First, third, fourth, tenth, eleventh, and twelfth authors: Department of Apple Research, National Institute of Fruit Tree Science, 92 Nabeyashiki, Shimokuriyagawa, Morioka 020-0123, Japan; second, sixth, seventh, eighth, and thirteenth authors: National Institute of Agro-Environmental Science, Tsukuba 305-8604, Japan; fifth author: National Institute of Fruit Tree Science, Tsukuba 305-8605, Japan; and ninth author: Hiroshima Agricultural Research Center, Akitsu 729-2402, Japan
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Accepted for publication 16 January 2004.
White root rot, caused by Rosellinia necatrix, is a serious soilborne disease of fruit trees and other woody plants. R. necatrix isolate W370 contains 12 segments of double-stranded RNA (dsRNA) that is believed to represent a possible member of the family Reoviridae. W370 was weakly virulent and its hyphal-tip strains became dsRNA free and strongly virulent. The 12 segments of W370dsRNA were transmitted to hygromycin B-resistant strain RT37-1, derived from a dsRNA-free strain of W370 in all or none fashion through hyphal contact with W370. The W370dsRNA-transmitted strains were less virulent than their parent strain RT37-1 on apple seedlings, with mortality ranging between 0 to 16.7% in apple seedlings that were inoculated with the W370dsRNA-containing strains and 50 to 100% for seedlings inoculated with the dsRNA-free strains. Some W370dsRNA-containing strains killed greater than 16.7% of seedlings, but these were found to have lost the dsRNA in planta. These results indicate that W370dsRNA is a hypovirulence factor in R. necatrix. In addition, a strain lost one segment (S8) of W370dsRNA during subculture, and the S8-deficient mutant strain also exhibits hypovirulence in R. necatrix.
© 2004 The American Phytopathological Society