counties. Twenty-seven out of the 29 isolates in these two types were from hybrid rice whereas four out of the five isolates in Type 1 were from conventional rice.

5 324 Rice Science, Vol. 19, No. 4, 2012 Fig. 4. Virulent frequencies of restorer lines in Manaporthe oryzae in Sichuan Province, China. DISCUSSION Virulence composition of M. oryzae in Sichuan Lijiangxintuanheigu has proven to be a widespectrum susceptible variety, therefore only six complementary differential host varieties were able to differentiate the virulence of M. oryzae. Forty-one virulence types of M. oryzae from Sichuan were reported by Ji et al (2008) based on the inoculation results of 243 isolates onto the same varieties, and 56 virulence types had been detected when the tested isolates increased to 638. The occurrence frequencies of the four major virulence types reported by Ji et al (2008) decreased to lower than 0.5% and one minor type disappeared in this study. The increased number of tested isolates and repeated inoculation contributed to the better dissection of the virulence of M. oryzae. In accordance with the preceding results, the majority of the isolates were virulent to Minghui 63, which indicated the impacts of the extensive plantation of Minghui 63 as a restorer line of hybrid rice on the virulence of M. oryzae in Sichuan. The virulence to IR24, Minghui 63 and Duohui 1 of the majority of isolates infecting any restorer lines posterior to Duohui 1, together with the fact that Type 127 had been the predominant virulence type, indicated that accumulation of the virulence to the sequentially released resistant restorer lines had been the main Table 2. Virulence types of 123 isolates of Magnaporthe oryzae from different origins in Sichuan Province, China in Location in Sichuan No. of isolates Virulence type Variety Dazhu 5 15, 63, 77, 111, 125 QYou 6, QYou 108, Gangyou 188, Gangyou 364 Dujiangyan 5 47, 49, 63, 127 Jinyou 188, Jinyou 718, Fuyou 718 Gongxian 6 65, 75, 79, 95, 127 Xinongyou 30 Hongya 2 7, 79 Unknown hybrid rice Huidong 3 1, 63, 65 Hexi 22 a Huili 4 95, 119, 121 Yijin 1 a, Hexi 22-2 a Jiajiang 6 75, 77, 79, 95 Xinongyou 30 Jiang an 6 73, 77, 79, 93, 107 Yixiang 1577, Xinongyou 30 Jiangyou 11 1, 7, 9, 15, 31, 47, 69, 75, 127 Unknown hybrid rice Peng an 4 11, 95, 127 Fuyou 802 and unknown hybrid rice Pujiang 7 63, 75, 127 DYou 177, IIYou 448 and unknown hybrid rice Puge 2 13, 23 Zhaojuemagu a Shehong 5 77, 79, 95, 119, 125 Unknown hybrid rice Xichang 5 1, 37, 57, 63 Hexi 22-2, Hexi 39 a Xuanhan 5 31, 99, 111, 127 Q You 2, Yixiang 2292, Fuyou 1, Fuyou 802 Ya an 11 1, 71, 77, 101, 111, 115, 127 Glutinous landraces and unknown hybrid rice Yilong 2 15, 63 Gangyou 725 Yibin 6 75, 77, 79, 127 Fuyou 802, Xinongyou 30 Yinshan 16 1, 11, 13, 15, 23, 25, 33, 63, 81, Glutinous landraces and unknown hybrid rice 95, 111, 125, 127 Zigong 4 11, 93, 127 Xinongyou 30 Quxian 3 35, 95 Xinongyou 30 Yanting Unknown hybrid rice Zizhong Unknown hybrid rice Junlian 2 67, 95 Xinongyou 30 a Conventional rice varieties.

