Evaluation expérimentale de stratégies de déploiement de gènes de résistance pour la gestion durable des nématodes à galles

Dans le cadre de projets soutenus par l'ANR Systerra et le GIS PICLeg (projets "Sysbiotel" et "Neoleg") menés en collaboration entre l'INRA PACA, l'IRD, l'APREL, la Chambre d'Agriculture du 06 et des entreprises privées de sélection de semences, plusieurs stratégies de déploiement de gènes de résistance ont été évaluées pendant 3 ans sur le terrain en conditions agronomiques pour mettre au point une gestion raisonnée des cultivars résistants permettant de gérer de manière durable les problèmes de nématodes à galles des racines. L'alternance des gènes de résistance dans la rotation et le "pyramiding" de gènes dans un même cultivar se sont révélés extrêmement efficaces pour supprimer l'émergence de populations virulentes et réduire les taux d'infestation du sol de plus de 80% en 3 ans. Un nouveau projet INRA "Gedunem", mis en place dans le cadre du Métaprogramme INRA SMaCH (Sustainable Management of Crop Health), vise maintenant à associer ces innovations variétales aux autres méthodes de lutte disponibles (gestion de l'interculture, plantes non hôtes, prophylaxie) afin de maintenir une pression parasitaire faible, tout en évaluant ces nouveaux systèmes de culture du point de vue agronomique et socio-économique

Importance of soil characteristics for plant-parasitic nematode communities in European coastal foredunes: Importance of soil characteristics for plant-parasitic nematode communities in European coastal foredunes

International audiencePlant-parasitic nematodes are involved in soil fatigue processes in coastal foredunes and, therefore, have an impact on the growth of plants such as marram grass (Ammophila arenaria) that enhance sand accretion and the stabilisation of dunes. Transects were sampled in European Atlantic and Mediterranean foredunes at different locations. Plant-parasitic nematode communities and physico-chemical soil properties were analysed, and multivariate techniques were used to link them. Although all the dunes are of sandy texture, the Atlantic and Mediterranean dunes mainly differ by their ratio of coarse and fine sands. Mediterranean dunes, although more disturbed by storms than the Atlantic dunes, have fine-textured soils that can accumulate organic matter. They trap a large amount of minerals and have a high carbonate concentration due to the salinity of the Mediterranean Sea. As a result of the higher content of coarse sand in the top horizons, probably due to a higher accretion of sand, Atlantic foredune soils are low in organic matter and minerals. Soils from North and Irish Sea dunes have intermediate characteristics. These soil contrasts lead to specific plant-parasitic nematode communities. Thus, nematodes such as Hemicycliophora spp., Neodolichorhynchus spp., Longidorus spp. and Merlinius spp. generally colonise the carbonated and mineralised soils of the Mediterranean dunes, and do not seem to be affected by high salinity. Conversely, populations of Meloidogyne spp. and Pratylenchus spp. nematodes are more commonly found in coarse textured soils and the oligotrophic conditions that occur in Atlantic dunes. Specific local conditions such as the presence of paleosols do not seem to disturb them. Considering that both nematode species are major pests in cropping systems, higher population levels due to their fit to such soil characteristics can explain their contribution to soil fatigue encountered in Atlantic dunes

Plant-parasitic nematodes associated with olive tree (Olea europaea L.) with a focus on the Mediterranean Basin : a review

The olive tree (Olea europaea ssp. europaea.) is one of the most ancient cultivated trees. It is an emblematic species owing to its ecological, economic and cultural importance, especially in the Mediterranean Basin. Plant-parasitic nematodes are major damaging pests on olive trees, mainly in nurseries. They significantly contribute to economic losses in the top-ten olive-producing countries in the world. However, the damages they induce in orchards and nurseries are specifically documented only in a few countries. This review aims to update knowledge about the olive-nematode pathosystem by: (1) updating the list of plant-parasitic nematodes associated with olive trees; (2) analysing their diversity (taxonomic level, trophic groups, dominance of taxa), which allowed us (i) to assess the richness observed in each country, and (ii) to exhibit and describe the most important taxa able to induce damages on olive trees such as: Meloidogyne, Pratylenchus, Helicotylenchus, Xiphinema, Tylenchulus, Rotylenchulus, Heterodera (distribution especially in the Mediterranean Basin, pathogenicity and reactions of olive trees); (3) describing some management strategies focusing on alternative control methods; (4) suggesting new approaches for controlling plant-parasitic nematodes based on the management of the diversity of their communities, which are structured by several environmental factors such as olive diversity (due to domestication of wild olive in the past, and to breeding now), cropping systems (from traditional to high-density orchards), irrigation, and terroirs

