6533b85cfe1ef96bd12bc5b7

RESEARCH PRODUCT

Les pourritures racinaires du pois potager, caractérisation et biocontrôle du complexe parasitaire d’origine tellurique incluant Aphanomyces euteiches

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description

Pea root rots are a major concern for pea growing regions around the world. The disease is caused by a parasitic complex composed of many species of fungi and oomycetes of soil origin. In France, the main pathogen is the oomycete Aphanomyces euteiches. The identity of the other components of the parasitic complex and their respective contributions to the disease have never been investigated. No means of control is currently available to effectively control the disease except for a predictive bioassay that is questioned by some users. However, this test allows the avoidance of infested plots, which limits the multiplication of the major pathogen A. euteiches in the soil. In this context, the objectives of my thesis work were to i) develop a reliable prediction tool of the risk of root rots due to A. euteiches for pea growers ii) characterise the parasitic complex responsible for root rots of garden pea in France and iii) identify effective microbial biocontrol agents to fight against this parasitic complexA molecular quantification tool based on digital PCR (ddPCR) was developed in order to accurately quantify low inoculum densities of A. euteiches in soils. The tool was validated on 200 soil samples taken from four infested plots in northern France. A significant relationship between disease severity determined by bioassays and A. euteiches inoculum density in these soils was established and a model is proposed to predict disease severity from inoculum density. Spatial analysis of these two variables confirmed the aggregative structuring of the A. euteiches inoculum in the infested plots. A statistical analysis based on simulations performed on these spatial data led to the definition of a sampling strategy adapted to the heterogeneous distribution of A. euteiches in agricultural soils. This two-W head-to-head strategy provides a representative measure of inoculum density in plots and thus improves prediction accuracy.The composition of the pest complex was determined by molecular identification and pathogenicity testing of a collection of 317 isolates of fungi and oomycetes from symptomatic pea roots collected from infested fields in northern France, at the end of the crop and during one growing season. The analysis revealed the prevalence of the fungi Fusarium oxysporum, F. solani, and F. redolens, known in other countries to be involved in the disease. It also revealed unexpected pathogens such as Clonostachys rhizophaga, which had never been reported as a pathogen of pea before. Furthermore, this analysis showed that fungal and oomycete communities associated with root rots evolve over the course of a cropping season. A molecular analysis based on a metabarcoding approach was used to compare the diversity of these communities between symptomatic and asymptomatic plants.Finally, various biocontrol agents were tested in vitro, in greenhouses and in the field, but no convincing results were obtained, highlighting the difficulty of developing biocontrol methods for a pest complex in field crops.In conclusion, this work will have provided pea growers with a reliable methodology for predicting the risk of root rots caused by A. euteiches. It also allowed us to verify the hypothesis that a parasite complex with a high specific richness is involved in the disease in France, opening up research prospects for its control. Translated with www.DeepL.com/Translator (free version)