Howard F. Harrison

Forty-nine new host plant species for Bemisia tabaci (Hemiptera: Aleyrodidae)

The sweetpotato whitefly, Bemisia tabaci (Gennadius), is a worldwide pest of numerous agricultural and ornamental crops. In addition to directly feeding on plants, it also acts as a vector of plant viruses of cultivated and uncultivated host plant species. Moreover, host plants can affect the population dynamics of whiteflies. An open‐choice screening experiment was conducted with B‐biotype B. tabaci on a diverse collection of crops, weeds, and other indigenous plant species. Five of the plant species were further evaluated in choice or no‐choice tests in the laboratory. The results reveal 49 new reproductive host plant species for B. tabaci. This includes 11 new genera of host plants (Arenaria, Avena, Carduus, Dichondra, Glechoma, Gnaphalium, Molugo, Panicum, Parthenocissus, Trianthema, and Triticum) for this whitefly. All species that served as hosts were acceptable for feeding, oviposition, and development to the adult stage by B. tabaci. The new hosts include three cultivated crops [oats (Avena sativa L.), proso millet (Panicum miliaceum L.), and winter wheat (Triticum aestivum L.)], weeds and other wild species, including 32 Ipomoea species, which are relatives of sweetpotato [I. batatas (L.) Lam.)]. Yellow nutsedge, Cyperus esculentus L., did not serve as a host for B. tabaci in either open‐choice or no‐choice tests. The results presented herein have implications for whitefly ecology and the numerous viruses that B. tabaci spreads to and among cultivated plants.

High frequency oligonucleotides: targeting active gene (HFO-TAG) markers revealed wide genetic diversity among Citrullus spp. accessions useful for enhancing disease or pest resistance in watermelon cultivars

There is a continuous need to enhance watermelon cultivars for disease and pest resistance. Different U.S. Plant Introductions (PIs) of Citrullus lanatus subsp. lanatus var. lanatus [also known as C. lanatus (Thunb.) Matsum. et Nakai subsp. lanatus var. citroides (Bailey) Mansf. ex Greb.] (CLC) collected in southern Africa are a useful source for enhancing disease or pest resistance in watermelon cultivars. They are also valuable as rootstocks for grafted watermelon, particularly in fields infested with root-knot nematodes or Fusarium wilt. However, there is little information about genetic relationships among these PIs. In this study, genetic diversity was examined among 74 CLC PIs collected from their center of origin in southern Africa. Also, 15 Citrullus lanatus subsp. lanatus (CLL) PIs and the American heirloom cultivars Charleston Gray and Black Diamond (Citrullus lanatus subsp. vulgaris (Schrader ex Eckl. et Zeyh.) Fursa) (CLV) and five Citrullus colocynthis (L.) Schrader (CC) PIs collected in different locations throughout the world were used as out-groups in the phylogenetic analysis for the CLC PIs. Twenty-three high frequency oligonucleotides—targeting active gene (HFO-TAG) primers were used in polymerase chain reaction (PCR) experiments to produce a total of 562 polymorphic markers among the Citrullus PIs and cultivars. Cluster and multidimensional scaling plot analysis produced distinct groups of CLC, CLL, and CC PIs. Several PIs that were designated as CLC or CLL were in transitional positions, indicating that they are the result of gene flow between the major Citrullus groups or subgroups. Population structure analysis indicated that CLC comprises two subgroups; each containing a set of unique alleles. Also, unique alleles exist in the CLL and the CC genotypes. Overall, broad genetic diversity exists among the Citrullus PIs. The data in this study should be useful for identifying PIs with a wide genetic distance between them that could be used in breeding programs aiming to develop heterotic F1 hybrid rootstock lines for grafted watermelon.

Effects of a Killed-Cover Crop Mulching System on Sweetpotato Production, Soil Pests, and Insect Predators in South Carolina

Abstract 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.

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.

Insect Resistance in Sweetpotato Plant Introduction Accessions

ABSTRACT 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 boniatotype 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.

Differential response of collard and kale cultivars to preemergence application of metolachlor

Abstract The feasibility of using preemergence application of metolachlor for weed control in direct-seeded collard and kale was assessed by determining the response of eight collard and three kale cultivars to the herbicide in greenhouse and field experiments. The lowest doses tested, 0.75 and 1.5 kg ha−1 caused injury and reduced the seedling weights of some cultivars in the greenhouse and field; however, yield was reduced in the field only at doses of 3.0 kg ha−1 and only when seedling injury and weight reduction were severe. There were differences between cultivars in injury ratings and seedling weight reduction in the greenhouse and field. Shoot weights of one of the most tolerant cultivars, ‘Flash’ were not reduced at metolachlor doses that reduced the weights of one of the least tolerant cultivars, ‘Georgia’. These studies indicate the metolachlor at 1.5 kg ha−1 or lower can be used safely for weed control in direct-seeded collard and kale if susceptible cultivars are avoided.

Insect Resistance in Traditional and Heirloom Sweetpotato Varieties

ABSTRACT 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 (Chaetocnema 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.

Management of Sweet Potato Leaf Curl Virus in Sweetpotatoes Using Insecticides1,2

ABSTRACT Sweet potato leaf curl virus (SPLCV), which is transmitted by the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), species complex, can severely affect yields of sweetpotatoes, Ipomoea batatas (L.) Lam. (Convolvulaceae). This virus is endemic in sweetpotato fields at the U.S. Vegetable Laboratory (USVL), Charleston, S.C. In 2010 and 2011, experiments were conducted to determine if repeated insecticide applications were useful for protecting ‘Beauregard’ sweetpotato from SPLCV infection. In 2010, plots were untreated or treated twice weekly with imidacloprid. A row of SPLCV-infected sweetpotato genotype ‘W-258’ was planted between ‘Beauregard’ plots to serve as a source of whiteflies and SPLCV. A similar test was performed in 2011, except that the plots were sprayed only once a week, and a rotation of four insecticides (in the order of imidacloprid, pyriproxyfen, acetamiprid, and pymetrozine) was used. Yellow sticky traps were placed horizontally in the center of each plot at canopy height to monitor whitefly abundance. Leaf samples were taken every other week to test for SPLCV infection using real-time polymerase chain reaction (PCR) techniques. Over the two-year period, there were significantly fewer whiteflies on sticky cards in the sprayed treatment for only two of the 36 weekly samples, indicating that insecticides were largely ineffective in reducing whitefly populations moving into these plots. By the end of the growing season each year, all of the unsprayed plots were infected with SPLCV as determined by real-time PCR. However only about one-half of the sprayed plots were infected with SPLCV. This indicates that insecticides could be useful in protecting sweetpotatoes from SPLCV. The insecticide sprays would likely be more effective under normal production practices where sources of the virus are not in such close proximity to the uninfected crop.