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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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Anne M. Gillen and Fred A. Bliss

An F2 population from a single F1 plant from the cross of peach [Prunus persica (L.) Batsch] rootstock cultivars Harrow Blood (HB) × Okinawa (Oki) was used to locate the Mi locus, which conditions resistance to Meloidogyne incognita (race 1) (Kofoid and White) Chitwood. These data and comparison of common markers among published genetic linkage maps placed the Mi locus on Prunus L. linkage group 2. Two restriction fragment length polymorphisms (RFLPs) [linked at 4.8 and 6.8 centimorgan (cM), repulsion phase] and one random amplified polymorphic DNA (RAPD) marker (linked at 9.5 cM, coupling phase) were linked to Mi. The RAPD marker was cloned, sequenced, and converted to a polymerase chain reaction (PCR)-based cleaved amplified polymorphic sequence (CAPs) marker. Clones of resistance gene analogs (RGA) developed from Oki were highly polymorphic when used as RFLP probes. The RGA's mapped to four linkage groups but clustered on two of the four linkage groups, providing limited coverage of the genome. Even so, they may be useful as markers for disease resistance genes that occur in other populations. The linkage maps of the HB × Oki F2 population and a peach × almond (Prunus amygdalus Batsch) F2 population were colinear in certain regions, however, a significant number of markers mapped to different linkage groups among the two populations. The locus for the blood-flesh trait (red-violet mesocarp) mapped to the top of linkage group 4.

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Richard L. Fery and Judy A. Thies

Greenhouse tests were conducted to compare the levels of resistance to the southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] exhibited by recently released Capsicum chinense Jacq. Scotch Bonnet-type germplasm lines PA-353, PA-398, and PA-426 to the levels of resistance exhibited by C. annuum L. `Carolina Cayenne' and `Mississippi Nemaheart'; to determine the inheritance of the resistance in C. chinense germplasm line PA-426; and to determine the genetic relationship between the resistances exhibited by C. chinense germplasm line PA-426 and C. annuum `Carolina Cayenne'. The results of a replicated test indicated that the level of resistances exhibited by the resistant released C. chinense germplasm lines is equal to the level of resistances exhibited by the resistant C. annuum cultivars. Evaluation of parental, F1, F2, and backcross populations of the cross PA-426 × PA-350 (a susceptible Habanero-type C. chinense cultigen) indicated that the resistance in C. chinense is conditioned by a single dominant gene. The results of an allelism test indicated that this dominant gene is allelic to the dominant gene that conditions much of the southern root-knot nematode resistance in the C. annuum `Carolina Cayenne'. The ease and reliability of evaluating plants for resistance to root-knot nematode and the availability of a simply inherited source of outstanding resistance makes breeding for southern root-knot nematode resistance a viable objective in C. chinense breeding programs.

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Susan L.F. Meyer, Inga A. Zasada, Mario Tenuta and Daniel P. Roberts

The biosolid soil amendment N-Viro Soil (NVS) and a Streptomyces isolate (S 99-60) were tested for effects on root-knot nematode [RKN (Meloidogyne incognita)] egg populations on cantaloupe (Cucumis melo). Application of 3% NVS (dry weight amendment/dry weight soil) in the soil mixture resulted in significant (P ≤ 0.01) suppression of RKN egg numbers on cantaloupe roots compared to all other treatments, including 1% NVS and untreated controls. Ammonia accumulation was higher with the 3% NVS amendment than with any other treatment. Adjustment of soil pH with calcium hydroxide [Ca(OH)2] to the same levels that resulted from NVS amendment did not suppress nematode populations. When cultured in yeast-malt extract broth and particularly in nutrient broth, S 99-60 was capable of producing a compound(s) that reduced RKN egg hatch and activity of second-stage juveniles. However, when this isolate was applied to soil and to seedling roots, no suppression of RKN egg populations was observed on cantaloupe roots. Combining S 99-60 with NVS or Ca(OH)2 did not result in enhanced nematode suppression compared to treatments applied individually. The results indicated that NVS application was effective at suppressing RKN populations through the accumulation of ammonia to levels lethal to the nematode in soil.

