2012 Annual Report
1a.Objectives (from AD-416):
1. Identify or develop tolerant germplasm lines, study the inheritance of tolerance, and utilize genetic differences in herbicide tolerance to improve crop safety and enhance weed management in watermelons and sweetpotato.
2. Develop vegetable crop production systems that utilize competitive crop genotypes and innovative cultural practices to reduce the impact of weeds.
1b.Approach (from AD-416):
Develop watermelon germplasm lines with enhanced tolerance to clomazone herbicide using a conventional plant breeding strategy, study the inheritance of clomazone tolerance, and identify or develop tolerant lines for use in watermelon breeding and as rootstocks for grafted watermelon production. Assess watermelon germplasm for differences in halosulfuron methyl tolerance, and investigate the feasibility of developing tolerant cultivars using conventional plant breeding approaches. Identify sweetpotato clones that differ in clomazone and flumioxazin tolerance, use a recurrent mass selection breeding approach to generate clones with high levels of tolerance and good horticultural characteristics, and make them available for use by sweetpotato breeders. Assess the competitive advantage against weeds of sweetpotato genotypes with more vigorous growth habits in comparison to less competitive conventional cultivars, identify competitive genotypes with good horticultural quality, and evaluate them as component in integrated weed management systems for conventional and organic growers. Evaluate weed management practices utilizing competitive southernpea cultivars as a component in integrated weed management systems for conventional and organic growers. Investigate the impact of poultry and swine litter biochar soil amendment on southernpea and weed growth and on herbicide activity.
Progress toward the goals of Sub-objective 1.A. included the following: Second and third crosses between tolerant and susceptible wild watermelon (citron melon) lines were completed to assess the heritability of clomazone tolerance. Greenhouse experiments to assess the inheritance of clomazone tolerance in citron melon lines were completed. A recurrent selection procedure to produce non-segregating clomazone tolerant and susceptible citron melon lines was completed. A backcross breeding project to transfer clomazone tolerance from citron melon into watermelon cultivars was continued with second and third backcrosses completed. A field screening experiment was conducted to assess halosulfuron tolerance in a genetically diverse collection of approximately 400 watermelon varieties, germplasm lines and related species. Thirteen watermelon lines that were more tolerant than commercial cultivars were selected, and seeds of these lines are being produced so that their tolerance can be further evaluated. Under sub-objective 1.B., a field screening procedure was used to identify clomazone tolerant sweetpotato seedlings. Seedlings were grown from seeds that were obtained from a crossing block last year. Ninety-six clomazone tolerant seedlings were selected from the preliminary screening experiment and evaluated in a secondary screen to affirm tolerance and to assess horticultural quality. Clomazone tolerance was assessed for broccoli and cabbage varieties in a field experiment. Under objective 2.A., a field experiment was conducted to determine the competitiveness of 13 sweetpotato lines against weeds. Under Sub-objective 2.B., the competitiveness of determinate, semi-determinate, and non-determinate cowpea varieties against weeds and their response to weed interference were assessed in a field experiment. A field investigation was completed in collaboration with the location nematologist to determine the effect of nematode resistant and susceptible cover crop cowpea varieties on nematodes and weeds.
Clomazone tolerance in wild watermelon (citron melon) is highly heritable.
Clomazone, an effective preemergence herbicide, is registered for use in watermelon; however, crop tolerance to the herbicide is marginal. Thus, clomazone use in watermelon is restricted to lower rates than are used on other crops, and weed control with lower rates is not as effective. ARS researchers in Charleston, SC, demonstrated that clomazone tolerance can be transferred from tolerant to susceptible lines, using conventional plant breeding approaches. The distribution of tolerance in seedlings obtained from crosses of tolerant and susceptible parents indicated that tolerance may be conferred by a dominant gene. This study demonstrates that increasing clomazone tolerance in watermelon using traditional plant breeding approaches is feasible and that the citron melon lines are potential sources of resistance. Highly tolerant watermelon varieties developed using knowledge generated by this study may result in more effective use of one of the few selective herbicides available for the crop.
Harrison Jr, H.F., Kousik, C.S., Levi, A. 2011. Identification of citrullus lanatus germplasm lines tolerant to clomazone herbicide. HortScience. 46(5):684-687.
Harrison Jr, H.F., Jackson, D.M. 2011. Greenhouse assessment of differences in clomazone tolerance among sweetpotato cultivars. Weed Technology. 25:501-505.
Harrison Jr, H.F., Wechter, W.P., Kousik, C.S. 2012. Inhibition of bacterial, fungal and plant growth by testa extracts of Citrullus genotypes. HortScience. 47:448-451.