Impact of Sweet Potato Feathery Mottle Virus and Micropropagation on Yield, Root Quality and Virus Incidence in Commercial Sweetpotato Production Systems

Abstract

Reduction in yield and root quality (decline) in sweetpotato has been attributed to the accumulation of viruses and mutations. Three studies were initiated to assess the effects of mutations and viruses in sweetpotato. In the first study, to document the effects of decline over time, two micropropagated, virus-indexed, greenhouse produced "G1" 'Beauregard' mericlones (clones obtained from meristem-tip culture) B94-14 and B94-34, were compared with B94-14 and B94-34 clones propagated over five years via adventitious field propagation (G2, G3, G4, G5) and non-micropropagated NCSU 'Beauregard' seed in field trials during 1997-2001. The trials were located in primary sweetpotato producing regions in NC each year. Yield and root quality measurements were recorded at harvest. G1 plants consistently produced higher total yield, total marketable yield (TMY), No.1 root yield (the most valued grade), and percent No.1 yield (relative to total yield) than G2-G5 plants. G1 plants produced roots with higher shape uniformity and better overall appearance than G2-G5 plants. G2-G5 roots tended to be longer than G1 roots. Linear regression analysis used to model G1-G5 yield and root quality measurements over time indicated that total yield, TMY, No.1 yield, percent No.1 yield, shape uniformity, and overall appearance decreased gradually and length/diameter (L/D) ratios increased gradually with increased field generations of adventitious propagation.

The second study was conducted to determine the effects of Sweet potato feathery mottle virus (SPFMV) on yield and root quality of sweetpotato. To do this, virus-indexed mericlones (VI-), which tested free of known viruses, were compared with virus-infected clones (VI+), in two separate tests with three mericlones each of 'Beauregard' and 'Hernandez' in a two-year study. Tests were arranged in a split plot design with the initial presence or absence (+/-) of SPFMV as the whole plot factor and mericlone as the subplot factor. Yield and root quality measurements indicated that the presence of SPFMV prior to planting (VI+) reduced TMY, yield of No.1s, and percent No.1s and decreased overall appearance for 'Beauregard' mericlones. SPFMV-infected plants also produced roots with higher L/D ratios than VI- plants for mericlones of both cultivars.

The third study was initiated to assess the reinfection rate of SPFMV in the previously described experiments. Weekly observations of the number of SPFMV symptomatic plants were recorded for each plant per plot beginning 4 weeks after planting (WAP) in 2000 and 1 WAP in 2001. In addition, aphid traps were placed in the trials 1 WAP and weekly aphid counts per trap were recorded. The monitoring indicated that 100 % of the virus-indexed, micropropagated mericlones became infected by the end of the growing season as early as 5 WAP or as late as 10 WAP. In both years, the percentage of plants displaying symptoms the first week of monitoring was higher for VI+, G2-G5, and NCSU 'Beauregard' plants than VI- and G1 plants. The reinfection rate of micropropagated, virus-indexed mericlones (VI- and G1) in all trials was higher in 2000 than 2001, which may correlate with the higher number of aphids recorded per trap in 2000 than 2001.

In order to verify the presence of SPFMV in the field trials, one symptomatic vine was collected per plot at three sampling dates in 2000 and 2001, and grafted onto the indicator plant Ipomoea setosa Ker and tested for SPFMV and other viruses using an enzyme linked immunosorbant assay (NCM-ELISA). All field samples induced virus symptoms on I. setosa after grafting. Samples tested using NCM-ELISA confirmed only the presence of SPFMV. The results from these studies indicate that adventitious propagation methods used in commercial sweetpotato production allows SPFMV to accumulate and that this contributes to cultivar decline.