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.
Howard F. Harrison, Judy A. Thies, Richard L. Fery, and J. Powell Smith
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.
Perry E. Nugent and P.D. Dukes
The southern root-knot nematode, Meloidogyne incognita [(Kofoid & White) Chitwood], causes serious economic losses to melon (Cucumis melo L.) production in the United States. The present study was conducted to determine if separable differences in nematode resistance of Cucumis melo could be found at some inoculum level. Five C. melo lines were compared with Cucumis metuliferus Naud. (C701A), a highly resistant species, for root necrosis, galling, egg mass production, and reproduction when inoculated at 0, 500, 1000, 2000, or 5000 nematode eggs per plant. Using these criteria, melon line C880 inoculated with 1000 eggs per plant was highly susceptible, while PI140471, PI 183311, and the cultivars Chilton, Georgia 47, Gulf Coast, Planters Jumbo, and Southland were less susceptible. In greenhouse tests with an inoculum level of 1000 eggs per plant, low levels of resistance were evident. A thorough screening of the available germplasm against M. incognita may identify higher levels of root-knot nematode resistance for incorporation into improved melon cultivars.
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.
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.
Susan L.F. Meyer
Two strains of the fungus Verticillium lecanii (A. Zimmermann) Viégas were studied as potential biocontrol agents for root-knot nematode (Meloidogyne incognita (Kofoid & White) Chitwood) on cantaloupe (Cucumis melo L.). For the study, pots were filled with soil that had been inoculated with M. incognita (inoculum was applied at two levels: 1000 and 5000 eggs/pot). Each fungus strain was applied individually by pouring an aqueous suspension (made from a wettable granule formulation) into the inoculated soil. Controls received water only. One cantaloupe seedling was then transplanted into each pot. Plants were grown for 55 days in the greenhouse, and then harvested and assessed for root and shoot growth and for nematode egg production. In pots inoculated with 1000 eggs/plant, neither fungus strain affected nematode egg numbers. At the 5000 eggs/plant inoculum level, both strains of the fungus suppressed egg numbers (counts were 28% and 31% less than water controls). Neither strain of V. lecanii affected the number of eggs embedded in root galls; the fungus suppressed nematode population numbers overall solely by affecting the number of eggs located outside of root tissues. Both fungus strains were also autoclaved and then applied to soil, to test for effects of nonviable fungus. In pots inoculated with 5000 eggs, application of one autoclaved strain resulted in a 35% suppression in egg numbers after 55 days, suggesting that the fungus produced a heat-stable substance deleterious to the nematode.
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.
Richard L. Fery and Judy A. Thies
Scotch Bonnet and Habanero peppers, extremely pungent cultivar classes of Capsicum chinense Jacq., are increasing in popularity in the United States. Because the southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood, is a major pest of many C. annuum cultivars, a series of greenhouse and field experiments was conducted to determine if Scotch Bonnet and Habanero peppers from available commercial and private sources also are vulnerable to the pest. In an initial greenhouse test, a collection of 59 C. chinense cultigens was evaluated for reaction to M. incognita race 3. All cultigens obtained from commercial sources were moderately susceptible or susceptible. However, four accessions obtained through Seed Savers Exchange listings exhibited high levels of resistance. Three of these cultigens (PA-353, PA-398, and PA-426) were studied in subsequent greenhouse and field plantings, and each was confirmed to have a level of resistance similar to that available in C. annuum. All three of the resistant cultigens are well-adapted and each is potentially useful in commercial production without further development. None of the Habanero cultigens was resistant to the southern root-knot nematode. The resistant Scotch Bonnet cultigens may serve as sources of resistance for development of root-knot nematode—resistant Habanero peppers.
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.
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.