@article{oai:repository.naro.go.jp:00001594,
 author = {平江, 雅宏 and HIRAE, Masahiro},
 journal = {中央農業総合研究センター研究報告, Bulletin of the National Agricultural Research Center},
 month = {Nov},
 note = {The green rice leafhopper (GRH) Nephotettix cincticeps (Uhler) is a serious pest in temperate Asia. Populations of this insect, which multiply rapidly around the heading stage of rice, can considerably increase and cause considerable losses of rice yield in northern Japan, although the peak density of population is relatively low in southwestern Japan. The GRH also damages rice indirectly by transmitting virus and phytoplasma causing plant diseases. Utilization of crop resistance is effective in integrated pest management. The advantages of this method are that it has a relatively small effect on the environment and involves minimum labor and low expenditure. In the present study, I developed a simple and rapid method for evaluating the resistance of rice to GRH on the basis of nymphal growth. Further, (1) the development of virulence in GRH was examined by subjecting the Joetsu (northern Japan) population to artificial selection on several resistant rice varieties, and (2) the virulence of selected GRH lines was evaluated to clarify the differences in virulence among resistant varieties. Moreover, I investigated the density of GRH population in relation to GRH-resistant near-isogenic lines (NILs) of rice plants in paddy fields and the resistance of NILs to the GRH populations. The results are summarized as follows. 1. A method for examining the resistance of rice to GRH on the basis of nymphal growth A simple and rapid method based on nymphal growth was developed in order to evaluate the resistance level of rice to GRH. Nymphs grew to the second instar on seedlings of susceptible varieties of rice within 3 days and 8 hours to 4 days. Therefore, the proportion of nymphs that developed into second instars within 4 days can be considered as a reliable index. The B_1F_1 population produced from a cross between Norin-PL6, a resistant rice line, and a susceptible variety of Toyonishiki was segregated into resistant and susceptible populations; the segregation was performed on the basis of the proportion of nymphs developing into second instars, as determined in a test using rice plant seedlings. The results of a leaf blade test conducted during the heading stage of rice plants correlated well with those of the seedling test. This indicates that determining the proportion of nymphs developing into second instars is effective for the accurate individual evaluation of rice plants with varying degrees of resistance. The evaluation method developed in this study could be used in various tests for rice plants, such as selection of crossed progeny and analysis of genes at the seedling stage. 2. GRH biotypes virulent to resistant rice varieties Biotypes of pest insects virulent to resistant crop varieties pose a serious problem for resistant crops; therefore, it is important to (1) investigate the potential of pests to overcome resistance and (2) predict the emergence of such biotypes. A population of GRH collected from Joetsu was artificially selected on 5 resistant rice varieties in the laboratory. The GRH lines selected on Saikai 164, Saikai 182, and Kanto-PL 6 were able to survive and reproduce on their respective varieties. In these lines, the developmental period of nymphs was shortened by continuous selection, although in the first generation the developmental period was longer than that of the GRH line reared on Nipponbare, which did not carry a resistance gene. This result shows that the GRH population from Joetsu has genetic variations to the resistance, which leads to development of the virulent. It also suggests that certain biotypes virulent to resistant varieties in the Joetsu district can overcome GRH resistance. It is important to note that I have not been able to establish GRH lines virulent to Norin-PL 5 or Norin-PL 6, which carry 2 complementary resistance genes Grh2 and Grh4. This suggests that pyramiding resistance genes would be effective for providing durable resistance. Six lines that are reared on IR 24, Chugoku 105, Saikai 164, Saikai 182, Kanto-PL 6, and Aichi 80 were assessed for virulence among different resistant varieties by conducting a seedling test. All the 6 GRH lines were highly virulent to the varieties on which they were selected. Virulence was similar for the IR 24 and Chugoku 105 lines, Saikai 164 and Saikai 182 lines, and Kanto-PL 6 and Aichi 80 lines. The Kanto-PL 6 and Aichi 80 lines were moderately virulent to Tadukan and Rantaj-emas 2. No GRH lines were virulent to Norin-PL 5, Norin-PL 6, and Pebi-hun. The results of the leaf blade test were similar to those of the seedling test. An allele test confirmed that the Kanto-PL 6 and Aichi 80 have the same GRH-resistance genes, and that the locus of the resistance gene in Norin-PL 