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Abstract

The current restrictions on the use of chemical nematicides and the specialized and intensive agriculture in Europe have contributed to increase soil-born pest problems in horticultural crops, especially due to root-knot nematodes, Meloidogyne spp. (RKN). These microscopic roundworms are difficult to control particularly because of their highly polyphagous nature, their ability to remain hidden in the soil or in plants, and their resistance to abiotic stresses. A recent survey I conducted in France underlines the particular importance of these nematodes in the south-east of France, with over 40% of horticultural farms affected. Combined with prophylactic methods, several cropping techniques can be used, including heat based methods (solarisation), biological control (nematophagous fungi, parasitic bacteria, symbiotic mycorhizes), trap plants or biofumigation. These methods are still experimental or used on a limited scale, but all demonstrate a highly variable and limited efficacy due to poor knowledge of their mode of action and of the factors limiting their effectiveness.

In this context, plant resistance to RKN is now promoted in breeding programs. Thanks to the collaboration developed with the GAFL Unit of INRA in Avignon, my early work focused on the characterization of major resistance genes in pepper, a Solanaceae of agronomic interest. We developed co-dominant markers linked to the broad spectrum R-genes, localized these genes and pointed out a R-genes cluster on pepper chromosome 9. Then, we showed that 1 / the choice of the R-gene is of crucial importance to reduce the frequency of resistance breakdown, 2 / the plant genetic background plays an important role on R-genes efficiency and can slow down the adaptation of pathogen populations to the R-gene, responsible QTLs being identified and located, some outside the R-genes cluster, 3 / there is no significant dosage allele of the R-genes, 4 / the specificity of virulence leads to consider the alternating use of distinct R-genes in rotation, and 5 / the pyramiding of two different R-genes in one genotype totally suppresses the emergence of virulent isolates.

To validate these results, we evaluated several R-gene deployment strategies in field experiments with traditional rotations done in vegetable cropping systems. Looking at 3 components of crop protection, i.e., R-efficiency, R-durability, and sustainability of rotating cultivation, a constant hierarchy of management strategies was observed, with Pyramyding > Alternating > Mixture of R-genes > Sequential use of a single R-gene introgressed in a susceptible background. Looking to the adaptation of xenobiotic to drugs and pesticides, a very similar hierarchic efficiency of strategies was observed. These results are also in good agreement with concepts recently developed from the analysis of very different plant-pathogen interactions, and thus contribute to generalize strategies for the breeding and management of R-cultivars strengthening and increasing the durability of qualitative resistances.

To preserve the resistance in the long-term and sustainably manage RKN problems, I am currently developing an approach combining varietal resistance in crop rotations with agronomic practices over pluri-annual crop sequences to reduce the parasitic pressure in the soil. The impact of such innovations on ecological diversity in the soil including other nematode species, on  yield and economic viability at farm level, are also taken into account. To foster synergistic and long-term goals, I rely on a strong collaboration with the diversified network of R & D partners that I could implement in previous projects and that I have expanded and strengthened for this new project. They bring all the complementary expertise needed for answering specific short-term questions as well as generic mid- to long-term expectations. Moreover, to improve the understanding of interactive processes and mode of action of alternative control methods can lift constraints on their use in the field, increase their efficiency and help to implement integrated pest management (IPM) in our innovative systems. For this purpose, various contracts are thus committed or being negotiated, and a new project is being considered taking into account more globally the soil health.

Composition of the jury

• Dr Pierre Abad, President, Inra, Institut Sophia Agrobiotech, Sophia Antipolis
• Dr Didier Merdinoglu, Rapporteur, INRA–UDS, Santé de la Vigne et Qualité du Vin, Laboratoire de Génétique et d'Amélioration des Plantes, Colmar
• Dr Régine Delourme, Rapporteur, INRA, Institut de Génétique, Environnement et Protection des Plantes, Le Rheu
• Dr Thierry Dupressoir, Rapporteur, INRA, Diversité, Génomes & Interactions Microorganismes – Insectes, Professeur au Laboratoire EPHE "Pathologie comparée des invertébrés", Université Montpellier 2, Montpellier
• Dr Thierry Mateille, Examiner, IRD, Centre de Biologie pour la Gestion des Populations, Montferrier /Lez Cedex