Studies were carried out to assess the potential of the plant pathogenic fungus <i>Ascochyta caulina</i> as a mycoherbicide against <i>Chenopodium album</i> L., a worldwide weed in arable crops. <i>A. caulina</i> is associated with necrosis of leaves and stems of plants belonging to genera of <i>Chenopodium </i>L. and <i>Atriplex</i> L. The research of this thesis was focused on recognition of factors that may enhance pathogenicity and determine the impact of the pathogen not only on its target host (growing both alone and in competition), but also on a range of temperate crop species. These studies showed that formulation of <i>A. caulina</i> spores with nutrients, especially V-8, and surfactants resulted in faster and greater <i>in vitro</i> germination. During an investigation of <i>in vitro</i> pathogen growth, it was observed that <i>A. caulina</i> grows most prolifically in the nutrient rich liquid medium, V-8. Simple nitrogen and carbon sources also induce a positive effect on the pathogen's growth. Nutrients also promoted the pathogen's enzyme activity of a-amylase and cellulase. The <i>in vitro</i> and <i>in planta</i> enzyme activity varied significantly between <i>A. caulina</i> isolates and plant growth stage affected the a-amylase activity but not cellulase. Observations of <i>A. caulina</i> on detached leaves and whole plants of <i>C. album </i>and several crop species, showed that the surface deposits on <i>C. album </i>leaves are part of a defensive mechanism, and that the pathogen can develop pycnidia on the dead leaves of the non-host plants. <i>A. caulina</i> caused no infection symptoms on any of the crop species. However, under some circumstances it reduced the dry biomass accumulation of some of the non-target plants. Results of the three-way interaction between <i>Beta vulgaris, C. album</i> and the potential mycoherbicide <i>A. caulina</i> showed that the pathogen can reduce the competitive effects of the weed over <i>B. vulgaris </i>by suppressing the growth of the former.

Development of microbial herbicides is constrained by unreliability in the field where conditions are often sub-optimal for infection. Crucially, sufficient moisture, often dew, is required to establish infection. Two model systems, Colletotrichum dematium (Pers. ex Fr. ) Grove on Chenopodium album L. and Mycocentrospora acerina (Hartig) Deighton on Viola arvensis Murr., have been investigated and formulation requirements for each system identified, principally to reduce the dew period necessary for infection. Effects of adjuvants (surfactants, stickers and humectants) on spore germination and appressorium formation were investigated in vitro. Few were toxic and then, principally, at high concentration. The surfactants Tween 40,60 and 80 were compatible with both pathogens. Similarly, the stickers acacia, ghatti, guar, karaya, locust bean and xanthan gums and low viscosity alginic acid were all non-toxic as was the humectant glycerol. Each pathogen reacted differently to the adjuvants and any potential microbial herbicide will need individual matching of adjuvants to give an effective formulation. A working formulation (rapeseed oil-in-water (1: 10 v/v) emulsion using 0.1 % v/v Tween 40 as the emulsifier) was found to reduce the dew period requirement of M. acerina from 36 to 18 hours. The formulation protected spores from desiccation for 24 hours after application, or for 16 hours following a sub-optimum dew period occurring immediately after application. Scanning electron microscopy showed that the applied spores, and the developing mycelium, were immersed in the oil deposit. Transmission electron microscopy of sections through formulation deposits on the leaf revealed that some inversion of the emulsion, to form a water-inoil deposit, had occurred, suggesting a mechanism of protection against desiccation. The oil phase infiltrated the cortical intercellular spaces only when the leaf was infected. This intercellular oil contained more water than that on the leaf surface. Emulsion-formulation applied to run-off with an 'air brush', consistently gave significantly better weed control under sub-optimal dew conditions than a formulation of surfactant only. When applied with a conventional hydraulic nozzle at 400 1 ha" the emulsion was only occasionally superior to the surfactant alone. Such interactions require further in-depth investigation. The importance of correct inoculum placement for maximum effectiveness, independent of formulation type, was highlighted. Unless all meristems are killed, survivors quickly grow, despite the death of neighbouring leaves and petioles, and the weed suffers merely a growth check. Formulation as emulsion improved diseasee stablishmenta nd diseasee xpressioni n the target weedo nly in somec ircumstancesF. urther researchin to spraya pplicationm ethodsa ndt heir interactions with formulation, host and environment is clearly necessary

