Apparent absence of viruses in most symptomless field-grown sweet potato in Uganda

Symptomless sweet potato (Ipomoea batatas) plants obtained from farmers' fields in each of the main sweet potato growing regions of Uganda were tested by nitro-cellulose membrane enzyme-linked immunosorbent assays (NCM-ELISA) for sweet potato feathery mottle virus (SPFMV), sweet potato mild mottle virus (SPMMV), sweet potato chlorotic fleck virus (SPCFV), sweet potato latent virus (SPLV), C-6 virus (C-6) and sweet potato caulimo-like virus (SPCa-LV). Most of the plants tested gave negative reactions in all NCM-ELISA, only c. 5% giving positive reactions to SPFMV and < 1% to SPMMV or SPCFV. Graft inoculations of cuttings from symptomless sweet potato plants, obtained either at Namulonge Agricultural and Animal Production Research Institute (NAARI) or from farmers' fields, onto seedlings of the near-universal indicator plant for sweet potato viruses, Ipomoea setosa, caused symptoms in c. 15% of indicator plants; in all such cases, SPFMV alone was identified by NCM-ELISA. Supplementary tests, involving further grafting of sweet potato to I. setosa seedlings or grafting sweet potato plants themselves with sweet potato sunken vein virus (SPSVV), which in the presence of SPFMV causes sweet potato virus disease (SPVD), confirmed the absence of infection in sweet potato cuttings which had not caused symptoms on I. setosa in the initial graft test. Possible mechanisms explaining the prevalence of virus-free sweet potato plants in farmers' fields are discussed.