Increased susceptibility to bacterial wilt in tomatoes by nematode galling and the role of the Mi gene in resistance to nematodes and bacterial wilt

The soil-borne bacterial pathogen Ralstonia solanacearum commonly coexists with polyspecific nematode populations in tropical and subtropical areas. The wounding of roots by nematodes is usually invoked to explain the correlation between nematode infection and bacterial wilt, since this wounding increases the number of sites for bacterial entry. Bacterial wilt development on tomato was investigated in a controlled environment on the susceptible tomato cultivar Floradel and the polygenically wilt-resistant cultivar Caraïbo. The bacterial isolate GMI 8217 and two different sedentary plant parasitic nematodes, the endoparasitic root-knot nematode, Meloidogyne incognita, and semi-endoparasitic reniform nematode, Rotylenchulus reniformis, were cross-infected. At low temperatures (22-27°C), the bacterium GMI 8217 was slightly pathogenic on all tomato lines, except on Floradel coinfected by root-knot nematode. At high temperatures (27-32°C), the root-knot nematode greatly increased wilt severity in susceptible Floradel and resistant Caraïbo, but the reniform nematode had no such effect regardless of temperature x cultivar combination. This showed that infection of tomato roots by root-knot nematodes reduced genetic resistance to bacterial wilt. The effects of combining bacterial wilt resistance with the Mi gene for resistance to root-knot nematode (Mi, resistant ; Mi+, susceptible) was investigated using the near-isogenic lines Caraïbo (Mi+/Mi+)/Carmido (Mi/Mi) and CRA 66 (Mi+/Mi+/Cranita (Mi/Mi), which differ by the size of the segment of L. peruvianum DNA carrying the Mi gene. The presence of the Mi gene was associated with a marked decrease in bacterial wilt resistance. It is suggested that at least one gene governing part of the bacterial wilt resistance is closely linked or allelic to the Mi gene in the tomato lines used in this experiment

Evaluation of bacterial wilt resistance in tomato lines nearly isogenic for the Mi gene for resistance to root-knot

Resistance to bacterial wilt, caused by #Ralstonia solancearum, in tomato lines CRA 66 and Caraïbo is reported to be decreased by root-knot nematode galling and by introduction of the #Mi gene for nematode resistance. The #Mi gene is located on tomato chromosome 6, which also carries a major quantitative trait locus (QTL) for resistance to bacterial wilt. Bacterial wilt resistance was evaluated in F3-progenies derived from two crosses between near-isogenic lines Caraïbo x Carmido and CRA 66 x Cranita, differing for small and large introgressions from #Lycopersicon peruvianum that carry the #Mi gene, respectively. These introgressed regions were mapped using RFLP markers. Plants homozygous Mi+/Mi+ (susceptible to the nematode) and homozygous Mi/Mi (resistant) for the #Mi gene were selected in F2 and used to produce F3 progenies. Parents and F3-lines with Mi/Mi had resistance to bacterial wilt reduced by 30% in Caraïbo x Carmido and by 15% in CRA 66 x Cranita. Caraïbo and Carmido were demonstrated to be isolines and the small introgression from #L. peruvianum resulted in loss of the QTL for bacterial wilt resistance, which is probably allelic or linked in repulsion to the #Mi gene. In contrast, resistance to bacterial wilt segregated in the F3 lines from the cross CRA 66 x Cranita, giving families varying in resistance between the levels shown by the parents. Consequently, two hyopotheses were considered : (i) after only four backcrosses, the parents were not isolines and the genes for resistance to bacterial wilt from CRA 66 were still segregating, and (ii) the parents were isolines and variation in resistance to bacterial wilt in F3 was due to recombination events among the large "L. peruvianum$ introgressed chromosome region from Cranita. (Résumé d'auteur

Host range as an axis of niche partitioning in the plant-feeding nematode community of banana agroecosystems

The stability and positive functions (such as pest regulation) of intensively managed agroecosystems can be enhanced by increasing biodiversity (in particular, by introducing cover plants or associated crops). Therefore, understanding modes of interspecific interactions among the phytophagous species, such as coexistence via host-plant partitioning, may allow manipulation of the balance between pest and non-pest species. In the present study, we tested the hypothesis that a community of six plant-feeding nematode species in banana agroecosystems is structured by host plants. This was done using extensive data on the abundance of the different nematodes species on banana and associated weeds in Martinique. Because the purpose of this study was to focus on host-plant range, we eliminated the effects of temperature, rainfall, and soil type using a partial canonical correspondence analysis. Host plants in banana agroecosystems have a marked influence on the plant-feeding nematode community structure. Host plants allow niche partitioning between some but not all pairs of species. The most evident pattern is the niche partitioning among the three nematodes considered long established in Martinique. For pairs of species showing no host-plant niche partitioning, additional information regarding the colonisation and population dynamics over time both at the root and agroecosystem scale would allow better understanding of the equilibrium among species of the plant-feeding nematode community. For management purposes, the modification of abundance of some host plants, which could be used as cover plants in the field, could change the balance among nematode species

Factors influencing the survivorship of the burrowing nematode, Radopholus similis (Cobb.) Thorne in two types of soil from banana plantations in Martinique

