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J.C. Cervantes-Flores, G.C. Yencho and E.L. Davis

Sweetpotato [Ipomoea batatas (L.) Lam.] genotypes were evaluated for resistance to North Carolina root-knot nematode populations: Meloidogyne arenaria (Neal) Chitwood races 1 and 2; M. incognita (Kofoid & White) Chitwood races 1, 2, 3, and 4; and M. javanica (Treub) Chitwood. Resistance screening was conducted using 150-cm3 Conetainers containing 3 sand: 1 soil mix. Nematode infection and reproduction were assessed as the number of egg masses produced by root-knot nematodes per root system. Host suitability for the root-knot nematode populations differed among the 27 sweetpotato genotypes studied. Five genotypes (`Beauregard', L86-33, PDM P6, `Porto Rico', and `Pelican Processor') were selected for further study based on their differential reaction to the different root-knot nematodes tested. Two African landraces (`Tanzania' and `Wagabolige') were also selected because they were resistant to all the nematode species tested. The host status was tested against the four original M. incognita races, and an additional eight populations belonging to four host races, but collected from different geographical regions. The virulence of root-knot nematode populations of the same host race varied among and within sweetpotato genotypes. `Beauregard', L86-33, and PDM P6 were hosts for all 12 M. incognita populations, but differences in the aggressiveness of the isolates were observed. `Porto Rico' and `Pelican Processor' had different reactions to the M. incognita populations, regardless of the host race. Several clones showed resistance to all M. incognita populations tested. These responses suggest that different genes could be involved in the resistance of sweetpotato to root-knot nematodes. The results also suggest that testing Meloidogyne populations against several different sweetpotato hosts may be useful in determining the pathotypes affecting sweetpotato.

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William B. Rutter, Chandrasekar S. Kousik, Judy A. Thies, Mark W. Farnham and Richard L. Fery

PA-593 is a new sweet cherry-type pepper line containing the N gene, providing resistance to the most prevalent root-knot nematodes (RKN) in the southern United States. PA-593 has shown comparable growth, fruit, and yield characteristics to

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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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R.L. Fery and J.A. Thies

The peanut root-knot nematode (Meloidogyne arenaria race 1) is potentially a major pest of pepper cultivars belonging to the species Capsicum chinense. Greenhouse tests were conducted to: 1) compare the level of resistance to the peanut root-knot nematode exhibited by the recently released C. chinense germplasm line PA-353 to that exhibited by the C. annuum cv. Carolina Cayenne; 2) to determine the inheritance of the resistance in the C. chinense germplasm line PA-353; and 3) to determine the genetic relationship between the resistance exhibited by the C. chinense germplasm line PA-353 and that exhibited by the C. annuum cv. Carolina Cayenne. The level of resistance exhibited by the C. chinense germplasm line PA-353 was equal to the high level of resistance of the C. annuum cv. Carolina Cayenne. Evaluation of parental, F1, F2, and backcross populations of the cross between the resistant C. chinense germplasm line PA-353 and the susceptible C. chinense accession PA-350 indicated that the resistance in C. chinense is conditioned by a single dominant gene. The F2 population of the interspecific cross between the resistant C. chinense germplasm line PA-353 and the resistant C. annuum cv. Carolina Cayenne did not segregate for resistance, indicating that the dominant resistance gene in C. chinense is likely allelic to or closely linked to a gene conditioning resistance in C. annuum. The availability of a simply inherited source of outstanding resistance makes breeding for peanut root-knot nematode resistance a viable objective in C. chinense breeding programs.

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Jorge Pinochet, Cinta Calvet, Adriana Hernández-Dorrego, Ariadna Bonet, Antonio Felipe and Marian Moreno

Two trials involving 20 Prunus rootstocks were conducted under greenhouse conditions to screen for resistance to root-knot nematode [Meloidogyne javanica (Treub.) Chitwood]. Many of the tested materials are interspecific hybrid rootstocks and represent new commercial peach (P. persica Batsch) and plum (Prunus sp.) releases or experimental genotypes of Spanish, French, and Italian origin. In the first trial, the rootstocks Adesoto 101 (P. insititia L.), Bruce (P. salicina Lindl. × P. angustifolia Marsh.), Ishtara, AC-952 (P. insititia), Garnem [P. dulcis (Mill.) D.A. Webb × P. persica], Cadaman [P. persica × P. davidiana (Carr.) Franch], and Orotava (P. salicina) were immune or resistant to a mixture of 10 isolates of M. javanica. The remaining rootstocks, Myrocal (P. cerasifera Ehr.), Montclar (P. persica), and Adafuel (P. dulcis × P. persica), were susceptible. In the second screening trial, the plum rootstocks Adesoto 101, Adara (P. cerasifera), Myro-10 (P. cerasifera), Constantí (P. domestica L.), and AD 105 (P. insititia) were immune to the root-knot nematode. Cadaman, G × N No. 17 (P. dulcis × P. persica), and Tetra (P. domestica) were resistant. Laroda F1OP (P. salicina), Myro-almond (P. cerasifera × P. dulcis), and the peach–almond hybrids Mayor, Adafuel, and Sirio were susceptible.

