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Howard F. Harrison, Judy A. Thies, Richard L. Fery and J. Powell Smith

A preliminary screening experiment was conducted to evaluate 47 cowpea [Vigna unguiculata (L.) Walp.] genotypes for use as a weed-suppressing cover crop. Of these, 11 were selected for further testing on the basis of vigorous growth and weed-suppressing ability. In a field experiment repeated over 4 years, the selected genotypes were not different from the leading cover crop cultivar `Iron Clay' in biomass production. Vigor ratings, vine growth ratings, and canopy widths of some genotypes exceeded those of `Iron Clay' Vigor ratings and canopy measurements were efficient selection criteria that could be useful for breeding cover crop cowpea cultivars. All except one selection were highly resistant to southern root knot nematode [Meloidogyne incognita (Kofoid and White) Chitwood], and the selections varied in seed size, photoperiod, and response to foliar diseases.

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J. A. Thies and A. Levi

Root-knot nematodes (Meloidogyne incognita, M. arenaria, and M. javanica) cause severe damage to watermelon and resistance has not been identified in any watermelon cultivar. In greenhouse tests, we evaluated 265 U.S. plant introductions (PIs) for nematode resistance (based on root galling and nematode reproduction), and identified 22 PIs of Citrullus lanatus var. citroides as moderately resistant to M. arenaria race 1. In subsequent tests, these 22 PIs exhibited low to moderate resistance to M. incognita race 3 and M. arenaria race 2. Three watermelon (C. lanatus var. lanatus) cultivars (Charleston Gray, Crimson Sweet, and Dixie Lee), three C. colocynthis PIs, and four C. lanatus var. citroides PIs, all previously shown to be susceptible to M. arenaria race 1, were susceptible to M. incognita race 3 and M. arenaria race 2. The C. lanatus var. citroides PIs that are most resistant to both M. incognita and M. arenaria should be useful sources of resistance for developing root-knot nematode resistant watermelon cultivars.

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Zhen-Xiang Lu, G.L. Reighard, W.V. Baird, A.G. Abbott and S. Rajapakse

Eighteen peach rootstock cultivars, most of Prunus persica (L.) Batsch, were screened for diagnostic random amplified polymorphic DNA (RAPD) markers using synthetic decamer oligonucleotide primers. Twenty of the 80 primers were informative, and 40 amplified DNA bands from the informative primers were selected as RAPD markers. Based on combined banding patterns, all 18 rootstock cultivars were identified with only six of the 20 informative primers. Cluster analysis of the 18 peach rootstock cultivars using 40 RAPD markers produced a dendrogram of genetic relatedness in good agreement with their putative pedigrees. The first major bifurcation in the dendrogram divided these rootstock cultivars into two groups according to their resistance or susceptibility to root-knot nematodes [Meloidogyne incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood].

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Kittipat Ukoskit, Paul G. Thompson, Gary W. Lawrence and Clarence E. Watson

The inheritance of root-knot nematode race 3 [Meloidogyne incognita (Kofoid & White) Chitwood] resistance was studied in 71 progenies of the F1 backcross population produced from the resistant parent `Regal' and the susceptible parent `Vardaman'. The distribution frequency of the progenies measured on total nematode number (eggs + juveniles) indicated a bimodal distribution with a ratio of 4 resistant: 1 susceptible. Based on this phenotypic ratio, the proposed genetic model was duplex polysomic inheritance (RRrrrr = resistant). Bulk segregant analysis in conjunction with the RAPD technique was employed to identify RAPD marker linked to the root knot nematode-resistant gene. Nine of 760 random decamer primers screened showed polymorphic bands. Primer OPI51500 produced a band in the resistant bulk, but not in the susceptible bulk. Estimated recombination frequency of 0.24 between the OPI51500 marker and the root-knot nematode-resistant gene indicated linkage.

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A.G. Hunter, O.L. Chambliss and J.C. Williams

Four southernpea (cowpea) [Vigna unguiculata (L.) Walp.] cultivars representing various combinations of resistance and susceptibility to blackeye cowpea mosaic virus (BlCMV) and southern root-knot nematode [Meloidogyne incognita (Kofoid and White) Chitwood] were used to determine effectiveness of simultaneous screening of plants for resistance to both pathogens. Plants were inoculated with both pathogens simultaneously, each pathogen separately, or left uninoculated as controls. The resistance classification of the cultivars based on treatments with only one pathogen was not different from that based on the treatment with both pathogens. Virus × nematode interaction was not a significant source of variation in BlCMV symptoms and root-knot nematode galls. Simultaneous screening for both pathogens in southernpeas appears to be a feasible option.

