Characterization of Watermelon ( Citrullus lanatus var . citroides ) Germplasm for Resistance to Root - knot Nematodes

Root-knot nematodes (Meloidogyne spp.) cause extensive damage to watermelon ICitrz,llus lanatus (Thunb.) Matsum. & Nakai var. lanatusl, and resistance to rootknot nematodes has not been identified in any watermelon cultivar. Twenty-six U.S. Plant Introductions (Pis) of Citrullus lanatus (Thunb.) Matsum. & Nakai var. citroides (L. H. Bailey) Mansf., one P1 of C. lanatus var. lanatus, and three Pis of Citrullus colocynthis (L.) Schrad. were evaluated in greenhouse tests for resistances to Meloidogyne incognita (Kofoid & White) Chitwood race 3 and Meloidogyne arenaria (Neal) Chitwood race 2. Twenty-three of the C. lanatus var. citroides Pis and the C lanatus var. lanatus Pis were previously identified as moderately resistant to M. arenaria race 1. Overall, the C. lanatus var. citroides Pis exhibited low to moderate resistance, and the C. lanatus var. lanatus and C. colocynthis Pis were susceptible to both M. incognita race 3 and M. arenaria race 2. The C. lanatus var. citroides P1 482303 was the most resistant P1 with gall index (GI) = 2.88 and reproductive index (RI) = 0.34 for M. incognita race 3 and GI = 3.46 and RI = 0.38 for M. arenaria race 2 (1 = no galling; 5 = 26% to 38% root system galled; 9 = 81% to 100% root system galled). These results demonstrate that there is significant genetic variability within C. lanatus var. citroides for reaction to M. incognita and M. arenaria race 2, and several C. lanatus var. citroides Pis may provide sources of resistance to root-knot nematodes. 1530 HORTSCiENCE VOL. 42(7) DECEMBER 2007 red food color (Thies etal., 2002) for 15 to 20 min to stain the egg masses. The root systems were carefully rinsed under running tap water and evaluated for galling severity and egg mass production using a 1 to 9 scale in which l=0,2=100to3%,3=4%to1200,4_1300 to25%,5=26°0to38°o,6=39°oto50°o,7 51%to65°o,8 = 66°oto8O°o, and 9 81°oto 100 0 0 of root system galled or covered with egg masses, respectively (Thies and Eery, 1998). Ratings were Ito 2.9 = high resistance, 3.0 to 4.0 moderate resistance, 4.1 to 4.9 low resistance, 5.0 to 6.9 susceptible; and 7.0 to 9.0 = highly susceptible (Thies and Levi, 2003). Then, the entire root systems of all plants of each accession in a replicate were cut into 1-to 2-cm pieces, the total root weight was recorded, and root-knot nematode eggs were extracted from the root sample using 1.00o NaOCI (Hussey and Barker, 1973). The 0.5°ö NaOCI was used to extract eggs for inoculum to avoid damage to the eggs; however, 1.0% NaOC1 was used to extract eggs for estimating nematode reproduction because extraction is more efficient with a higher concentration of NaOC1. Numbers of eggs were counted using a stereomicroscope. Nematode reproduction was assessed by calculating the reproductive index (RI) in which RI P i/P i , where P the initial inoculum level and P f final egg recovery (Sasser etal., 1984). Eggs per gram fresh root and nematode reproductive index data were 1og i (x + 1) transformed before analysis. Data were analyzed using the GLM procedure of SAS for Windows, v.8.0 (SAS Institute, Cary, NC), and means were separated using Fisher's protected least significant difference test. Results and Discussion Overall, the C. lanatus var. citroides P1 exhibited higher resistance to M. incognita and M. arenaria race 2 than the watermelon cultivars and C. coloci'nthi,s P1. The C. lanatus var. citroides root systems exhibited minimal to moderate galling; the root systems were more fibrous and had fewer and smaller galls than the root systems of the watermelon cultivars and the C. colocinthis PIs (Tables 1 and 2). Like in a previous study with M. arenaria race 1 (Thies and Levi, 2003), all three Pis of the desert species C. colocynthis also were highly susceptible to M. incognita race 3 and M. arenaria race 2 (Tables 1 and 2). Meloidogyne incognita race 3 tests I and 2. The C. lanatus var. citroides Pt 482303 exhibited high resistance in both tests with M. incognita race 3; gall index (GI) was 2.80 and GI 2.97 in tests I and 2, respectively, with 1535 M. incognita eggs per gram fresh root and RI = 0.34 (Table 1). An additional 21 C. lanatus var. citroides PIs exhibited low to moderate resistance to M. incognita race 3 based on root gall severity (GI ranges: 3.07 to 4.80 in test 1 and 3.00 to 4.25 in test 2). However, some of these PIs had relatively high numbers of eggs per g fresh root (greater than 5000) and RIs greater than 1.0 (Table I). For example, P1 189225 supported 16,508 eggs of M. incognita per gram fresh root and RI = 2.17. The PIs 248774, 288316, and 532666 exhibited susceptible reactions to M. arenaria race 1 in previous tests (Thies and Levi, 2003) and were also susceptible to M. incognita race 3 in the current tests (Table 1). The P1 542114 exhibited low resistance to M. arenaria race 1 in a previous Table 1. Gall indices, egg mass indices, numbers of Meloidogvne incognita race 3 eggs per gram flesh root, and reproductive indices for selected PIs of Citrulluc lunatics var. citroides. C lunatics var. lanatus. and C. colocs'ntlns and control watermelon cultiars inoculated with M. incognita race 3 in replicated greenhouse tests.'