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R.L. Fery, P.D. Dukes Sr., and J.A. Thies

The southern root-knot nematode (Meloidogyne incognita) is a major pest of bell peppers (Capsicum annuum) in the United States. Since none of the leading bell pepper cultivars grown in the U.S. exhibit adequate levels of resistance, a breeding program was initiated to incorporate the N root-knot nematode resistance gene into commercial bell pepper germplasm. A backcross breeding procedure was used. The donor parent of the N gene was the open-pollinated, pimiento pepper cultivar Mississippi Nemaheart, and the recurrent parents were the open-pollinated bell pepper cultivars Keystone Resistant Giant and Yolo Wonder. A large number of homozygous resistant BC6 populations were evaluated in field tests in 1995, and two lines (PA-440, an isoline of `Keystone Resistant Giant', and PA-453, an isoline of `Yolo Wonder') were selected for further field evaluation and seed multiplication in 1996. Results of replicated field and greenhouse tests conducted in 1996 indicate that root-knot nematode resistance has been incorporated successfully in `Keystone Resistant Giant' and `Yolo Wonder' backgrounds.

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W. R. Maluf, S. M. Azevedo, and V.P. Campos

Heritabilities for resistance to root knot nematodes (Meloidogyne javanica and Meloidogyne incognita races 1, 2, 3, and 4) were studied in a population of 226 sweetpotato clones of diverse origin. For each nematode isolate tested, 128-cell speedling trays were filled with previously inoculated substrate (30000 eggs/1000 mL substrate). Sweetpotato clones suitably tagged and identified were randomly planted in the cells (one plant/cell), with a total of four plants per clone per isolate. Ninety days after inoculation, sweetpotato plants had their roots washed for substrate removal, and treated with 150 mg·L–1 Phloxine B to stain nematode egg masses. The number of egg masses per root was recorded, and plants were accordingly assigned scores from 0 (highly resistant) to 5 (highly susceptible). Broad-sense heritability estimates were 0.87, 0.91, 0.81, 0.95, and 0.93 respectively for resistance to M. javanica and races 1, 2, 3, and 4 of M. incognita. The frequencies of resistant genotypes were higher for M. javanica and lower for M. incognita race 2. Genotypic correlations (rG) among the resistances to the various Meloidogyne isolates utilized were weak, ranging from 0.11 to 0.57, suggesting independent genetic controls. Clones could be selected, however, with high levels of resistance to all nematode isolates tested. (This work was supported by CNPq, CAPES, FAPEMIG, and FAEPE/UFLA.)

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Aref A. Abdul-Baki, Sanaa A. Haroon, and David J. Chitwood

Resistance to root-knot nematodes (Meloidogyne spp.) in tomato (Lycopersicon esculentum Mill.) plants has been reported to break down at soil temperatures >28C. We evaluated in vitro root explants of tomato heterozygous (Mimi), homozygous (MiMi) at the Mi locus, or lacking the Mi-1 gene for resistance to Meloidogyne incognita (Kofoid & White) Chitwood and Meloidogyne arenaria (Neal) Chitwood at 28, 31, 34, and 37C. Genotypes Ace-55 UF and Rutgers, lacking the dominant allele, were susceptible to M. incognita and M. arenaria at all temperatures. Genotypes possessing the dominant allele (heterozygous or homozygous) were equally resistant to both nematode species. The resistance level in these genotypes was maintained fully at 31C, partially maintained at 34C, and lost at 37C. Resistance in the heat-tolerant Mi-heterozygous accession CLN 475-BC1F2-265-4-19 was not different from that of the heat-sensitive genotypes. As temperature increased, the genotypes differed in their sensitivity to resistance conferred by the Mi-1 locus.

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

A 3-year field study was conducted at Blackville, S.C., to evaluate the potential of using resistant pepper (Capsicum annuum L.) cultivars as a rotation crop for managing the southern root-knot nematode [Meloidogyne incognita (Kofoid and White) Chitwood]. The experiment was a split-plot with main plots arranged in a randomized complete-block design. In 1993, the entire experimental site was infested with M. incognita by inoculating a planting of susceptible PA-136 cayenne pepper with eggs of M. incognita race 3. In 1994, the main plots were planted to either highly resistant `Carolina Cayenne' or its susceptible sibling line PA-136. In 1995, `Carolina Cayenne' and the susceptible bell cultivars California Wonder and Keystone Resistant Giant were grown as subplots in each of the original main plots. `Carolina Cayenne' plants were unaffected by the previous crop. Previous cropping history, however, had a significant impact on the performance of the bell cultivars; the mean galling response was less (P < 0.01) and the yield was 2.8 times greater (P < 0.01) in the main plots previously cropped with `Carolina Cayenne' than in those previously cropped with PA-136. These results suggest that resistant pepper cultivars have considerable merit as a rotation crop for managing M. incognita infestations in soils used for growing high-value vegetables.

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Richard L. Fery, Philip D. Dukes, and Judy A. Thies

A series of greenhouse and field studies was conducted over 9 years to characterize three new sources of resistance in cowpea [Vigna unguiculata (L.) Walp.] to the southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] and to determine if the resistances are conditioned by genes allelic to the Rk root-knot nematode resistance gene in `Mississippi Silver'. Three plant introductions (PI), PI 441917, PI 441920, and PI 468104, were evaluated for reaction to M. incognita in four greenhouse tests, and in every test each PI exhibited less galling, egg mass formation, or egg production than `Mississippi Silver'. F2 populations of the crosses between `Mississippi Silver' and each of the three resistant PIs were also evaluated for root-knot nematode resistance in a greenhouse test. None of the F2 populations segregated for resistance, indicating that PI 441917, PI 441920, and PI 468104 each has a gene conditioning resistance that is allelic to the Rk gene in `Mississippi Silver'. Our observations on the superior levels of resistances exhibited by PI 441917, PI 441920, and PI 468104 suggest that the allele at the Rk locus in these lines may not be the Rk allele, but one or more alleles that condition a superior, dominant-type resistance. The availability of additional dominant alleles would broaden the genetic base for root-knot nematode resistance in cowpea.

