Abstract
A new filamentous virus (designated VC) was isolated from sweet potato plants that seemed to be tolerant to russet crack. It infected Ipomoea spp., Chenopodium quinoa and C. amaranticolor. Ipomoea nil infected with the virus developed typical vein-clearing on the first true leaf. The infectivity of VC in sap was lost at dilutions between 1:10, 000 and 1:100, 000 and after storage for one day at 20°C. Its thermal inactivation point was between 50 and 60°, and virus particles were 850-880nm long and 13nm wide. VC was nonpersistently transmitted by the aphid Myzus persicae, and was serologically related but not identical to sweet potato feathery mottle virus (SPFMV, abbreviated as Mo) that belongs to Potyvirus. Coat proteins of both purified VC and Mo migrated as three major bands of proteins with Mr 32, 000, 30, 000 and 29, 000. However, electro-blot immunoassay revealed one major protein of Mr 38, 000 and an additional, more slowly migrating protein of Mr 97, 000 in leaves infected with VC, and one major protein of Mr 38, 000 in leaves infected with Mo. Sweet potato inoculated with VC developed symptoms of russet crack on the fleshy roots. These results indicate that VC is the causative agent of russet crack that occurs in some Japanese cultivars of sweet potato and that VC is a new strain of SPFMV. Therefore, we named VC and Mo severe strain (SPFMV-S) and ordinary strain (SPFMV-O), respectively, of SPFMV.
