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Leptosphaeria maculans (the perfect form of Phoma lingam) is responsible for cruciferous stem canker, one of the main diseases of oilseed rape (Brassica napus) in all growing regions.

The interaction between L. maculans and B napus is a model for INRAe because of the agro-economic importance of the disease, which has led to the development, within the Institute, of diversified approaches to controlling its harmfulness (plant breeding, resistance gene mapping, agronomic approaches, resistance management strategies), and justifying investment by both the seed industry and public research in improving rapeseed resistance and characterizing the pathogen. L. maculans also serves as a valuable study model, given the complexity of the developmental and phytopathological strategies implemented during its life cycle.

L. maculans is a hemibiotrophic fungus whose life cycle comprises a saprophytic phase followed by a long parasitic phase. In Europe, the life cycle of L. maculans begins in autumn with the aerial dissemination of ascospores projected from perithecia formed on the residues of previous crops. The ascospores germinate on plant cotyledons or leaves, and the fungus colonizes the leaf mesophyll, inducing the formation of leaf spots. From the leaf spots, the mycelium systemically colonizes the intercellular spaces of leaf, petiole and stem tissues, without causing symptoms. After a long latent phase, during which the fungus is systemic and endophytic, L. maculans becomes necrotrophic and causes necrosis of the stem base, which can lead to parasitic lodging of the plants. This alternation of parasitic modes involves a highly sophisticated molecular dialogue with the host plant, and the implementation of complex, finely regulated biological programs.

Given the absence of suitable fungicide treatments against L. maculans, the main means of control is the use of resistant B. napus varieties. In the L. maculans/B. napus interaction model, specific resistances are widely used in combination with increasingly high levels of general resistance, which can maximize the lifespan of specific resistances

Our objectives are to :

• decipher the molecular dialogue involved in biotic interactions between the host plant and the pathogen,
• determine the molecular mechanisms by which L. maculans adapts to different Brassica species and resistances, and the impact of sexual reproduction on this adaptation,
• propose management strategies based on the knowledge acquired.

At present, the EPLM team's project focuses on the interactions of L. maculans with Brassica species (host or non-host) and with the microbiota associated with infected plants. We also aim to decipher in greater detail the effect of the environment (biotic and abiotic) on these interactions. Our aim is to propose diversified control strategies against L. maculans and, if possible, against other fungal pathogens of Brassica leaves, by identifying specific and quantitative plant resistances, limiting the impact of sexual reproduction on primary inoculum production and on the rapid evolution of the L. maculans genome, and identifying microorganisms capable of limiting L. maculans infection.