Trend to explain the distribution of root-knot nematodes Meloidogyne spp. associated with olive trees in Morocco

Knowing which factors affect the distribution of organisms provides useful insights for the management of biodiversity. For root-knot nematodes Meloidogyne (RKN), a critical threat in agriculture, an understanding of the constraints that impact their diversity is a fundamental prerequisite for their control. RKN are reported to be major pests on olive trees, mainly in high-density orchards and in nurseries. This study was undertaken in order to characterise the Meloidogyne species associated with olive trees in Morocco and to consider how anthropogenic stresses and some edaphic and climatic factors may be involved in their spatial distribution. A soil sampling was conducted in the main olive growing areas in Morocco, including wild, feral and cultivated olive trees. RKN were detected in 12.2% of the samples. The analysis of biochemical (esterase patterns) and molecular (SCARS, D2D3) markers revealed the occurrence of M. javanica (as the predominant species), M. arenaria, M. hapla,M. spartelensis and of an undescribed species Meloidogyne n.sp. Some esterase variability was detected in M. javanica and M. hapla populations. The RKN species were distributed everywhere on an aggregated basis. However, M. javanica was recorded in high abundance throughout the cultivated areas in Morocco in high-density and traditionally cultivated olive orchards and rarely on some feral olive, while the other species were detected in low abundance on wild olive in mountain plant refuge areas in northern and southern Morocco. Nevertheless, the large distribution of M. javanica in cultivated areas could be primarily due to widespread introductions from nurseries, followed by multiple founder processes favoured by local conditions such as irrigation and planting density. In contrast, the detection of the other species hosted by wild olive only in plant refuge areas suggest that these areas host ancestral nematode species that did not disperse in the absence of human activities

Early Detection of the Root-Knot Nematode Meloidogyne hapla Through Developing a Robust Quantitative PCR Approach Compliant With the Minimum Information for Publication of Quantitative Real-Time PCR Experiments Guidelines

International audienceRoot-knot nematodes (RKNs) are major threats to crops through attacking the roots, which induces an abnormal development of the plant. Meloidogyne hapla Chitwood is of particular concern as it is currently expanding its distribution area and displays a wide host range. Effective plant protection against this RKN requires early detection as even a single individual can cause severe economic loses on susceptible crops. Molecular tools are of particular value for this purpose and among them, qPCR presents all the advantages, i.e. sensitivity, specificity, rapidity of diagnosis at a reduced cost. Although few studies already proposed detecting M. hapla through this technique, they lack experimental details and performance testing, and suffered from low taxonomic resolution and/or required expensive hydrolysis probes. Here we propose a qPCR detection method that uses SybrGreen with developed primers amplifying a fragment of COI mitochondrial region. The method is developed and evaluated following the MIQE guidelines to ensure its quality, i.e. sensitivity, specificity, repeatability, reproducibility, robustness. The results demonstrate that the newly developed method fulfills its goals as it proved specific to M. hapla and allowed for a reproductible detection level as low as 1.25 equivalent of a juvenile individual. All criteria associated with the MIQE guidelines were also met, what makes it of general use for the reliable early detection of M. hapla

Characterising the herbivore community and its impact on Sonchus oleraceus (Asterales: Asteraceae) in its invaded range in Australia

International audienceSonchus oleraceus is an annual species native to Eurasia and Northern Africa that has become a major weed of cultivated fields and ruderal areas in Australia. Populations are difficult to manage in cropping systems because of the development of resistance to common herbicides. Biological control is being investigated as an additional tactic for managing the weed. A literature review was conducted to identify the phytophagous arthropod species already associated with the species in Australia to guide such a biological control programme. To identify opportunities for biological control agents to aid in management, we undertook field surveys across Queensland and South Australia in different environments. We also investigated factors that may influence their performance in Australia. Both the literature review and field surveys identified 21 arthropod species associated with S. oleraceus in Australia, most of them being generalist species and pests of exotic origin. Capitula were the most damaged plant part while stems were relatively free from insects, except aphids. The field surveys recorded an unexpected new interaction between S. oleraceus and the gall midge, Contarinia jongi (Diptera: Cecidomyiidae). This association was surprising as the midge, only known to develop on Alstroemeria (Liliales: Alstroemeriaceae), a very distant relative to S. oleraceus (Asterales: Asteraceae), was reported in Australia only a few years ago under greenhouse conditions. The midge and the moth Eublemma cochylioides (Lepidoptera: Erebidae) were the two species that occurred most frequently in developing flower head samples. We considered their infestation rate as a proxy of herbivory and tested whether the environment surrounding the plant may influence herbivory. Both E. cochylioides and C. jongi showed the highest infestations in ruderal sites compared with the sites located in conventional farming areas. We discuss the implications of our results in the context of selecting and releasing candidates for the biological control of the weed, especially in agricultural landscapes