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Freddi A. Hammerschlag, Ghazala Hashmi, Robin Huettel, Dennis Werner and David Ritchie

One approach for obtaining useful genetic variation is to select for somaclonal variants generated by tissue culture techniques. Increased levels of resistance to bacterial leaf spot (Xanthomonas campestris pv. pruni) have been observed in toxin-selected and unselected peach regenerants in vitro, in the greenhouse and under field conditions. Peach regenerants have also demonstrated increased levels of bacterial canker (Pseudomonas syringae pv. syringae) and root-knot nematode (Meloidogyne incognita) resistance. Random amplified polymorphic DNA (RAPD) primers have been used to study genetic variation at the DNA level among the somaclonal variants. Sixty RAPD primers (10-mers) were screened and 10 proved useful as markers to detect polymorphisms, thus establishing a genetic basis for somaclonal variation. These studies demonstrate the feasibility of using tissue culture techniques to generate fruit trees with increased levels of disease resistance.

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J.A. Thies, J.D. Mueller and R.L. Fery

The southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] is a serious pest of pepper (Capsicum annuum L.). Currently, methyl bromide is used for nematode control, but the pending withdrawal of this fumigant from the United States market has resulted in a need for effective alternative root-knot nematode management measures. We evaluated the effectiveness of resistance of `Carolina Cayenne' relative to the susceptible genotypes `Early Calwonder' and PA-136 in greenhouse, microplot, and field studies. In all tests, `Carolina Cayenne' exhibited exceptionally high resistance (minimal galling, minimal nematode reproduction, and no yield reduction) to M. incognita; `Early Calwonder' and PA-136 were highly susceptible. In a test conducted in a heavily infested field, `Carolina Cayenne' outyielded PA-136 by 339%. The exceptionally high resistance exhibited by `Carolina Cayenne' provides an alternative to methyl bromide and other fumigant nematicides for managing root-knot nematodes in pepper.

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Kathryn E. Brunson, Sharad C. Phatak, J. Danny Gay and Donald R. Summer

Velvetbean (Mucuna deeringiana L.) was used in crop rotation to determine the influence on southern root-knot nematode (Meloidogyne incognita) in sustainable vegetable production. Replicated trials were conducted at four locations. Two cover crop treatments, crimson clover and subterranean clover, were used in the sustainable plots and rye was the plow-down cover crop for the conventional plots. Selected as the vegetable crops were tomato, pepper, and eggplant. Following the final harvest, velvetbean was planted into the sustainable plots and disked under after 90 days. Results from soil samples before and after velvetbean, indicated the sustainable plots had substantially reduced nematode densities, while most conventional plots showed increases. A correlation between location, treatment, root-gall indexes and nematode density occurred in all crops for 1992. In 1993 there was only a correlation between root-gall index and nematode density in pepper. However, root-gall indexes were significant for location and treatment in all crops.

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Kathryn E. Brunson, Sharad C. Phatak, J. Danny Gay and Donald R. Sumner

Velvetbean (Mucuna deeringiana L.) has been used as part of the crop rotation in low-input vegetable production in southern Georgia to help suppress populations of root-knot nematode (Meloidogyne incognita) for the past 2 years. Over-wintering cover crops of crimson and subterranean clovers were used the low-input plots and rye was the plow-down cover crop in the conventional plots. Tomatoes, peppers, and eggplant were the vegetable crops grown in these production systems. Following the final harvest in 1992, use of nematicides in the low-input plots was discontinued and velvetbean was then planted into the low-input plots and disked in after 90 days. Results from the 1993–94 soil samples taken before and after velvetbean showed a continuing trend of reduced nematode numbers where velvetbean had been, while most conventional plots that had nematicides applied resulted in increases in nematode populations.

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G. Craig Yencho, Kenneth V. Pecota, Jonathan R. Schultheis, Zvezdana-Pesic VanEsbroeck, Gerald J. Holmes, Billy E. Little, Allan C. Thornton and Van-Den Truong

‘Covington’ is an orange-fleshed, smooth-skinned, rose-colored, table-stock sweetpotato [Ipomoea batatas (L.) Lam.] developed by North Carolina State University (NCSU). ‘Covington’, named after the late Henry M. Covington, an esteemed sweetpotato scientist at North Carolina State, was evaluated as NC98-608 in multiple state and regional yield trials during 2001 to 2006. ‘Covington’ produces yields equal to ‘Beauregard’, a dominant sweetpotato variety produced in the United States, but it is typically 5 to 10 days later in maturity. ‘Covington’ typically sizes its storage roots more evenly than ‘Beauregard’ resulting in fewer jumbo class roots and a higher percentage of number one roots. Total yields are similar for the two clones with the dry matter content of ‘Covington’ storage roots typically being 1 to 2 points higher than that of ‘Beauregard’. ‘Covington’ is resistant to fusarium wilt [Fusarium oxysporum Schlect. f.sp. batatas (Wollenw.) Snyd. & Hans.], southern root-knot nematode [Meloidogyne incognita (Kofoid & White 1919) Chitwood 1949 race 3], and moderately resistant to streptomyces soil rot [Streptomyces ipomoeae (Person & W.J. Martin) Wakswan & Henrici]. Symptoms of the russet crack strain of Sweet Potato Feathery Mottle Virus have not been observed in ‘Covington’. The flavor of the baked storage roots of ‘Covington’ has been rated as very good by standardized and informal taste panels and typically scores as well or better in this regard when compared with ‘Beauregard’.