Sweet potato viruses in South Africa and the effect of viral infection on storage root yield

A survey was conducted during 2001-2003 of sweet potato grown by small-scale farmers in the seven main sweet potato-producing provinces of South Africa to determine the incidence of viral diseases in the field. Based on symptoms observed, disease incidence was relatively low. The highest levels of infection observed were between 9-10 % in the Western Cape and Mpumalanga. Symptoms were less prevalent or absent in the Eastern Cape, Limpopo, Gauteng, KwaZulu-Natal and North West. However, grafting onto Ipomoea setosa and subsequent testing by ELISA indicated that 81%of the samples were infected by viruses. Nine viruses were identified. Sweet potato feathery mottle virus (SPFMV) was the most common and occurred in 73 % of the samples tested. Sweet potato mild mottle ipomovirus (SPMMV) and Sweet potato latent potyvirus (SPLV) were detected in 3 % and 5 % of the samples, respectively. Five viruses were found for the first time in South Africa, viz. Sweet potato chlorotic fleck virus (SPCFV), Sweet potato mild speckling virus (SPMSV), Sweet potato chlorotic stunt crinivirus (SPCSV-East and West African strains), Sweet potato virus Y (SPVY) and Sweet potato virus G (SPVG). The effect of viral infection on storage root yield of ten sweet potato varieties was also studied over two seasons. Total yield was on average reduced by between 12 and 22 %, and marketable yield by 21 to 38 %. Reduction in marketable yield was mainly due to increased cracking of the storage roots. The new Agricultural Research Council varieties Monate and Ndou, and the 1994-5-1 breeding line produced marketable yields of 34-48 tonnes ha-1 even when infected with viruses, whereas the established South African varieties Mafutha and Natal Red yielded only 4-16 tonnes ha-1.