Cucumber Green Mottle Mosaic Virus (Watermelon Strain) in Watermelon and its Bearing on Deterioration of Watermelon Fruit Known as “Konnyaku” Disease

Abstract

Many watermelon plants cultivated in the Chiba and Ibaraki prefectures showed symptoms similar to those of mosaic disease in June 1968. In these districts it became evident that apparently healthy fruit that were harvested harboured some form of abnormality. Infected plants were retarded in growth. The young leaves showed mosaic symptoms but in the majority of cases the symptoms were not distinct and often disappeared as the leaves grew older. Fruit stalks of infected plants generally showed necrotic lesions and sometimes the fruit showed mosaic-like dark green elevated areas on the surface. The white peripheral regions of infected fruit which were cut open appeared to be water-soaked and whitish yellow. The inner pulp also turned to water-soaked dirty red and contained crescent-shaped crevices while there was an increase in the yellow fibres.The above symptoms were induced on fruit as well as leaves of healthy watermelon by juice inoculation with a virus isolated from infected plants showing the abnormalities described. It became apparent that the symptoms in watermelon were most marked when inoculated at the time of fruit set.The virus was identified with cucumber green mottle mosaic virus (CGMMV) by mechanical inoculation and observation under electron microscope using the dip method. This virus was, however, distinguishable from that isolated from cucumber in the western part of Japan in 1966, on the basis of differential reaction on Datura stramonium, Chenopodium amaranticolor and from serological comparison. The virus isolated from watermelon was designated as watermelon strain and the one isolated from cucumber in 1966 as cucumber strain.Specimens of fruit exhibiting the fruit pulp symptoms were collected from various localities and virus isolations were made. From most of them CGMMV was isolated, while from certain specimens watermelon mosaic virus (WMV) and cucumber mosaic virus (CMV) also were isolated, and from some no virus was isolated. The fruit abnormality referred to here is certainly caused by CGMMV, but WMV, CMV and some physiological disorder may also cause similer fruit abnormalities. These, however, can be distinguished from the symptoms caused by CGMMV.Bottlegourd, Lagenaria siceraria var. hispida, is generally employed as a root-stock in the cultivation of watermelon. The presence of CGMMV was detected in the seed coat of bottlegourd seeds which were collected in Chiba and Ibaraki prefectures from left-over stocks of one season, while it was not detected in the seeds of watermelon. Also in the Tochigi prefecture where bottlegourd is cultivated on a large scale the crop showed a high percentage of mosaic symptoms, and from these mosaic plants CGMMV was isolated.