Submitted to: Journal of Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 18, 2009
Publication Date: N/A
Interpretive Summary: Root-knot nematodes are microscopic roundworms that parasitize the roots of watermelon and many other crop plants. Root-knot nematodes invade and feed on the roots, causing disease and injury. Nematodes withdraw nutrients from the roots and inject hormone-like substances into roots, causing the roots to become swollen or “galled”. Nutrients from the watermelon foliage and vines are drawn to the galled roots, resulting in a stunted watermelon plant with smaller and fewer watermelon fruit. Currently, soil is fumigated with methyl bromide gas before planting watermelon in order to control nematodes and other soil pathogens. However, because methyl bromide is harmful to the environment, its use is being restricted. Resistant varieties, if available, would provide an economical and environmentally friendly way to control root-knot nematodes in watermelon. Unfortunately, all watermelon varieties are susceptible to root-knot nematodes. Results of greenhouse experiments have shown that wild watermelons are potential sources of nematode resistance. Scientists at the US Vegetable Laboratory, Charleston, SC, tested 21 experimental lines of wild watermelon for resistance to southern root-knot nematode in field studies over two years. In the first year (2006), five wild watermelon lines were identified as potentially resistant to southern root-knot nematode. In 2007, the five selected wild watermelon lines were re-evaluated to assess performance when planted in soils treated with methyl bromide for nematode control and in untreated soils. In general, the wild watermelon populations performed as well in non-treated soils as in soils treated with methyl bromide. Additionally, these selected lines performed better than commercial watermelon varieties. Therefore, these wild watermelon lines serve as sources of nematode resistance for breeding nematode resistant watermelon varieties.
Technical Abstract: Southern root-knot nematode (Meloidogyne incognita) is a serious pest of cultivated watermelon (Citrullus lanatus var. lanatus) in southern regions of the US and no resistance is known to exist in commercial watermelon cultivars. Wild watermelon relatives (C. lanatus var. citroides) have been shown in greenhouse studies to possess various degrees of resistance to root-knot nematode species. Two experiments were undertaken over two years to assess resistance of southern root-knot nematode in C. lanatus var. citroides experimental lines in artificially infested field sites at the US Vegetable Laboratory in Charleston, SC. In the first study (2006), 21 experimental lines of C. lanatus var. citroides developed at the US Vegetable Laboratory were compared with reference entries of C. colocynthis and C. lanatus var. lanatus cultivars. Of the wild watermelon lines, two entries exhibited significantly less galling than all other entries. Five of the best performing C. lanatus var. citroides experimental lines were evaluated with and without methyl bromide at the same field site in 2007. C. lanatus var. citroides populations performed better than C. lanatus var. lanatus and C. colocynthis for most variables measured. Overall, most entries of C. lanatus var. citroides performed similarly with and without methyl bromide treatment in regard to root galling, visible egg masses, vigor, and amount of fibrous roots. In both years of field evaluations, most C. lanatus var. citroides lines showed lesser degrees of nematode reproduction and higher vigor and root mass. The results of these two field evaluations suggest wild watermelon populations may be useful sources of resistance to southern root-knot nematode.