2 differs from that of the resistance gene in Kanto-PL 6 and Aichi 80. These results suggest that the virulence of GRH biotypes is correlated to the resistance genes in the rice varieties; hence, the use of different biotypes allows the identification of groups of rice varieties that have similar resistance genes. I propose that the biotypes virulent to the Grh1-, Grh2-, and Grh3(t)-carrying varieties be designated "biotype 1," "biotype 2," and "biotype 3," respectively. The method for identifying resistance genes in resistant varieties was established on the basis of the relationship between resistance genes and GRH biotypes and could be used in the screening of new resistant varieties. The development and reproduction in 3 GRH biotypes were examined on resistant rice varieties of Chugoku 105 (carrying the resistance gene Grh1), Saikai 182 (Grh2), and Aichi 80 (Grh3(t)). Biotypes 1, 2, and 3 exhibited a high survival rate, short developmental period, long adult longevity and high fecundity when grown on the respective varieties to which they are virulent. The total sugar content of honeydew excreted by these biotypes was high; this observation suggests that each biotype has the ability to suck phloem sap from the resistant variety to which it is virulent. Nymph survival and development, adult longevity, fecundity, and total sugar content of excreted honeydew in the 3 biotypes were similar to those observed in the unselected line reared on the Nipponbare variety carrying no resistance gene. These results indicate that there is no difference in fitness with respect to development and reproduction between the 3 biotypes and the unselected line of GRH grown on susceptible rice varieties. The result also suggested that the use of sequential release of single resistance genes in rice would not be a practical strategy for providing durable resistance. 3. Resistance of NILs to GRH in paddy fields In order to evaluate resistance to GRH under field conditions, the density of GRH was investigated in relation to GRH-resistant NILs of rice plants in paddy fields in Joetsu, Niigata Prefecture. The GRH population on the rice cultivar "Kinuhikari," carrying no resistance gene, increased from late August and peaked during early to mid-September; however, the GRH population was suppressed in NILs carrying each of the GRH-resistance genes Grh1, Grh2, Grh3(t), and Grh2 and Grh4. The resistance of NILs to GRHs collected from Joetsu differed among NILs, as determined by the leaf blade test. Resistance decreased rapidly during the maturation stage of rice in lines carrying the Grh2 or Grh3(t) gene, whereas it remained high in lines carrying the Grh1 or Grh2 and Grh4 genes. Temporary resistance is effective for suppressing the GRH population in Joetsu paddy fields because in the maturation stage, the GRH density remained low on NILs with decreased resistance to GRH. The resistance of NILs to the GRH population collected from Joetsu, Mito (Ibaraki Prefecture), and Chikugo (Fukuoka Prefecture) was evaluated in terms of the proportion of nymphs developing into second instars that was determined by conducting a seedling test. The proportion of nymphs of the Chikugo population developing into second instars was higher than that of the Joetsu and the Mito populations on NILs carrying Grh1 or Grh2. In Chikugo, the number of GRH adults and nymphs on these NILs was slightly high in the field. This result indicates that the difference in the proportion of nymphs developing into second instars on NILs among GRH populations is related to the difference in the proportion of individuals that are virulent to the NILs among the populations. In Chikugo, a slightly high number of GRH adults and nymphs were found on NILs carrying the Grh1, Grh2, or Grh3(t) genes in the field. This study confirmed the resistance of GRH-resistant NILs to s under field conditions. It also revealed that GRH populations differ in genetic structure with respect to virulence to resistant rice varieties. The widespread use of a single resistance gene may lead to the development of resistance-breaking biotypes. Further, there is no difference in fitness between different GRH biotypes on susceptible rice varieties. Pyramiding resistance genes, for example, combining Grh2 and Grh4, is proposed to delay biotype development. In conclusion, to predict and prevent the development of resistance-breaking biotypes, it is important to use GRH-resistant rice varieties combined with monitoring the GRH population for variation in virulence to resistance genes before and after the use of resistant varieties. In addition, breeding GRH-resistant varieties with resistance genes from new sources and with more than one resistance gene is likely to be an effective strategy for achieving durable resistance against GRH.},
 pages = {51--93},
 title = {水稲の抵抗性を利用したツマグロヨコバイ管理技術に関する研究},
 volume = {15},
 year = {2010},
 yomi = {ヒラエ, マサヒロ}
}