6 BAI Yu-lian, et al. Virulence Types of Magnaporthe oryzae to Hybrid Rice in China 325 Table 3. Virulence types of 120 isolates of Magnaporthe oryzae from different origins in Sichuan Province, China in Location in Sichuan No. of isolates Virulence type Variety Anxian 3 7, 15 Unknown hybrid rice Bazhong 7 15, 31, 127 Unknown hybrid rice Changning 5 7, 39, 47, 79, 9 Fuyou 4, Fuyou 22, Fuyou 23 Daxian 4 1, 7, 95, 127 Glutinous landrace and Gangyou 1 Dazhu 10 5, 7, 11, 13, 15, 79, 95, 127 Gangyou 364, Gangyou 365 and unknown hybrid rice Jiange 5 39, 47, 79, 119 Gangyou 188, Gangyou 881, Gangyou 882 Mingshan 8 3, 9, 11, 35, 43, 73, 79 Gangyou 725, Gangyou 1313 and unknown hybrid rice Peng-an 9 5, 7, 11, 13, 77, 119, 125, 127 Unknown hybrid rice Pingchang 4 11, 15, 47, 85 Unknown hybrid rice Pingshan 7 3, 9, 15, 55, 123, 127 Unknown hybrid rice Pujiang 13 5, 7, 15, 39, 67, 73, 111, 123, 127 DYou 527, Gangyou 725, Jinyou 725, Jinyou 726 Puge 11 1, 5, 11, 13, 29, 53, 63, 71, 127 Zhaojuemagu a, Qiaonong 4 a, Qiaonong 5 a, Zhaojuebaigu a, Liangfengxiangnuo a Qingchuan 3 1, 127 Unknown hybrid rice Qionglai 3 67, 127 Unknown hybrid rice Wusheng 5 79, 119, 127 Unknown hybrid rice Ya an 4 7, 9, 95, 127 Gangyou D069, Gangyou 12, Gangyou 13, Yixiang 725 Yanting 6 7, 87, 127 Gangyou 725 and unknown hybrid rice Yanyuan Red Hongza a Lezhi 1 95 Unknown hybrid rice Yinshan 10 3, 13, 15, 53, 69, 87, 127 Unknown hybrid rice Yuexi Unknown hybrid rice a Conventional rice varieties. route for the virulence of M. oryzae to evolve in Sichuan. Roles of conventional rice in the evolution of virulence of M. oryzae to hybrid rice Conventional rice, mainly of japonica varieties, has been only extensively planted in the areas of high mountains in Liangshan of Sichuan. Inside Sichuan Basin, the majority of the conventional rice has been glutinous landraces mainly for festival food, Guichao 2 and other indica varieties for the processing of fast food. All these varieties had been susceptible in Sichuan Basin before the extension of hybrid rice. It has been reported that all of the isolates from hybrid rice could infect conventional rice, but only part of the isolates from conventional rice were able to infect hybrid rice. Because no isolates from grasses were found to infect rice and there were differences in the esterase isoenzymes and tolerance to isoprothiolane between Magnaporthe isolates from rice and the most popular grass Digitaria sanguinaris, susceptible conventional rice had been considered as the major origin of isolates infecting hybrid rice (Peng et al, 1995, 1998). Majority of currently planted hybrid rice had become susceptible in Sichuan Basin and the majority of the tested isolates of M. oryzae from conventional varieties were weak in virulence to hybrid rice. But the detection of Type 127 on conventional varieties did suggest they still played a role as the pool source of the virulence to hybrid rice, so is the case with drought in Inside Sichuan Basin, sporadic plots of conventional rice were surrounded by overwhelming susceptible hybrid rice, but Type 1 had been detected in large numbers on conventional rice. Whether the fitness of isolates in Type 1 on conventional rice was better than other types is still subjected to study, which could be important for the understanding of the cost for M. oryzae to evolve in virulence. Virulent frequency, virulence type and physiological race The virulence of M. oryzae is still in the process of enhancement. Hybrid rice Chenghui 448 and Neihui as restorer lines became susceptible respectively in 2008 and 2009 in Pujiang. However, the virulent frequencies to Chenghui 448 and Neihui decreased after Moreover, the virulent frequency to a variety could not reflect the whole virulence of any isolate as well as the population of M. oryzae. In this study, the change in the predominant virulence type from Type 63 to Type 127 since 2005 could reflect better the changes in the virulence of M. oryzae. The susceptibility of all the tested differential host varieties at blast nurseries and the emergence of new resistance varieties had called for adopting new differential host varieties and monitoring the new virulence types of M. oryzae in Sichuan. The classification of physiological races of the plant pathogens required certain standards and the differential host varieties were fixed at least for certain periods after verification (Li and Zeng, 2002). China differential host varieties for physiological

