Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2011
Publication Date: 1/31/2015
Citation: Thies, J.A., Ariss, J., Hassell, R., Buckner, S.A., Levi, A. 2015. Accessions of Citrullus lanatus var. citroides are valuable rootstocks for grafted watermelon in fields infested with root-knot nematodes. HortScience. 50:4-8.
Interpretive Summary: The fumigant methyl bromide has been used extensively by growers to control soil-borne diseases and pests of vegetable crops. However, following the Montreal Pact (2005) on banning this toxic fumigant, there is an existing need to identify and develop environmental friendly technologies to control soil-borne diseases and pests in vegetable crops. Watermelon is highly susceptible to soil-borne diseases and pests, including root-knot nematodes (RKN) that attack the roots of the plants and cause severe damage and loss of yield. Recently, scientists at the USDA, ARS U.S. Vegetable Laboratory (Charleston, SC) identified wild watermelon accessions that were collected in southern Africa as resistant to RKN. In this study, the researchers have tested these wild watermelons as rootstocks to graft elite watermelon cultivars. They found that the wild watermelon rootstocks are useful in controlling RKN and produce high watermelon yields in fields infested with RKN. The information in this study should be useful to plant breeders and plant pathologists interested in developing rootstocks for watermelon. It should also be useful to watermelon growers looking for alternatives to methyl bromide for controlling root-knot nematodes in watermelon.
Technical Abstract: Wild watermelon (Citrullus lanatus var. citroides) RKVL rootstock lines developed at the U.S. Vegetable Laboratory, USDA, ARS in Charleston, South Carolina, were compared to wild tinda and commercial cucurbit rootstock cultivars for grafting of seedless watermelon ‘Tri-X 313’ (C. lanatus var. lanatus) in fields infested with the southern root-knot nematode (Meloidogyne incognita). Tests were conducted in Charleston, SC, in 2009 and 2010. In both years, root-knot nematode (RKN) infection was severe in ‘Emphasis’ bottle gourd, ‘Strong Tosa’ hybrid squash, and wild tinda (Praecitrullus fistulosus) rootstocks (root galling = 86% to 100%). The RKVL wild watermelon rootstocks exhibited significantly lower (P<0.05) percentages of root galling (9.1% to 16.2%) than non-grafted ‘Tri-X 313’ (40.9%), ‘Emphasis’, ‘Strong Tosa’, and the wild tinda rootstocks in 2009. The grafted wild watermelon rootstock RKVL 318 produced significantly more (P<0.05) fruit (12 per plot) than all other entries (mean = 5.3 per plot), and a heavier (P<0.05) fruit yield (29.5 kg per plot) than all entries except self-grafted ‘Tri-X 313’ (21.5 kg per plot). In 2010, soil in one-half the plots was treated with methyl bromide (50%) : chloropicrin (50%) (350 lbs/A) before planting. The RKVL wild watermelon rootstocks exhibited resistance or tolerance to RKN with percentages of root system galled ranging from 11% for RKVL 316 to 56% for RKVL 301 in the untreated control plots. Fruit yields in the untreated plots were 21.9, 25.6, and 19.9 kg/plot for RKVL 301, RKVL 316, and RKVL 318, respectively. Yields were significantly greater (P<0.05) for the three RKVL rootstocks than for ‘Strong Tosa’ (3.0 kg), ‘Emphasis’ (7.2 kg), and ‘Ojakkyo’ wild watermelon rootstock (2.8 kg) in the untreated plots. Yields of watermelon grafted on ‘Strong Tosa’ were nearly 7X greater (P<0.05) in the methyl bromide treated plots than in the untreated plots. In contrast, yields of RKVL 301, RKVL 316, and RKVL 318 were similar in both treatments. RKVL 316 had low root galling and produced the heaviest fruit yield and greatest numbers of fruit of any rootstock evaluated. The RKVL lines should be useful sources of RKN-resistance for rootstocks for grafted watermelon.