Cultivar Decline in Sweetpotato: II. Impact of Virus Infection on Yield and Storage Root Quality in 'Beauregard∑ and 'Hernandez∑

To determine the effects of Sweet potato feathery mottle virus (SPFMV), and possibly other newly described potyviruses, on sweetpotato yield and storage root appearance, virus-indexed 'Beauregard∑ and 'Hernandez∑ mericlones testing free of known viruses were compared with virus-infected mericlones in two separate experiments over two years. The experiments were arranged in a split-plot, randomized, complete-block design with the initial presence (VI+) or absence (VI-) of SPFMV as the whole plot factor and mericlone as the subplot factor. Plants were monitored weekly for symptoms of SPFMV and vine samples were taken for virus-indexing on Ipomoea setosa. Additional testing for selected sweetpotato viruses was done using a nitrocellulose membrane enzyme-linked immunosorbant assay. SPFMV was the only virus detected in the study, using available testing methodologies. Field monitoring indicated that s 100% of the VI- plants were reinfected with SPFMV by 9 weeks after planting. The presence of virus before planting reduced yields of No. 1 roots by 26% and decreased overall appearance ratings for the three 'Beauregard∑ mericlones. In addition, VI+ planting materials resulted in increased storage root length and reduced storage root width of both cultivars leading to increased storage root length/diameter ratios, further detracting from overall storage root appearance. The results of this study demonstrate that SPFMV contributes to cultivar decline in sweetpotato. However, the interaction of SPFMV with other newly described potyviruses, which may result in synergistic negative effects on sweetpotato yield and quality, needs further research. Decline in yield and root quality in sweetpotato has been at- tributed to the accumulation of mutations and viruses (Clark et al., 2002; Dangler, 1994; Villordon and La Bonte, 1995, 1996). In the United States and other mild temperate climates where sweetpotato is produced, growers save seed roots from each crop. In early spring, seed roots are planted in beds, and adventitious sprouts from bedded roots are cut and transplanted to the field to produce the sweetpotato crop (Wilson and Averre, 1989). The disadvantage of this method of propagation is that mutations and viruses acquired by sweetpotato plants during the previous grow- ing season will be present in the storage roots, and consequently, in the adventitious sprouts produced. Adventitious sprouts of sweetpotato storage roots, which are used as planting materials for subsequent crops, have higher phe- notypic variability than planting materials derived from nodal stem cuttings of sweetpotato, and this may contribute to decreases in yield and root quality of sweetpotato (Templeton-Somers and