IDENTIFICATION OF DISTINCT POTYVIRUSES IN MIXEDLY-INFECTED SWEET-POTATO BY THE POLYMERASE CHAIN-REACTION WITH DEGENERATE PRIMERS

A combined assay of reverse transcription and the polymerase chain reaction utilizing degenerate primers derived from conserved regions in the genome of potyviruses was designed to amplify the variable 5'-terminal region of the coat protein cistron. Amplification on total RNA extracted from two sweetpotato clones originating from China (Guangdong Province) yielded a 1.35-kb fragment amplified from a sweetpotato feathery mottle virus (SPFMV) isolate from China (SPFMV-CH) associated with one or two other fragments (1.30 and 1.45 kb), suggesting the presence of two other potyviruses in these sweetpotato clones. The 1.30- and 1.45-kb amplified fragments were cloned into the Bluescript plasmid and partially sequenced. Comparison of the deduced partial amino acid sequences derived from the amplified fragments with those of the C-terminal region of the nuclear inclusion b protein and the N-terminal region of the coat protein of SPFMV-CH and of several other potyviruses indicated mixed infections by distinct potyviruses in the sweetpotato clones we investigated. Dot blot assays and preliminary sequence analysis indicate that the virus corresponding to the 1.30-kb fragment is closely related to sweetpotato latent virus.