7 326 Rice Science, Vol. 19, No. 4, 2012 races of M. oryzae had been proposed in 1976, and no changes could be expected in the near future. Five restorer lines in this study and their derivatives had been widely planted in Sichuan and other parts of China. They have shown obvious differentiation of the virulence of M. oryzae from Sichuan. Although high temperature and low humidity during the seedling cultivation could affect the symptoms in the experiments, such affection could be relieved by the control of experimental conditions. The adoption of these restorer lines as the differential host varieties could complement the weakness in the differentiating ability of the national differential host varieties. No variety derived from RHR-1 had been registered and planted in Sichuan. In addition, RHR-1 had become susceptible in blast nurseries, and the prospect for it to be the differential host in Sichuan had further diminished. ACKNOWLEDGEMENTS The research was supported by the Special Fund for Agro-scientific Research in the Public Interest (Grant No ) and Sichuan Program for Major Crop, Poultry and Livestock Breeding, China (Grant No. 2012YZGG-25-3). REFERENCES All China Corporation of Research on Physiological Races of Pyricularia oryzae. Research on physiological races of rice blast fungus in China. Acta Phytopathol Sin, 1980, 10(2): (in Chinese with English abstract) Chai R Y, Jin M Z Introduction of a method to pick up single spores of Pyricularia oryzae for isolation. Plant Prot, (5): 53. (in Chinese) Ebbole D Magnaporthe as a model for understanding host-pathogen interactions. Annu Rev Phytopathol, 45: Ji H L, Liao H M, Gong A Q, Lang D M, Liu W D, Zhang Y, Tu J H, Peng Y L New changes in populational virulence of rice blast fungus (Magnaporthe grisea) in Sichuan Province of China. Southwest Chin J Agric Sci, 18(3): (in Chinese with English abstract) Ji H L, Shen L, Xiang Y J, Zhang Z Y, Liao H M, Luo L M, Qin Z, Peng Y L Studies on the complementary differential varieties and local physiologic races of Magnaporthe grisea in Sichuan. In: Wang G L, Valent B. Advances in Genetics, Genomics and Control of Rice Blast Disease. Netherlands: Springer: Li Z Q, Zeng S M Wheat Rust Diseases in China. Beijing: China Agriculture Press: (in Chinese) Ling Z Z, Lei C L, Wang J L Review and prospect for study of physiologic races on rice blast fungus (Pyricularia grisea). Sci Agric Sin, 37(12): (in Chinese with English abstract) McDonald B A, Linde C Pathogen population genetics, evolutionary potential, and durable resistance. Annu Rev Phytopathol, 40: Ou S H Rice Disease. 2nd edn. Kew, Surrey, UK: Commonwealth Mycological Institute: Peng G L, Feng D G, Luo Q M, Liu Z J The relationship between the resistance of rice varieties to rice blast and the variation in the biological strains of the pest. J Southwest Agric Univ, 14(1): (in Chinese with English abstract) Peng Y L, Chen G H, Yang X R, Shen Y, He M, Zen D C Studies on the change and composition of Pyricularia oryzae pathogenic strains on hybrid rice. J Plant Prot, 22(3): (in Chinese with English abstract) Peng Y L, Chen G H, He M A possible mechanism for the breakdown of resistance in hybrid rice to blast disease. Med Fac Landbouw Univ Gent, 63: Shen Y, Zhu P L, Yuan X P, Levy M, Decker M, Talbot N, Hamer J E Genetic diversity of rice blast fungus in China. Acta Phytopathol Sin, 23(4): (in Chinese with English abstract) Sun G C, Du X F, Tao R X, Sun S Y Control tactics and prospect of rice blast research in 21st century. Acta Phytopathol Sin, 28(4): (in Chinese)