Insect resistance in traditional and heirloom sweetpotato varieties.

Fifty-nine sweetpotato cultivars, including 16 heirlooms, 11 near-heirlooms (developed in the 1960s and 1970s), 19 cultivars from the 1980s, and 13 modern varieties (since 1990), were evaluated for resistance to soil insects in field experiments during 2010-2011 at the U.S. Department of Agriculture-Agriculture Research Service, U.S. Vegetable Laboratory (USDA-ARS, USVL), Charleston, SC. These experiments included two insect-susceptible control cultivars ('Beauregard' and 'SC1149-19') and four insect-resistant control cultivars ('Charleston Scarlet,''Regal,' 'Ruddy,' and 'Sumor') that were developed by the USDA-ARS, USVL sweetpotato breeding program. Sweetpotato genotypes differed significantly in resistance measured by the overall percentage of injured roots, WDS (Wireworm, Diabrotica, and Systena) index, the percentage of roots damaged by the sweetpotato weevil (Cylas formicarius F.), the percentage of roots damaged by the sweetpotato flea beetle (Chaetonema confinis Crotch), and the percentage of roots damaged by white grub larvae (including Plectris aliena Chapin and Phyllophaga spp.). Twenty-three sweetpotato cultivars had a lower percentage of injured roots than the susceptible control genotype, SC1149-19, while 14 varieties had a lower percentage of injured roots than Beauregard, one of the leading commercial orange-fleshed cultivars in the United States. Over the 2-yr period, Ruddy (7.6%) had the lowest percentage of injured roots and 'Carolina Ruby' (84.6%) the highest percentage of injured roots. Carolina Ruby (1.07) also had the highest WDS index, but 15 genotypes had a significantly lower WDS index than either susceptible control, SC1149-19 (1.03) or Beauregard (0.82). Ruddy (0.07) and 'Murasaki-29' (0.09) had the lowest WDS indices. Forty-five genotypes had a significantly lower percentage infestation by flea beetles than SC1149-19 (12.3%), and the highest level of flea beetle infestation was for 'Bonita' (18.9%). The highest percent white grub infestation was for 'Caromex' (19.6%), however none of the genotypes had significantly less white grubs than the susceptible controls. The highest infestation of sweetpotato weevils was observed for SC1149-19 (17.9%), while 29 genotypes had significantly lower percentage of sweetpotato weevil infestation than SC1149-19. The moderate to high levels of resistance to soil insect pests exhibited by many of these traditional and heirloom cultivars may provide useful sources of germplasm for sweetpotato breeding programs.

Insect resistance in sweetpotato plant introduction accessions.

Fifty-five sweetpotato cultivars, experimental breeding clones, and plant introduction (PI) accessions were evaluated in 17 field experiments at the USDA, ARS, U.S. Vegetable Laboratory (Charleston, SC; 12 evaluations, 1997-2010), the Clemson University, Edisto Research and Education Center (Blackville, SC; two evaluations, 1998-1999), and the University of Florida, Tropical Research and Education Center (Homestead, FL; three evaluations, 2005-2007). These experiments included two insect-susceptible control entries ('Beauregard' and 'SC1149-19') and three insect-resistant control cultivars ('Regal,' 'Ruddy,' and 'Sumor'). At each location, genotypes differed significantly in the percentage of uninjured roots WDS (wireworm, Diabrotica, Systena) index, the percentage of roots damaged by the sweetpotato weevil (Cylas formicarius (F.)), the percentage of roots damaged by the sweetpotato flea beetle (Chaetocnema confinis Crotch), and the percentage of roots damaged by white grub larvae (including Plectris aliena Chapin and Phyllophaga spp.). 'SC1149-19' had a significantly lower percentage of uninjured roots, a significantly higher WDS index rating, and significantly higher percentages of infestation by flea beetles, grubs, and sweetpotato weevils than most other sweetpotato genotypes in this study. In addition, 43 of 55 genotypes had significantly less overall insect damage than 'Beauregard,' one of the leading commercial orange-fleshed cultivars in the United States. Ten genotypes had significantly less insect injury than 'Picadito,' a commercial boniato-type sweetpotato grown extensively in southern Florida. Many of these sweetpotato genotypes have high levels of resistance to soil insect pests, and they may be useful as sources of insect resistance for use in sweetpotato breeding programs.

Effects of a killed-cover crop mulching system on sweetpotato production, soil pests, and insect predators in South Carolina.

