No abstract

Sweet potato virus disease (SPVD) is the name used to describe a range of severe symptoms in different cultivars of sweet potato, comprising overall plant stunting combined with leaf narrowing and distortion, and chlorosis, mosaic or vein-clearing. Affected plants of various cultivars were collected from several regions of Uganda. All samples contained the aphid-borne sweet potato feathery mottle potyvirus (SPFMV) and almost all contained the whiteflyborne sweet potato chlorotic stunt closterovirus (SPCSV). SPCSV was detected by a mix of monoclonal antibodies (MAb) previously shown to react only to a Kenyan isolate of SPCSV, but not by a mixture of MAb that detected SPCSV isolates from Nigeria and other countries. Sweet potato chlorotic fleck virus (SPCFV) and sweet potato mild mottle ipomovirus (SPMMV) were seldom detected in SPVD-affected plants, while sweet potato latent virus (SPLV) was never detected. Isolates of SPFMV and SPCSV obtained by insect transmissions together induced typical symptoms of SPVD when graft-inoculated to virus-free sweet potato. SPCSV alone caused stunting and either purpling or yellowing of middle and lower leaves when graft-inoculated to virus-free plants of two cultivars. Similarly diseased naturally inoculated field plants were shown consistently to contain SPCSV. Both this disease and SPVD spread rapidly in a sweet potato crop.

The movement function of poa semilatent hordeivirus (PSLV) is mediated by the triple gene block (TGB) proteins, of which two, TGBp2 and TGBp3, are membrane proteins. TGBp3 is localized to peripheral bodies in the vicinity of the plasma membrane and is able to re-direct TGBp2 from the endoplasmic reticulum (ER) to the peripheral bodies.

Four hundred and forty-eight symptomatic and 638 asymptomatic samples were collected from sweet potato fields throughout Kenya and analysed serologically using antibodies to Sweet potato feathery mottle virus (SPFMV), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato mild mottle virus (SPMMV), Cucumber mosaic virus (CMV), Sweet potato chlorotic fleck virus (SPCFV), Sweet potato latent virus (SwPLV), Sweet potato caulimo-like virus (SPCaLV), Sweet potato mild speckling virus (SPMSV) and C-6 virus in enzyme-linked immunosorbent assays (ELISA). Only SPFMV, SPMMV, SPCSV, and SPCFV were detected. Ninety-two percent and 25% of the symptomatic and asymptomatic plants respectively tested positive for at least one of these viruses. Virus-infected plants were collected from 89% of the fields. SPFMV was the most common and the most widespread, detected in 74% of the symptomatic plants and 86% of fields surveyed. SPCSV was also very common, being detected in 38% of the symptomatic plants and in 50% of the fields surveyed. SPMMV and SPCFV were detected in only 11% and 3% of the symptomatic plant samples respectively. Eight different combinations of these four viruses were found in individual plants. The combination SPFMV and SPCSV was the most common, observed in 22% of symptomatic plants. Virus combinations were rare in the asymptomatic plants tested. Incidence of virus infection was highest (18%) in Kisii district of Nyanza province and lowest (1%) in Kilifi and Malindi districts of Coast province.

In a survey of most sweetpotato-growing areas of Uganda, virus-like diseases were observed in all districts surveyed. Out of 338 fields sampled in 35 of the then 42 districts, 219 (65%) had some plants with symptoms. The most common symptoms included vein clearing, mottling, leaf distortion, yellowing, stunting and leaf strapping. Particularly high viruslike disease incidences (means of 34-86%) were encountered in districts around Lake Victoria and in the Rift Valley in southern and western parts of Uganda; particularly low incidences were encountered in the east and north of Uganda. Using four formats of enzyme-linked immunosorbent assay in combination with immunoelectron microscopy and polymerase chain reaction assays, five viruses were identified. Sweet potato feathery mottle virus (SPFMV) and Sweet potato chlorotic stunt virus (SPCSV) were most commonly detected, being found in about 90% of samples. Sweet potato mild mottle virus at 10%, Sweet potato chlorotic fleck virus (SPCFV) at 8% and Sweet potato caulimo-like virus (SPCaLV) at 0·07% were more rarely detected. Most infections were multiple, SPCSV + SPFMV constituting > 90% of all double infections. Triple infections, involving mainly SPFMV, SPCSV and either SPMMV or SPCFV, and quadruple infections of SPFMV + SPCSV + SPMMV + SPCFV were observed in < 10% of the diseased samples. The identification of SPCaLV is the first evidence of its occurrence in Africa.