2009 Annual Report
1a.Objectives (from AD-416)
Investigate the role of plant growth habit and allelopathic potential on weed suppression by cole crops, sweetpotato, and watermelon, and identify vegetable crop genotypes that are competitive against weeds. Investigate the use of living and killed cover crop mulches in combination with other control measures for weed management in cole crops and sweetpotato. Select aggressive growth habit cowpea genotypes that are most suited for use as weed suppressive cover crop varieties.
1b.Approach (from AD-416)
Develop and use bioassay experiments to identify allelopathic and non-allelopathic cole crop and watermelon genotypes. Evaluate allelopathic and non-allelopathic lines in field and greenhouse experiments to assess the importance of allelopathic potential on weed suppression by the crops. Utilize bioassay guided extraction and chromatography procedures to isolate allelopathic substances for identification by collaborating chemists. Develop rapid techniques to identify allelopathic genotypes using bioassays or simple chemical analyses. Survey watermelon and sweetpotato germplasm collections and identify accessions with aggressive, weed suppressing growth habit. Assess the impact of growth habit on weed interference in greenhouse and field studies. Use the knowledge attained from studies on the effect of allelopathy and growth habit on weed suppression to develop guidelines for use by plant breeders to develop genotypes that are less susceptible to weed interference. Evaluate highly allelopathic sweetpotato lines for yellow nutsedge suppression in field and greenhouse studies. Evaluate ladino clover mulch for weed suppression in sweetpotato and cowpea-sorghum cover crop mulch for weed suppression in collard and cabbage. Compare the weed suppressing ability of several cowpea genotypes in field and greenhouse experiments in order to select those most suited for use as cover crops.
Sweetpotato varieties with vigorous, compact growth habits (bunch type) are more competitive against weeds than varieties with the more common viny growth habit. An ongoing project conducted in cooperation with the sweetpotato breeder at the U.S. Vegetable Laboratory is directed toward developing highly competitive sweetpotato varieties and assessing the effect of vigorous growth habit on weed interference. Forty-two germplasm lines with vigorous bunch type canopy growth were evaluated in a field experiment to identify those with high yields and good horticultural characteristics. The lines included in this evaluation were generated in prior years employing a recurrent selection procedure. Studies on the allelopathic potential of substances associated with watermelon seed coats were continued. Extracts of seed coats of wild watermelons caused greater inhibition in seed germination and bacterial growth bioassays than extracts from domesticated watermelon varieties. Most of the seed germination and bacterial growth inhibiting potential was found in polar fractions of the seed coat extract. A greenhouse experiment was conducted to assess the impact of allelopathic and non-allelopathic watermelon accessions on weed growth. Clomazone is registered for use in watermelon; however, tolerance to the herbicide is marginal and the possibility of crop injury restricts its use by growers. Preliminary experiments demonstrated that there are differences among watermelon genotypes in clomazone tolerance. African watermelon lines belonging to the watermelon subspecies, citroides were shown to be highly tolerant to the herbicide in subsequent greenhouse and field experiments. The citroides lines readily cross with domestic watermelon. Research was initiated in collaboration with the watermelon breeder at the U.S. Vegetable Laboratory to evaluate the feasibility of breeding watermelon varieties with increase clomazone tolerance by transferring tolerance genes from the citroides lines utilizing conventional plant breeding techniques. The difference in clomazone response between susceptible and tolerant lines was established in greenhouse and field experiments, and lines homozygous for tolerance were generated from USPI accessions. A field study to assess the effect of cole crop residues on annual weed growth indicated that residues of collard, kale, mustard, and turnip do not provide adequate control of annual weeds.
Three cover crop cowpea germplasm lines (US1136, US-1137, and US-1138) developed for public release. Cover crops are important in organic and sustainable vegetable production to suppress weeds, provide nitrogen for subsequent rotational crops, and improve soil quality. Forage type cowpea is an excellent warm season cover crop species, because it tolerates hot and dry conditions, grows vigorously in low fertility soil, produces high biomass, fixes high amount of nitrogen, and suppresses weeds. Currently the only cowpea variety widely available for cover crop use is Iron Clay, and it produces seeds with impermeable seed coats which allow viable seeds to overwinter in the soil and create a weed problem in subsequent crops. A collection of cowpea genotypes were evaluated in comparison to Iron Clay, and after repeated field trials, three were selected for release as cover crop genotypes. Growth rates and biomass production of these lines were equal or superior to Iron Clay in field trials. None of the three lines produce seeds with impermeable seed coat. The adoption of these lines for use as a cover crop will eliminate the weed problem created by overwintering cowpea seeds and provide superior alternatives to Iron Clay for cover crop use.