6533b859fe1ef96bd12b7459

RESEARCH PRODUCT

Pea root rot diseases : characterization and biocontrol of the disease complex including Aphanomyces euteiches.

subject

description

Root rots in peas are a major concern in most growing regions around the world. The disease is caused by a parasitic complex made up of many species of soil-borne fungi and oomycetes. In France, the main pathogen involved until recently was the oomycete Aphanomyces euteiches. The identity of the other components of the parasitic complex and their respective contributions to the disease have not been investigated. No control method is currently available to effectively control the disease, apart from a predictive biological test questioned by some users. However, this test allows the avoidance of infested plots, which furthermore limits the multiplication in soils of A. euteiches, i.e. the major pathogen. In this context, the objectives of my thesis work were to i) develop a tool for reliable prediction of the risk of root rots due to A. euteiches for pea producers ii) characterize the composition of the parasitic complex responsible for root rots in garden peas in France and iii) identify effective microbial biocontrol agents towards this parasitic complex. A digital PCR-based molecular quantification tool (ddPCR) was developed to accurately quantify low levels of A. euteiches in soils. The tool was validated using two hundred soil samples taken from four infested plots in northern France. A significant relationship between the disease severity and the inoculum density of A. euteiches in these soils was established and a model is proposed to predict the disease severity from the inoculum density. A spatial analysis of these two variables (inoculum density and disease severity) confirmed the aggregative structuring of the inoculum of A. euteiches in the form of foci in infested plots. A statistical analysis based on simulations carried out from these spatialized data led to the definition of a sampling strategy adapted to the heterogeneous distribution of A. euteiches in agricultural soils. This strategy (two W head-to-tail) makes it possible to obtain a representative measurement of the inoculum density in plots up to 12 ha and thus improves the accuracy of the disease risk prediction. The composition of the parasitic complex was determined by molecular identification and pathogenicity tests of a large collection of 317 isolates of fungi and oomycetes isolated from symptomatic pea roots collected from infested fields in northern France, at the end of cultivation and during a growing season. The analysis revealed the prevalence of fungi belonging to the genus Fusarium, including F. oxysporum, F. solani, and F. redolens known in other countries for their involvement in the disease. It also revealed unexpected pathogens like Clonostachys rhizophaga, which has never been reported as a pea pathogen before. In addition, this analysis showed that fungal and oomycete communities associated with root rots evolve during the growing season. A molecular analysis based on a metabarcoding approach was implemented to compare the diversity of fungal and oomycetes communities between symptomatic and asymptomatic plants. Finally, various biocontrol agents have been tested in vitro, in greenhouses and in the field, but no convincing results have been obtained, highlighting the difficulty of developing methods for the biocontrol of a parasitic complex in field crops. To conclude, this work provides the pea producers with a reliable methodology for predicting the risk of the appearance of root rots due to A. euteiches. It has also made it possible to verify the hypothesis according to which a parasitic complex with a high specific richness is involved in the disease in France too, opening up research perspectives for its control.