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Judy A. Thies, Richard F. Davis, John D. Mueller, Richard L. Fery, David B. Langston and Gilbert Miller

Root-knot nematode-resistant `Charleston Belle' bell pepper (Capsicum annuum L. var. annuum) and metam sodium treatment were evaluated for managing the southern root-knot nematode [Meloidogyne incognita (Chitwood) Kofoid and White] in fall-cropped cucumber (Cucumis sativus L.). `Charleston Belle' and its susceptible recurrent parent, `Keystone Resistant Giant', were planted as spring crops at Blackville, S.C., and Tifton, Ga. `Charleston Belle' exhibited high resistance and `Keystone Resistant Giant' was susceptible at both locations. After termination of the bell pepper crop, one-half of the plots were treated with metam sodium delivered through the drip irrigation system. Cucumber yields and numbers of fruit were highest for cucumber grown in plots treated with metam sodium following either `Charleston Belle' or `Keystone Resistant Giant'; however, root gall severity and numbers of M. incognita eggs in the roots were lowest for cucumber grown in plots treated with metam sodium following `Charleston Belle'. Conversely, root gall severity and nematode reproduction were highest for cucumber grown in plots following `Keystone Resistant Giant' without metam sodium treatment. Application of metam sodium through the drip irrigation system following a spring crop of root-knot nematode-resistant bell pepper should reduce severity of root galling and reproduction of M. incognita as well as increase fruit yield of fall-cropped cucumber.

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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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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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Peter Cousins and M. Andrew Walker

The grape Vitis champinii Planchon is one source of nematode resistance in grape rootstocks. Several selections valued for their resistance to the root-knot nematode (Meloidogyne incognita), a serious pest of grape production, are used as rootstocks and in rootstock variety development. However, V. champinii-based rootstock varieties are faulted for their excess vigor and susceptibility to other root pests. Root-knot nematode populations with the ability to damage important V. champinii-based rootstocks have been identified and may become more common. Other V. champinii accessions might be sources of nematode resistance genes with different specificities or might have more suitable horticultural characteristics than V. champinii varieties in commercial use. Nine V. champinii accessions from the National Clonal Germplasm Repository, Davis, Calif., and a V. champinii rootstock variety were screened for resistance to M. incognita. Resistance was assessed by counting eggs produced per root system. Eight of ten V. champinii accessions did not support nematode reproduction. Susceptible accessions supported lower nematode reproduction than susceptible V. vinifera control varieties. Progeny testing from crosses of resistant and susceptible accessions suggests that a dominant and a recessive gene may condition root-knot nematode resistance.

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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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R.L. Fery and J.A. Thies

Scotch Bonnet and Habanero peppers, extremely pungent cultivar classes of Capsicum chinense, are becoming popular in the United States. Since the southern root-knot nematode (Meloidogyne incognita) is a major pest of many C. annuum cultivars commonly grown in the United States, a series of greenhouse and field studies was conducted to determine whether Scotch Bonnet and Habanero peppers also are vulnerable to the pest. An effort was made to collect Scotch Bonnet and Habanero seeds from all available commercial and private sources. In an initial greenhouse test, a collection of 59 C. chinense accessions was evaluated for reaction to M. incognita (race 3). All accessions obtained from commercial sources were moderately susceptible or susceptible. However, four accessions obtained via Seed Savers Exchange listings exhibited high levels of resistance. Three of these accessions (identified by the seed sources as Yellow Scotch Bonnet, Jamaica Scotch Bonnet, and Red Habanero) were studied in subsequent greenhouse and field plantings, and each was confirmed to have a level of resistance similar to the level of resistance exhibited by the C. annuum cv. Mississippi Nemaheart. Each of the resistant lines has good fruit and yield characteristics. The two Scotch Bonnet accessions produce yellow, bonnet-shaped fruit. The Red Habanero accession does not produce the lantern-shaped fruit typical of Habanero cultivars; the fruit have a bonnet shape.