Sweet potato viruses in Uganda: identification of a new virus, a mild strain of an old virus and reversion

In 2009, a sweet potato begomovirus (sweepovirus) was detected for the first time in Uganda. An isolate was sequenced, providing the first full sequence of a sweepovirus from mainland Africa which differed from other sweepoviruses by at least 13%, discriminating this isolate as a new species, ‘Sweet potato leaf curl Uganda virus’ (SPLCUV). SPLCUV was quite common in cultivars (cvs) Ejumula, New Kawogo and 318L having uneven distribution in infected plants and reversion to healthy occurred, especially in cv New Kawogo. SPLCUV was observed not to be synergised by Sweet potato chlorotic stunt virus (SPCSV), apparently making it the first report of a sweet potato virus not synergised by SPCSV. Besides SPLCUV, a ‘mild’ SPCSV strain that induced purpling symptoms and 50% yield reduction similar to wild type SPCSV when infecting alone was identified from Busia district, Uganda. ‘Mild’ SPCSV was never observed to be co-infected with Sweet potato feathery mottle virus (SPFMV) in farmers’ fields. Experimentally, ‘mild’ SPCSV induced mild symptoms in Ipomoea setosa and sweet potato plants and SPFMV titre was greater in co-infections of SPFMV and wild type SPCSV than in co-infections of SPFMV and ‘mild’ SPCSV. Both RNase3 (accession No. HE575406) and p22 (accession No. HE575409) genes on RNA1 of ‘mild’ SPCSV compared closely to those reported previously. Instead, RNA1 region appears to be expressed less in the ‘mild’ SPCSV infection than in the wild type SPCSV infection though RNA2 continues to be more expressed in the ‘mild’ SPCSV infection than in the wild type SPCSV infection. Recovery from SPVD symptoms and reversion from SPFMV were observed in cv Kampala White co-infected with ‘mild’ SPCSV and SPFMV. Reversion from SPFMV single infections occured in several landraces with higher rates observed in shoots of resistant than susceptible cultivars. Overall, cv NASPOT 11 was the fastest to revert while cv Beauregard was the slowest