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Richard L. Fery and Philip D. Dukes

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Richard L. Fery and Philip D. Dukes

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Maureen M.M. Fitch, Terryl C.W. Leong, Xiaoling He, Heather R.K. McCafferty, Yun J. Zhu, Paul H. Moore, Dennis Gonsalves, Herb S. Aldwinckle, and Howard J. Atkinson

., 2002 ). Nematode resistance was sought using genes reported to improve resistance, rice cystatin in banana ( Atkinson et al., 2004 ), and a dual construct, rice cystatin and cowpea trypsin inhibitor, in Arabidopsis ( Urwin et al., 1998 ). Materials

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Laurie E. Boyden* and Peter Cousins

Development of rootstocks resistant to root-knot nematodes (Meloidogyne spp.) is a priority in grape breeding. The N allele, present in Harmony and Freedom rootstocks, confers resistance to N-avirulent strains of Meloidogyne. Extensive planting of rootstocks containing N has led to the development of N-virulent nematode strains, prompting a search for new resistance alleles. A seedling population derived from Vitis nesbittiana Comeaux was evaluated to investigate the genetic control of nematode resistance in this species. Hybridization with easily propagated rootstock selections will be required to utilize nematode resistance found in V. nesbittiana, a native of Mexico. The female parent of the population was 161-49C, a V. riparia × V. berlandieri hybrid rootstock. 161-49C does not contribute nematode resistance to its progeny. The male parent was V. nesbittiana DVIT 2236.12, an accession held in the U.S. National Plant Germplasm system. Nematode resistance of the 161-49C × V. nesbittiana DVIT 2236.12 population was assessed in greenhouse pot culture. Seedling roots were stained in an eosin-Y solution six weeks after inoculation with 1500 N-virulent M. arenaria juveniles. Resistance classes of seedlings were determined by assessing the degree of galling and number of egg masses per root system. Segregation in the seedling population was consistent with a 1:1 ratio of resistance to susceptibility, indicating that the V. nesbittiana accession is heterozygous for a dominant allele conferring resistance to N-virulent root-knot nematodes. The genetic relationship between this allele and the N allele has yet to be determined.

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Richard L. Fery and Philip D. Dukes

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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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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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Tridate Khaithong, Brent S. Sipes, and Adelheid R. Kuehnle

Lack of a conclusive evidence of ingestion of plastid components by plant-parasitic nematodes cautions the use of plastid transformation technology for nematode resistance. Nematode-resistant effector proteins generally require ingestion by the nematode to be effective. Transgene-encoded proteins produced in plastids are not known to be exported into the cytoplasm. Disintegration of plant cell organelles after nematode feeding suggests that nematodes possibly ingest plastid components. Proof of ingestion will validate the development of plastid transformation for nematode resistance. Small subunit ribulose-1,5-bisphosphate carboxylase/oxygenase (SSRubisco) protein is prevalent in chloroplasts and thus chosen as a study molecule. The migratory endoparasitic nematodes Pratylenchus penetrans and Radopholus similis were cultured on green carrot callus containing chloroplasts. Total nematode proteins were extracted and subjected to western blot analysis using cross-reactive polyclonal antibody raised against spinach SSRubisco. Positive detection of SSRubisco occurred in protein extracts from nematodes fed on green carrot callus, but not in extracts from nematodes cultured on non-green alfalfa root callus as negative control. These results confirm the ingestion of plastid components of MW 14 kDA by migratory endoparasitic nematodes.

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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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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.