Abstract:

Sweet potato is a vegetatively propagated crop where vine cuttings from previous crops or sproutslips from roots are used as planting material. This practice can lead to the accumulation of systemic pathogens, especially viruses when the planting material used are not virus tested. The contribution of this practice to degeneration of sweetpotato cultivars in Uganda has been only speculative, and with different views in high and low sweetpotato virus disease pressure zones of central and eastern Uganda. The overall objective of this study was to establish the importance of viruses in mediating cultivar decline in Uganda. Four sweetpotato cultivars of 'Beauregard’, ‘Dimbuka’, ‘Ejumula’ and ‘NASPOT 1’ were planted in a series of Held trials in central (Kabanyolo; MUARIK) and eastern (Serere; NaSARRI) Uganda over five generations (Gl, G2, G3, G4 and G5). The trials started with virus-free planting material (GO). Each succeeding trials retained planting material from the previous one, plus GO material. Virus symptoms were observed 1 month after planting in all the generations, with sweet potato feathery mottle virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV) being the most frequently detected viruses. The cultivars ‘Beauregard’ and ‘Ejumula’ had highest disease incidence and severity and degenerated after a single season in both locations. Storage root yields and numbers were greatest in Gl and gradually reduced in subsequent generations (G2, G3, G4 and G5) for each cultivar in both locations. Field plots were planted with vines of cultivars Beauregard, Dimbuka, Ejumula, Kabode and NASPOT 1 graft inoculated with SPFMV and or SPCSV alongside GO as control at both Kabanyolo and Serere. SPFMV spread rapidly to control plots at Kabanyolo, and these plots had similar yields to those singly infected with SPFMV. At Serere where SPFMV spread slowly, plots infected with SPFMV yielded 40% less than the control. Recovery and reversion from SPFMV infection were more frequent at Serere than at Kabanyolo. Infection by SPCSV alone resulted in yield losses of 14-60% and no reversion was observed, while mixed infections of SPFMV and SPCSV resulted in yield losses of 60-95% in both locations depending on the cultivar. SPCSV and SPFMV were found to translocate from infected vines to roots and then to sprouts. When roots obtained from Kabanyolo and Serere fields were evaluated for possible virus accumulation, over 70 % of the roots from Kabanyolo tested positive for both SPFMV and SPCSV, and their sprouts had visible virus symptoms. Roots obtained from Serere fields did not show symptoms on sprout and only 25% tested positive for SPFMV. Roots from graft inoculated plants (either with SPCSV or SPFMV alone) under screen house condition produced both infected and non-infected roots as well as sprouts. Sprouts infected with SPFMV alone reverted to healthy status while those infected by SPCSV only showed recovery, and none infected by both viruses even showed recovery. This study has proved that viruses rapidly accumulate in sweetpotato through vines and roots which are used for propagation resulting into cultivar decline. Both single and dual infection by SPFMV and SPCSV contributed to the observed cultivar declines in both locations. Susceptible cultivars like Bearegard and Ejumula degenerated within one season when grown in high disease pressure zone. Cultivars Dimbuka and NASPOT 1 (and also Ejumula at Serere) gradually declined in yield and can be grown for up to three seasons in central and up to five seasons in eastern Uganda before replacing with virus indexed planting material. Virus indexing services and rapid multiplication of clean planting material needs to be established to support sustainable production of sweetpotato in Uganda.