The burrowing nematode, Radopholus similis (Cobb.) Thorne, causes the most damage to bananas. To minimize nematicide applications, cropping systems that use fallow, crop rotation and clean planting material have been developed in the French West Indies. In order to optimize the benefit of the intercropping period, we studied the survivorship of R. similis in different soil types and conditions. We monitored the survivorship of calibrated populations of R. similis in the laboratory on a Nitisol and on an Andosol, two soils derived from volcanic ashes and pumices. We studied water potentials ranging from 0 to -700 kPa on undisturbed soil and on soil previously frozen to get rid of living nematodes. Mortality of adult R. similis decreased regularly, and was fairly well described by Teissier's model. In the previously frozen soils, R. similis survived longer in wet soils (half-life of 21-46 days at 0 to -5 kPa) than in dry soils (half-life of less than 10 days between -80 and -250 kPa). In contrast, in undisturbed soils, R. similis survived longer in dry soils: half-lives ranged from 57 days at -273 kPa to 17 days at water saturation in the Andosol, and 36 days at -660 kPa to 14 days at water saturation in the Nitisol. These results are consistent with the absence of anhydrobiosis in R. similis, unlike Pratylenchus coffeae. A coffeae survivorship curves over time do not follow a model derived from exponential decrease like Teissier's model. These results also show that the recommended one year host-free period required to sanitize soils cannot be shortened without risk, even if flooding the soil could improve it

Nematode resistance in bananas : screening results on some new Mycosphaerella resistant banana hybrids

Banana hybrids with resistance to Yellow Sigatoka and Black Leaf Streak disease were evaluated for resistance to the burrowing nematode Radopholus similis and to the lesion nematode Pratylenchus coVeae in a growth chamber at 24-28. Plants produced by tissue culture were acclimatised for 6 weeks prior to inoculation. Forty-five days after inoculation with nematodes, the root systems were processed and nematode numbers assessed. Two cultivars of Grande Naine (Musa AAA, Cavendish subgroup, ITC1256 and cv902) and one cultivar of Yangambi Km5 (Musa AAA, Ibota subgroup, ITC1123) were used respectively as susceptible and resistant controls. Results based on multiplication rates and root infestations showed that three of these hybrids (FB918, FB919 and FB924) were not significantly different from the resistant control Yangambi Km5 with a lower multiplication of R. similis. Similarly four of these hybrids (FB918, FB919, FB920, FB924) showed a lower multiplication of P. coffeae, not significantly different from the same resistant control. This is the first study that shows a partial resistance to both nematode species, R. similis and P. coffeae within synthetic hybrids of M. acuminata, adding an important extra value to these dessert banana hybrids formerly bred to resist to Mycosphaerella leaf spot diseases

Six known plant-feeding nematodes from Guadeloupe, Martinique and French Guiana (Nemata: Tylenchina)

Descriptions, notes and illustrations are given for six known tylenchid species from Guadeloupe, French Guiana and Martinique viz. Criconema paraguayense (Andrássy, 1968 Raski & Luc, 1985, Discocriconemella degrissei Loof & Sharma, 1980, Mesocriconema denoudeni (De Grisse, 1967) Loof & De Grisse, 1989, M. oostenbrinki (Loof, 1964) Andrássy, 1965, Paratylenchus bilineatus (Brzeski, 1995) Brzeski, 1998 and Pratylenchus coffeae (Zimmermann, 1898) Filip’ev & Schuurmans Stekhoven, 1941. C. paraguayense and M. oostenbrinki are reported from the Caribbean for the first time while P. bilineatus and males of D. degrissei are reported from French Guiana for the first time. M. denoudeni is very widespread throughout the whole area. Several P. coffeae females, from one locality, were found to have a fully functional posterior ovary.Se presentan descripciones, comentarios e ilustraciones de seis especies de tilénquidos recolectadas en Guadalupe, Guayana Francesa y Martinica, a saber: Criconema paraguayense (Andrássy, 1968) Raski et Luc, 1985, Discocriconemella degrissei Loof et Sharma, 1980, Mesocriconema denoudeni (De Grisse, 1967) Loof et De Grisse, 1989, M. oostenbrinki (Loof, 1964) Andrássy, 1965, Paratylenchus bilineatus (Brzeski, 1995) Brzeski, 1998 y Pratylenchus coffeae (Zimmermann, 1898) Filip’ev et Schuurmans Stekhoven, 1941. C. paraguayense y M. oostenbrinki se citan por vez primera en el Caribe, en tanto que P. bilineatus y los machos de D. degrissei se citan por primera vez en la Guayana Francesa. M. denoudeni está muy ampliamente distribuida en toda el área estudiada. Varias hembras de P. coffeae, procedentes de una sola localidad, presentaron un ovario totalmente funcional

Differential response of two pineapple cultivars (Ananas comosus (L.) Merr.) to SAR and ISR inducers against the nematode Rotylenchulus reniformis

We hypothesized that inducing systemic resistances can contribute to the control of the nematode Rotylenchulus reniformis in pineapple. In greenhouse experiments conducted in Martinique, the pineapple cultivars Smooth Cayenne and MD-2 were treated with methyljasmonate (JAME) and salicylic acid (SA), elicitors of induced systemic resistance (ISR) and systemic acquired resistance (SAR). The efficacy of the elicitors was tested by inoculating plantlets grown in individual pots with a monospecific population of R. reniformis reared on Vigna unguiculata. The final nematode populations, 45 days after inoculation on MD-2 treated with JAME were reduced by 67.0% (p = 0.006). Nematode populations on MD-2 plants treated with SA were reduced by 55.8% (p = 0.016). Nematode populations on SC were not reduced by the elicitors. In a second experiment, using split-root systems, JAME was applied to MD-2 plantlets and enzymatic activities involved in plant defense and stress responses were monitored for 14 h after treatment. Additional pots were inoculated with nematodes 24 h after JAME treatment and examined ten days later. Transient stress was observed along with an increase in enzymatic response after inoculation with nematodes. These results showed that the MD-2 was primed through an ISR by JAME. The question now arises whether ISR can be specifically induced only in certain pineapple cultivars. Results are discussed from the perspective of introducing new strategies to manage pineapple nematodes