|Wechter, William - Pat|
|HASSEL, RICHARD - Clemson University|
Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2013
Publication Date: 3/15/2013
Citation: Levi, A., Thies, J.A., Wechter, W.P., Farnham, M.W., Weng, Y., Hassel, R. 2013. USVL-360, a novel watermelon tetraploid germplasm line. HortScience. 49:354-357.
Interpretive Summary: Watermelon is an important vegetable crop, grown in 44 states in the U.S. There is continuous need to enhance resistance to soil borne diseases and pests of this important crop, especially following the ban on the soil fumigant methyl bromide. USDA, ARS maintains a large number of watermelon accessions collected throughout the world and many of these accessions are wild species that were collected in their natural habitat in southern Africa. In recent studies, ARS identified several of these accessions having resistance to the soil borne diseases and pests on watermelon. In this study, scientists have converted one of these watermelon accessions into a tetraploid line having four sets of chromosomes instead of two. Scientists then conducted greenhouse and field experiments to test the new tetraploid line as a rootstock for grafted watermelon, and found it exhibits resistance to Fusarium wilt, a major disease of watermelon. Also, the tetraploid line contains resistance to root-knot nematodes producing higher watermelon yield than commercial cucurbit rootstocks available in the market. The rootstock line developed should be useful for researchers and plant breeders who are interested in developing rootstock lines for grafted watermelon, and should be useful in reducing losses to soil borne diseases or pests of important cucurbit crops.
Technical Abstract: A tetraploid line “USVL-360” derived from the United States Plant Introduction (PI) 299379 that belong to Citrullus lanatus (Thunb.) Matsum. & Nakai var. citroides (L. H. Bailey) Mansf. (CLC) was developed in this study. USVL-360 is a pure line selection from an autotetraploid plant generated by treating the apical meristem of a PI 299379 seedling with 0.01% ‘of the herbicide Oryzalin, inducing differentiation of the apical cells and regeneration of a tetraploid stem. Flow cytometry, chromosome counting, and fluorescent in situ hybridization (FISH) analyses of cells extracted from resulting S1 plants confirmed theme as tetraploids, having four sets of chromosomes (n=44). An S1 plant with tetraploid features, including large leaves with notched lobes and serrated edges, and flowers with large dark yellow petals was self-pollinated in three successive generations by using the single seed descent procedure to insure a highly homozygote line. Experiments in 2012 and 2013, in fields naturally infested with RKN confirmed the superior resistance of USVL-360 and its diploid CLC parent (PI 299379) over other commonly used cucurbit rootstocks (including the bottle gourd rootstock ‘Emphasis’ or the hybrid-squash rootstocks ‘Strong Tosa’ and ‘Shintosa Camel). In addition, the USVL-360 rootsock produced higher yields in grafted watermelon plants. Separate greenhouse experiments determined that the resistance level of USVL-360 to Fusarium wilt (FW) race 2 infection is similar to its diploid parent (PI 299379). In these tests, 45% of plants inoculated with FW race 2 survived 3 weeks post infection while the control checks ‘Sugar Baby’ and ‘Charleston Gray’ were highly susceptible (e.g., 0.0% and 12.5% of plants survived 3 weeks post inoculation, respectively). USVL-360 is readily crossed with tetraploid lines derived from the cultivated type watermelon, and may be considered a useful resource to breeding programs focused on enhancing resistance to soil-borne diseases in tetraploid watermelon lines, which are integral to the development of triploid-seedless watermelon varieties.