PEAR NECROTIC SPOT, A NEW VIRUS DISEASE IN JAPAN
Leaves of affected trees turn yellow and defoliate during summer, then trees seem to weaken for many years. The most sensitive cultivar is Nijisseiki, one of the best commercial cultivars in Japan.
Seven of 34 cultivars and five of 12 promising hybrid seedlings of Japanese pear showed symptoms by graft-inoculation with diseased scion. None of nine P y r u s root-stock species showed symptoms. One of five cultivars of Chinese pear (P. Bretschneideri REHDER and Pussuriensis MAXIM var. sinensis KIKUCHI) and three of 74 cultivars of pear (P. communis LINN.) also showed symptoms. All remaining cultivars without symptoms were proved to be not immune but tolerant. This means that the causal virus is harbored latently in many commercial cultivars of both Japanese pear and pear.
We tried to find a sensitive woody indicator plant. Sixteen of 173 Japanese pear hybrid seedlings tested showed necrotic leaf spots by graft-inoculation. Among them, HN-39 seedling was selected as the most suitable indicator. HN-39 is very sensitive to necrotic spots formation but highly resistant to fungal diseases (Alternaria black spot and Venturia nashicolascab) which are similar in symptoms to those caused by necrotic spot virus.