Feasibility of classical biological control of Sonchus oleraceus in Australia

International audienceSonchus oleraceus L. (Asteraceae), an annual species native to Eurasia and northern Africa, is among the most widely distributed plant species on Earth. In Australia, S. oleraceus, is a common weed in disturbed areas such as crop fields, pastures, gardens and roadsides. In agricultural settings, it can dominate fallows and cultivated fields where it uses stored soil moisture and reduces crop yield. This weed has also developed herbicide resistance, predicating the need for alternative management solutions. In this context, we undertook field surveys and preliminary host range studies in the native range of S. oleraceus to determine the feasibility of developing classical biological control solutions for Australia. Fifty-nine phytophagous arthropod species were recorded and nine pathogenic fungi were recovered from disease symptoms. Four arthropod species were selected for initial host-specificity testing based on information available in the literature. Preliminary host-specificity tests were also performed with representative isolate(s) of six of the pathogenic fungi. All these candidate agents were shown in the tests to affect key native species in Australia in the same subtribe as S. oleraceus (i.e. Sonchus hydrophilus and Actites megalocarpus). The results of our investigations suggest that classical biological control may not be a feasible option for the management of S. oleraceus in Australia, and that alternative integrated weed management tactics may need to be pursued to mitigate the impacts of this weed

An innovative approach combining metabarcoding and ecological interaction networks for selecting candidate biological control agents

International audienceClassical biological control (CBC) can be used to decrease the density of invasive species to below an acceptable ecological and economic threshold. Natural enemies specific to the invasive species are selected from its native range and released into the invaded range. This approach has drawbacks, despite the performance of specificity tests to ensure its safety, because the fundamental host range defined under controlled conditions does not represent the actual host range in natura, and these tests omit indirect interactions within community. We focus on Sonchus oleraceus (Asteraceae), a weed species originating from Western Palearctic that is invasive in many countries and notably in Australia. We explore how analyses of interaction network within its native range can be used to (a) inventory herbivores associated to the target plant, (b) characterize their ecological host ranges and (c) guide the selection of candidate biocontrol agents considering interactions with species from higher trophic levels. Arthropods were collected from plant community sympatric to S. oleraceus, in three bioclimatic regions, and interactions were inferred by a combination of molecular and morphological approaches. The networks reconstructed were structured in several trophic levels from basal species (plant community), to intermediate and top species (herbivorous arthropods and their natural enemies). The sub-network centred on S. oleraceus-related interactions contained 116 taxa and 213 interactions. We identified 47 herbivores feeding on S. oleraceus, 15 of which were specific to the target species. Some discrepancies with respect to published findings or conventional specificity tests suggested possible insufficient sampling effort for the recording of interactions or the existence of cryptic species. Among potential candidate agents, six exhibited interactions with natural enemies. Synthesis and applications. Adopting a network approach as prerequisite step of the classical biological control programme can provide a rapid screening of potential agents to be tested in priority. Once ecological host range defined, we suggest that priority should be given to agent used by a minimum species, and, when they exist, to agents that possess enemies from the most distant taxonomical group from those occurring in the range of introduction

Alternative cropping systems can have contrasting effects on various soil-borne diseases : relevance of a systemic analysis in vegetable cropping systems

Vegetable production makes an intensive use of pesticides, and a major challenge is to build alternative cropping systems that can control pests and diseases with fewer uses of chemical products. An on-farm analysis was conducted in Southeast France to assess the efficacy of several cropping systems in simultaneously controlling two major pests: root-knot nematodes (Meloidogyne spp.) and lettuce drop due to Sclerotinia sclerotiorum. Ten cropping systems resulting from the combinations of three crop sequences and two alternative techniques, solarization and green manure, were assessed during two years. The use of solarization once a year or once every two years limited the occurrence of S. sclerotiorum. Sorghum green manure tended to increase S. sclerotiorum incidence; the effect was positively correlated with green manure duration. Especially when no vegetable was cropped in summer, the green manure crop duration was lengthened and this probably created soil conditions favorable to the development of the fungus. The incidence of root-knot nematodes was largely dependent on crop rotation: a melon crop in summer increased its incidence on the subsequent lettuce crops whereas a summer sorghum cover crop had no effect. The cropping systems that limited Sclerotinia development in soil tended to support the root-knot nematode populations. These results should motivate farmers and advisers to adopt a systemic analysis and take into account the various interactions among inoculum level, soil characteristics, crop rotations, and technical management options for designing sustainable vegetable production systems