EFFECTS OF SWEET-POTATO FEATHERY MOTTLE VIRUS AND SWEET-POTATO SUNKENVEIN VIRUS ON SWEET-POTATO YIELDS AND RATES OF REINFECTION OF VIRUS-FREE PLANTING MATERIAL IN ISRAEL

Yield reductions of ca 50% or more were observed in field plots infect ed with both sweet potato feathery mottle virus (SPFMV) and sweet pota to sunken vein virus (SPSVV) ('complex'), compared with plots planted with virus-free propagation stocks. No yield reductions were observed in a plot planted with SPFMV-infected cuttings. In plots infected with SPSVV alone, no significant effect on tuber yields was observed in on e year, whereas in the second year there was a ca 30% reduction in yie ld compared with virus-free control plants. Reinfection in the field, in the absence of introduced infection sources, was observed only with SPSVV. However, natural spread resulted when SPFMV-infected source pl ants were introduced. This implies that aphid vectors were present dur ing the growing season, but that SPFMV infection sources were absent f rom the area.