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Southern root-knot nematode (RKN, Meloidogyne incognita) is a serious pest of cultivated watermelon (Citrullus lanatus var. lanatus) in southern regions of the United States and no resistance is known to exist in commercial watermelon cultivars. Wild watermelon relatives (Citrullus lanatus var. citroides) have been shown in greenhouse studies to possess varying degrees ofresistance to RKN species. Experiments were conducted over 2 yr to assess resistance of southern RKN in C. lanatus var. citroides accessions from the U.S. Watermelon Plant Introduction Collection in an artificially infested field site at the U.S. Vegetable Laboratory in Charleston, SC. In the first study (2006), 19 accessions of C. lanatus var. citroides were compared with reference entries of Citrullus colocynthis and C. lanatus var. lanatus. Of the wild watermelon accessions, two entries exhibited significantly less galling than all other entries. Five of the best performing C. lanatus var. citroides accessions were evaluated with and without nematicide at the same field site in 2007. Citrullus lanatus var. citroides accessions performed better than C. lanatus var. lanatus and C. colocynthis. Overall, most entries of C. lanatus var. citroides performed similarly with and without nematicide treatment in regard to root galling, visible egg masses, vine vigor, and root mass. In both years of field evaluations, most C. lanatus var. citroides accessions showed lesser degrees of nematode reproduction and higher vigor and root mass than C. colocynthis and C. lanatus var. lanatus. The results of these two field evaluations suggest that wild watermelon populations may be useful sources of resistance to southern RKN.

Four pepper genotypes classified as resistant and four pepper genotypes classified as susceptible to several avirulent populations of M. incognita were compared for their reactions against a population of Meloidogyne incognita (Chitwood) Kofoid and White which had been shown to be virulent to resistant bell pepper (Capsicum annuum) in preliminary tests. The virulent population of M. incognita originated from a commercial bell pepper field in California. The resistant pepper genotypes used in all experiments were the Capsicum annuum cultivars Charleston Belle, Carolina Wonder, and Carolina Cayenne, and the C. chinense cultigen PA-426. The susceptible pepper genotypes used in the experiments were the C. annuum cultivars Keystone Resistant Giant, Yolo Wonder B, California Wonder, and the C. chinense cultigen PA-350. Root gall indices (GI) were ≥ 3.0 for all genotypes in both tests except for PA-426 (GI=2.57) in test 1 and ‘Carolina Cayenne’ (GI=2.83) in test 2. Numbers of eggs per gram fresh root weight ranged from 20,635 to 141,319 and reproductive indices ranged from 1.20 to 27.2 for the pepper genotypes in both tests, indicating that all eight pepper genotypes tested were susceptible to the M. incognita population used in these tests. The M. incognita population used in these studies overcame resistance conferred by the N gene in all resistant genotypes of both C. annuum and C. chinense.