Rice black-streaked dwarf transmitted by the small brown planthopper, Laodelphax striatellus, has widely distributed in the Kamikawa district of Hokkaido. However, the percentages of diseased plants were generally low. The occurrence of the maize streaked dwarf was also recognized in Higashikagura town. From seasonal flactuation of rice black-streaked dwarf in paddy fields, it was considered that the overwintering generation adults and the 1st generation larvae played the important role to the

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... irus infection. The rice plant was susceptible to the virus until the 11th leaf stage. In the infected plant , the plant height was lower and the number of tillers increased as compared with the uninfected plant.