| Rice false smut is a kind of rice disease induced by fungus Villosiclava virens which is belong to Ascomycotina, Clavicipitaceae, Claviceps. Since the 1980s, rice false smut became serious gradually in China with the change of rice cultivation pattern, increase of nitrogen fertilizer application and use of compact panicle type of hybrid rice. This fungal disease is widespread across most main rice production areas and decreases yield in ricce mature period. Moreover, they produce mycotoxins which are harmful to health of people and livestock and may threaten the security of food. To provide rice cultivars of durable resistance to false smut, we refined inoculation technique based on statistical method. The studies of virulence differentiation, resistance of rice cultivars, and the genetic diversity and structure of Villosiclava virens were also carried out in this paper. The results are summarized as follows:1. In controlled environment, the influence of rice growth period, temperature, relative humidity and wetness duration on infection by Villosiclava virens was assessed by inoculating rice cultivar’ Yueyou938’with conidial suspension (1 × 106 conidia per ml). Rice plants, grew in greenhouse, were inoculated at a late stage of panicle development, and averaged more than 90% diseased incidence which was higher than those inoculated at an early stage of panicle development (66.5%). There were significant differences in effect of temperature and wetness duration on disease panicle incidence, which increased rapidly from 48 to 120 h at both 20 and 25 ℃. The highest level of disease panicle incidence at 92.9% was obtained at 25 ℃ and 120 h of wetness duration. Disease panicle incidence was modeled as a function of both temperature and wetness duration using a modified version of the Weibull equation giving R2= 0.951. The model was used to construct a risk chart that can be used to estimate the efficiency of inoculation under a certain temperature and wetness duration condition After incubation in a plant growth chamber for a wetness period of 120 h, inoculated rice plants were subjected to different combinations of temperature and relative humidity to determine the effects of the both factors on infection by Villosiclava virens. A higher temperature (28℃) during the incubation was found to induce higher disease panicle incidence than a lower temperature (20℃). Temperature is the primary factor for disease development during the two weeks after a wetness period of 120 h.2. To obtain resistant germplasm resources,259 rice cultivars were evaluated in the field during 2010 and 2011. According to the temporary evaluation standard of rice resistant to false smut, the 259 rice cultivars were identified as highly resistant (8 cultivars), resistant (125 cultivars), moderately resistant (57 cultivars), moderately susceptible (47 cultivars), susceptible (21 cultivars) and highly susceptible (1 cultivar). To confirm the resistance result, the 8 potential highly resistant cultivars were further identified via artificial inoculation in doors with both two highly pathogenic strains (HWD-2 and 44-A) respectively. As the result indicated, the cultivar HG25 (include resistant control cultivar IR28) showed consistent highly resistance to Villosiclava virens while other cultivars did not. For example, rice cultivars HG27 and Jieyou493 were susceptible to strain 44-A, but were resistant to strain HWD-2. This phenomenon probably resulted from the virulence differentiation between Villosiclava virens stains. To obtained rice cultivars of stable resistant to rice false smut, the evaluation process should be conducted by inoculation using multiple Villosiclava virens strains. Based on the analysis of climate record and resistance evaluation of 20 rice cultivars, the changes of temperature and relative humidity in rice booting stage maybe resulted in the difference of resistance evaluatuion in Yichang and Huangshi during 2010 and 2011.3. By the use of illumina sequencing technology and bio informatics method, the genome of Villosiclava virens strain HWD-2 was sequenced and assemblied. Based on the genome sequence, we detected 403 microsatellite loci by the program QDD and randomly selected 122 loci of them for a preliminary screening using 8 Villosiclava virens strains. As the result indicated,20 microsatellite loci showed polymorphism in preliminary screening. The microsatellite loci, characterized as more compound structure and big motif size, probably showed polymorphisms. The uvssr114 microsatellite locus showed highest polymorphism and the uvssrl2 microsatellite locus showed lowest polymorphism. The 95 strains from 10 provinces in the major rice-growing areas of China showed a certain genetic diversity and can be divided into two groups, A (include strains from Jilin, Liaoning, Shaanxi, Henan, Hubei, Hunan, Jiangxi and Zhejiang province) and B (include strains from Guangxi and Fujian province). Among the 10 geographical populations, the strains in Guangxi province showed highest polymorphism and the strains in Henan province showed lowest polymorphism. The Mantel test indicated that there was significant low correlation between geographic distance and genetic distance. But most genetic variation came from that within populations while the difference among populations contributed to 3% of the total genetic variation.4. In present study, the virulence of 40 strains isolated from 5 regions in Hubei province were evaluated by artificial inoculation on 3 rice cultivars (IR28, Huanghuazhan and Yueyou938) in doors. The variation of biomass, sporulation capacity and inhibition of crude toxicity to rice seedlings among different pathogenic populations were also accessed. The results indicated that:(ⅰ) the disease indexs were significantly different among both rice cultivars and strains. By clustering analysis based on virulence to 3 rice cultivars,40 strains can be divided into 4 pathogenic groups. Group Ⅰ represented moderate pathogenic strains which induced moderate disease on both cultivars Yueyou938 and Huanghuazhan and no symtom on cultivar IR28. Group Ⅱ represented highly pathogenic strains which induced severe disease on both cultivars Yueyou938 and Huanghuazhan and even moderate disease on cultivar IR28. Group Ⅲ and IV represented weak pathogenic strains which induced light disease on either cultivar Yueyou938 or Huanghuazhan; (ⅱ) there were significant difference in biomass, sporulation capacity and crude toxicity to rice seedling among different pathogenic Villosiclava virens populations; (ⅲ) there was a significant genetic variation among the strains from 4 pathogenic populations (P< 0.01). Genetic variation among populations contributed to 34% of the total genetic variation. By phylogenic analysis on the basis of shared-allele distance, the 40 strains can be also divided into 3 genetic populations which showed a correlation with the pathogenic populations. |
PDF Full Text Request