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Richard L. Fery and Judy A. Thies

Greenhouse experiments determined the inheritance of resistance to the peanut root-knot nematode [Meloidogyne arenaria (Neal) Chitwood race 1] in Capsicum chinense Jacq. germplasm lines PA-353 and PA-426. Evaluation of parental, F1, F2, and backcross populations of the crosses PA-353 × PA-350 and PA-426 × PA-350 (PA-350 is a susceptible cultigen) indicated that resistance in both C. chinense germplasm lines was conditioned by a single dominant gene. Evaluation of the F1 × resistant parent backcross populations in the cytoplasm of their respective resistant and susceptible parents indicated that the cytoplasm of the resistant parent is not needed for full expression of resistance. Allelism tests indicated that the dominant resistance gene in both PA-353 and PA-426 is allelic to a resistance gene in C. annuum L. `Carolina Cayenne'. However, these allelism tests did not demonstrate conclusively that the M. arenaria race 1 resistance gene in C. chinense is the N gene that conditions resistance to the southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] in C. annuum. The ease and reliability of evaluating plants for resistance to root-knot nematodes and the availability of simply inherited sources of resistance makes breeding for peanut root-knot nematode resistance a viable objective in C. chinense breeding programs.

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K. Ukoskit, P.G. Thompson, C.E. Watson Jr., and G.W. Lawrence

The inheritance of resistance to root-knot nematode race 3 [Meloidogyne incognita (Kofoid & White) Chitwood] in sweetpotato [Ipomoea batatas (L.) Lam.] was studied in 71 progenies of the F1 single-cross population produced from the cross of resistant parent `Regal' and susceptible parent `Vardaman'. The distribution frequency of the progenies based on log total nematode number (egg + juvenile counts) was 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 parent and rrrrrr = susceptible parent). Bulk segregant analysis in conjunction with the RAPD technique was used to identify a RAPD marker linked to a root-knot-nematode-resistance gene. Of 760 random decamer primers screened, 9 showed polymorphic bands between the two bulk DNA samples. Primer OPI51500 produced a band in the resistant bulk but not in the susceptible bulk, suggesting a linkage in coupling phase. An estimated recombination fraction of 0.2421 ± 0.057 between the marker and the root-knot-nematode-resistance gene indicated linkage.

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A. G. Hunter and O. L. Chambliss

Screening for resistance to blackeye cowpea mosaic virus (BlCMV) and rootknot nematode on the same plant is possible if the two pathogens do not interact significantly. To determine if such interactions were present four cultivars were planted in 72-cell styrofoam flats, with a combination of BlCMV and nematode inoculations (--, -+, +-, and ++). `Freezegreen' is known to be susceptible to both pathogens, `Mississippi Silver' is resistant to both, `California Blackeye #5' is susceptible to BlCMV, and `Worthmore' is resistant to BlCMV. Nematode treated seeds were inoculated at planting with 2,000 eggs of (Meloidogyne incognita Race 3); BlCMV was inoculated on primary leaves a week later. Plants were visually rated for symptoms: either negative or positive for BlCMV and 1-5, no galls and heavily galled respectively, for rootknot. Analyses of variance using percentage of plants negative for virus symptoms or average nematode score as the dependent variable, resulted in non-significant virus × nematode interactions. Results by cultivar indicated simultaneous screening did not change their resistance/susceptible classifications.

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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’.

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Judy A. Thies and Richard L. Fery

Expression of the N gene, which confers resistance to southern root-knot nematode (Meloidogyne incognita Kofoid and White) in bell pepper [(Capsicum annuum L. var. annuum (Grossum Group)], is modified at high temperatures (28 °C and 32 °C), but its expression in the heterozygous condition (Nn) has not been documented at moderate or high temperatures. Responses of the near-isogenic bell pepper cultivars, Charleston Belle and Keystone Resistant Giant (differing at the N locus), and the F1 and reciprocal F1 crosses between these cultivars to M. incognita race 3 were determined at 24, 28, and 32 °C in growth chamber experiments. `Keystone Resistant Giant' (nn) was susceptible at 24, 28, and 32 °C. `Charleston Belle' (NN) exhibited high resistance at 24 °C and resistance was partially lost at 28 and 32 °C. However, at 32 °C root gall and egg mass severity indices for `Charleston Belle' were still in the resistant range, and the number of M. incognita eggs per gram fresh root and reproductive index were 97% and 90% less, respectively, than for `Keystone Resistant Giant'. Responses of the F1 and F1 reciprocal hybrid populations to M. incognita were similar to the response of the resistant parent at all temperatures. Root fresh weights and top dry weights indicated that both hybrid populations tolerated M. incognita infections at least as well as `Charleston Belle'. These findings indicate that i) only one of the parental inbred lines needs to be converted to the NN genotype to produce F1 hybrid cultivars with fully functional N-type resistance to M. incognita; and ii) cytoplasmic factors are not involved in expression of N-type resistance and the resistant parental inbred can used to equal advantage as either the paternal or the maternal parent.