Watermelon is grown on over 60,000 ha in the United States and is an important crop in many states. The herbicide clomazone (Anonymous, 2005) is widely used in watermelon production in the United States, except California. It controls many important annual weeds, and the number of weed species controlled increases with the rate of application. The recommended use rates for watermelon are 0.17 for coarse soils and 0.28 kg·ha−1 for fine soils. Clomazone application rates are higher for other crops, for instance recommended rates for soybeans [Glycine max L. (Merr.)] grown in southern states on coarse soils are between 1.1 and 1.4 kg·ha−1. Even at low rates, clomazone can cause moderate injury to young watermelon plants that is expressed as chlorosis of leaves and growing tips.
We first observed differential clomazone tolerance among Citrullus germplasm accessions when the herbicide was used for weed control in research plots at the U.S. Vegetable Laboratory. Hines and Wilson (1992) reported that watermelon cultivars varied in clomazone injury in a greenhouse experiment; however, the cultivars included in their experiment were not listed. Cultivars of other cucurbit crops have also been shown to vary in tolerance. Staub et al. (1991) screened the U.S. germplasm collection of cucumber (Cucumis sativus L.) for clomazone tolerance in the field and identified tolerant and susceptible accessions based on injury ratings. In a greenhouse evaluation of selected accessions, the differences in tolerance were not as great as in the field screening experiment. Al-Khatib et al. (1995) demonstrated differences in clomazone tolerance among five cucumber cultivars in greenhouse and field experiments. The greatest differences in tolerance were observed with injury ratings taken at 4 weeks after planting in the field and with leaf area and plant dry weights in the greenhouse. After observation of differences in clomazone injury in a field cultivar trial, Harrison and Keinath (2003) evaluated 67 pumpkin [Cucurbita maxima Duchesne, C. moschata Duchesne (Poir.), C. pepo L.] cultivars for tolerance in the greenhouse. Differences in tolerance were greatest between the species marketed as pumpkins; however, differences in tolerance were also observed among cultivars within species. Watermelon was most susceptible to clomazone among six cucurbit crops (Figueroa and Kogan, 2005) based on early-season injury ratings and growth measurements. However, clomazone at up to 0.9 kg·ha−1 did not affect crop yields. Boyhan et al. (1995) reported that watermelon yields were reduced by clomazone applied pre-emergence at 1.1 and 1.7 kg·ha−1 to direct seeded watermelon. Grey et al. (2000) reported that clomazone at 0.84 kg·ha−1 applied preplant-incorporated, preplant to the soil surface, and post-transplant to the soil surface caused minor foliar bleaching (14% injury or less) but did not reduce the yields of seeded or transplanted ‘Royal Sweet’ watermelon. Cohen et al. (2008) concluded from a greenhouse study that clomazone at 1.0 and 2.0 kg·ha−1 was too injurious to grafted and non-grafted watermelons to be used safely. Variation among cultivars in clomazone tolerance has been reported in several non-cucurbit crops, including bean (Phaseolus vulgaris L.) (Sikkema et al., 2006), cabbage (Brassica oleracea L.) (Hopen et al., 1992), corn (Zea mays L.) (Keifer, 1989), and rice (Oryza sativa L.) (Mudge et al., 2005; Scherder et al., 2004; Zhang et al., 2004).
The first objective of this study was to evaluate clomazone tolerance in a diverse germplasm collection of watermelon cultivars and germplasm accessions of Citrullus lanatus and closely related species. Subsequent greenhouse and field experiments were conducted to assess the differences in tolerance of selected accessions identified in the germplasm evaluation experiment.
Al-Khatib, K., Kadir, S. & Libbey, C. 1995 Broadleaf weed control with clomazone in pickling cucumber (Cucumis sativus) Weed Technol. 9 166 172
Boyhan, G.E., Kovach, S.P., Norton, J.D., Abrahams, B.R., Hollingsworth, H.M. & Dangler, J.M. 1995 Preemergence herbicides for cantaloupe and watermelon J. Veg. Crop Prod. 1 79 92
Brunner, E., Domhof, S. & Langer, F. 2002 Nonparametric analysis of longitudinal data in factorial experiments John Wiley & Sons New York, NY
Cohen, R., Eizenberg, H., Edelstien, M., Horev, C., Lande, T., Porat, A., Achdari, G. & Herschenhorn, J. 2008 Evaluation of herbicides for selective weed control in grafted watermelon Phytoparasitica 36 66 73
Grey, T.L., Bridges, D.C. & NeSmith, D.S. 2000 Tolerance of cucurbits to the herbicides clomazone, ethanlfluralin, and pendimethalin. II. Watermelon HortScience 35 639 641
Jarret, R.L., Merrick, L.C., Holms, T., Evans, J. & Aradhya, M.K. 1997 Simple sequence repeats in watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] Genome 40 433 441
Levi, A., Thomas, C.E., Joobeur, T., Zhang, X. & Davis, A. 2002 A genetic linkage map for watermelon derived from a testcross population: (Citrullus lanatus var. citroides × C. lanatus var. lanatus) × C. colocynthis Theor. Appl. Genet. 105 555 563
Levi, A., Thomas, C.E., Keinath, A.P. & Wehner, T.C. 2001a Genetic diversity among watermelon (Citrullus lanatus and Citrullus colocynthis) accessions Genet. Resources Crop Evol. 48 559 566
Levi, A., Thomas, C.E., Wehner, T.C. & Zhang, X. 2001b Low genetic diversity indicates the need to broaden the genetic base of cultivated watermelon HortScience 36 1096 1101
Levi, A., Thomas, C.E., Trebitsh, T., Salman, A., King, J., Karalius, J., Newman, M., Reddy, O.U.K., Xu, Y. & Zhang, X. 2006 An extended linkage map for watermelon based on SRAP, AFLP, SSR, ISSR and RAPD markers J. Amer. Soc. Hort. Sci. 131 393 402
Mudge, C.R., Webster, E.P., Leon, C.T. & Zhang, W. 2005 Rice (Oryza sativa) cultivar tolerance to clomazone in water seeded production Weed Technol. 19 907 911
Staub, J., Crubaugh, L., Baumgartner, H. & Hopen, H. 1991 Screening of the cucumber germplasm collection for tolerance to clomazone herbicide Cucurbit Crop Genet. Rep. 14 22 24
Zhang, W., Webster, E.P., Blouin, D.C. & Linscombe, S.D. 2004 Differential tolerance of rice (Oryza sativa) varieties to clomazone Weed Technol. 18 73 77