Sweetpotatoes, Ipomoea batatas (L.) Lam. (Convolvulaceae), are typically grown on bare soil where weeds and erosion can be serious problems. Conservation tillage systems using cover crop residues as mulch can help reduce these problems, but little is known about how conservation tillage affects yield and quality of sweetpotato or how these systems impact populations of beneficial and pest insects. Therefore, field experiments were conducted at the U.S. Vegetable Laboratory, Charleston, SC, in 2002-2004 to evaluate production of sweetpotatoes in conventional tillage versus a conservation tillage system by using an oat (Avena sativa L. (Poaceae)-crimson clover (Trifolium incarnatum L.) (Fabaceae) killed-cover crop (KCC) mulch. The four main treatments were 1) conventional tillage, hand-weeded; 2) KCC, hand-weeded; 3) conventional tillage, weedy; and 4) KCC, weedy. Each main plot was divided into three subplots, whose treatments were sweetpotato genotypes: 'Ruddy', which is resistant to soil insect pests; and 'SC1149-19' and 'Beauregard', which are susceptible to soil insect pests. For both the KCC and conventional tillage systems, sweetpotato yields were higher in plots that received hand weeding than in weedy plots. Orthogonal contrasts revealed a significant effect of tillage treatment (conventional tillage versus KCC) on yield in two of the 3 yr. Ruddy remained resistant to injury by soil insect pests in both cropping systems; and it consistently had significantly higher percentages of clean roots and less damage by wireworm-Diabrotica-Systena complex, sweetpotato flea beetles, grubs, and sweetpotato weevils than the two susceptible genotypes. In general, injury to sweetpotato roots by soil insect pests was not significantly higher in the KCC plots than in the conventionally tilled plots. Also, more fire ants, rove beetles, and carabid beetle were captured by pitfall traps in the KCC plots than in the conventional tillage plots during at least 1 yr of the study. This study suggests that a sweetpotatoes can be successfully grown under a killed-cover crop production system.

United States Department of Agriculture-Agricultural Research Service research on natural products for pest management.

Recent research of the Agricultural Research Service of USDA on the use of natural products to manage pests is summarized. Studies of the use of both phytochemicals and diatomaceous earth to manage insect pests are discussed. Chemically characterized compounds, such as a saponin from pepper (Capsicum frutescens L), benzaldehyde, chitosan and 2-deoxy-D-glucose are being studied as natural fungicides. Resin glycosides for pathogen resistance in sweet potato and residues of semi-tropical leguminous plants for nematode control are also under investigation. Bioassay-guided isolation of compounds with potential use as herbicides or herbicide leads is underway at several locations. New natural phytotoxin molecular target sites (asparagine synthetase and fructose-1,6-bisphosphate aldolase) have been discovered. Weed control in sweet potato and rice by allelopathy is under investigation. Molecular approaches to enhance allelopathy in sorghum are also being undertaken. The genes for polyketide synthases involved in production of pesticidal polyketide compounds in fungi are found to provide clues for pesticide discovery. Gene expression profiles in response to fungicides and herbicides are being generated as tools to understand more fully the mode of action and to rapidly determine the molecular target site of new, natural fungicides and herbicides.

Quantity and potential biological activity of caffeic acid in sweet potato [Ipomoea batatas (L.) Lam.] storage root periderm.

The caffeic acid content of storage root periderm and cortex tissues of genetically diverse sweet potato [Ipomoea batatas (L.) Lam.] cultivars and breeding clones was quantified by high-performance liquid chromatography. Periderm caffeic acid content of the clones ranged from 0.008 to 7.97 mg/g dry weight, whereas the highest cortex content was 0.047 mg/g. Clones varied greatly in periderm caffeic acid content in all experiments, but there were also differences between experiments in content averaged for all clones. This indicates that periderm caffeic acid content is subject to genetic and environmental influences. Caffeic acid inhibited the growth of four sweet potato pathogenic fungi and germination of proso millet seeds in bioassays. Inhibitory activity in the bioassays suggests that high periderm caffeic acid levels contribute to the storage root defense chemistry of some sweet potato genotypes.

Increase in Populations of Rhizoctonia solani and Wirestem of Collard with Velvet Bean Cover Crop Mulch.

Velvet bean has been used traditionally as a summer cover crop in the southeastern United States. We investigated the use of killed velvet bean as a cover crop mulch left on the soil surface before collard was transplanted in the fall. Control treatments were weed-free fallow and velvet bean that was killed and disked into the soil before transplanting. Incidence of wirestem, caused by Rhizoctonia solani, reached a maximum of 25% in 2000 but only 4% in 2001 in cover crop mulch treatments. Nevertheless, in both years, the infection rate, area under the disease progress curve, and final incidence were significantly greater with cover crop mulch than in the fallow or disked treatments. Wirestem incidence did not differ between the disked and fallow treatments in either year. Populations of R. solani in soil were greater after cover crop mulch than in fallow plots in both years and greater in the disked treatment than in fallow soil in 2000 but not 2001. Velvet bean does not appear to be suitable as an organic mulch for fall collard production, but could be used as a summer cover crop if disked into the soil before transplanting collard.