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Min Wang and I.L. Goldman

The genetics of resistance to root-knot nematode (M. hapla Chitwood) was studied in crosses of three carrot inbred genotypes, two resistant genotypes (R1 and R2) and one susceptible genotype (S1) identified in previous screening tests. Seedlings of three parental genotypes, six F1 crosses including three reciprocal crosses, two BC1 populations, and three F2 populations were evaluated for their resistance and susceptibility to infestation of M. hapla Chitwood based on gall number per root, gall rating per root, and root rating per root in a greenhouse experiment carried out in 1994. All six F1 plants were susceptible, which indicated a lack of heterosis for resistance in these F1s. The R1 × S1 cross segregated 3 susceptible: 1 resistant in the F2, 1 susceptible: 1 resistant in the BC1R1, and did not segregate in the BC1S1. The R1 × R2 cross yielded 44 susceptible: 36 resistant seedlings in the F2 (R1R2), and 48 susceptible: 32 resistant in the reciprocal cross of R1 and R2, both of which closely fit a 9: 7 ratio (P ≤ 0.001). These results indicate these two resistant genotypes carry two different homozygous recessive genes conditioning root-knot nematode resistance. We propose a model of duplicate recessive epistasis control the reactions of host plants and nematode in these crosses.

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J. Farías-Larios, J.G. López-Aguirre, J.L. Miranda and L.A. Bayardo-Vizcaino

Acerola (Malpighia glabra L.) is a small, red fruit that is native to the West Indies, but is also grown in South and Central America. In western Mexico, this crop is very important because acerola is the richest known natural source of vitamin C, with a content of 1000 to 4500 mg/100 g of fruit. In nursery and field conditions, acerola growth is severely affected by root-knot nematode. The objective of this study was to evaluate the use of commercial formulations of Bacillus spp. on root-knot nematode management. This study was carried out in the Farm Santa Clara Maria in Colima State. Acerola plants, 60 days old were used. They were grown in 3-L pots with soil, compost, and pumice stone mixture as substrate. Treatments evaluated were: 5, 10, 15 and 30 mL/pot of Activate 2001, Tri-Mat (5 mL/pot) and control, without application. Activate 2001® is a concentrated liquid in water suspension of Bacillus chitinosporus, B. laterosporus, and B. licheniformis. Initial nematode population was of 3,305 in 50 g of roots. Acerola plants were harvested at 30, 60, and 90 days after application. Results show that Activate 2001 at 10 and 30 mL rates reduce significantly root-knot populations in acerola plants 60 days after application with 135 and 178 nematodes/50 g of roots, respectively. Diameter stem, shoot fresh and dry weight and root production were also increased by rhizobacteria application. These results are promising and confirmed the potential of Bacillus as a biological agent for nematode management.

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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 and Richard L. Fery

Heat stability of the N gene that confers resistance to the southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood in pepper (Capsicum annuum L.), was determined at 24, 28, and 32°C. Responses of resistant bell pepper cultivars Charleston Belle and Carolina Wonder (homozygous for the N gene) and their respective susceptible recurrent backcross parents, `Keystone Resistant Giant' and `Yolo Wonder B', to M. incognita were compared. Numbers of eggs/g fresh root, reproductive factor of M. incognita, numbers of second-stage juveniles in soil, egg mass production, and root galling increased (P < 0.05) for all cultivars as temperature increased. The response of the resistant cultivars to temperature increase was less dramatic than the response of the susceptible cultivars. Both `Charleston Belle' and `Carolina Wonder' exhibited a partial loss of resistance at 28 and 32 °C. Reproduction of M. incognita was minimal on the resistant cultivars at 24 °C, but increased at higher temperatures. However, at 32 °C reproduction of M. incognita on the resistant cultivars was only 20% of that on the susceptible cultivars and root gall indices were within the range considered moderately resistant. Unlike the susceptible cultivars, the shoot dry weights of the resistant cultivars were not suppressed at 32 °C. This suggests that `Charleston Belle' and `Carolina Wonder' may be somewhat tolerant to M. incognita at high soil temperatures. Although results indicate a partial loss of resistance occurred in `Charleston Belle' and `Carolina Wonder' under high soil temperatures, resistant cultivars may be a useful component of cropping systems designed to manage M. incognita in hot climates.

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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.