Un projet visant à identifier des mycoherbicides pour lutter contre les adventices a été initié entre l’UMR Agroécologie de Dijon et la société DE SANGOSSE® (Agen). Trois volets ont structuré ce projet à l’issue d’une collecte de prélèvement de 475 plantes représentatives de 23 espèces d’adventices symptomatiques et asymptomatiques en Bourgogne et en Beauce. Le 1er volet reposait sur une approche de type metabarcoding (technologie Illumina), pour évaluer et comparer la diversité des communautés fongiques endophytes des plantes symptomatiques et asymptomatiques. 542 genres fongiques ont ainsi été identifiés. Des taxons associés aux plantes symptomatiques ont été identifiés. Parmi ceux-ci, certains sont des pathogènes connus, d’autres non et ils constituent des pistes à exploiter pour la recherche de candidats mycoherbicides. Le deuxième volet repose sur une approche conventionnelle de microbiologie et pathologie. Une collection de 194 champignons associés aux symptômes des adventices a été constituée. La pathogénicité de ces isolats a été testée grâce à une série de screenings de plus en plus sélectifs qui ont abouti à la sélection de cinq souches, identifiées par séquençage de l’ITS ou d’autres marqueurs taxonomiques. Une souche appartient à l’espèce Boeremia exigua var exigua, une autre à l’espèce Alternaria alternata, deux appartiennent à l’espèce A. penicillata et la dernière au genre Alternaria. Le troisième volet visait à identifier le mode d’action d’une souche par une double approche, métabolomique et microscopique. La souche de B. exigua var exigua secrète des métabolites phytotoxiques mais également infeste et semble détruire les tissus végétaux sous-épidermique de la plante hôte.Ce projet exploratoire a fourni des pistes de taxons fongiques associés à des symptômes observés sur adventices en analysant la diversité par une approche moléculaire et a fourni des souches fongiques, mycoherbicides potentiels, par une approche microbiologique dont on voit bien qu’elle reste une méthode incontournable, malgré ses limites, pour obtenir des candidats fongiques à action herbicide. / A project aiming at identifying mycoherbicides to control weeds has been initiated between the UMR Agroécologie (Dijon) and the company DE SANGOSSE® (Agen, France). Three axes structured this project after a sampling collection of 475 plants representative of 23 species of symptomatic and asymptomatic weeds was carried out in Burgundy and Beauce. The first part was based on a metabarcoding approach (Illumina technology), to evaluate end compare the diversity of endophytic fungi communities of symptomatic and asymptomatic weeds. 542 fungal genera have been identified. Taxa associated with symptomatic plants have been identified. Of these, some are known pathogens, others are not, and both constitute avenues to exploit for the research of mycoherbicide candidates. The second axe is based on a conventional approach to microbiology and pathology. A collection of 194 fungi associated with weed symptoms was established. The pathogenicity of these isolates was tested through a series of increasingly selective screenings that resulted in the selection of five strains that were identified by sequencing of ITS or other taxonomic markers. One strain belongs to the species Boeremia exigua var exigua, another species Alternaria alternata, two belong to the species A. penicillata and the last to the genus Alternaria. The third axe aimed at identifying the mode of action of a strain by a dual metabolomics and microscopic approach. The strain of B. exigua var exigua produced phytotoxic secondary metabolites but also infested and apparently destroyed the sub-epidermal plant tissues of the host plant.This exploratory project provided tracks to exploit fungal taxa associated with observed weeds symptoms, by analyzing the diversity, by a molecular approach and provided fungal strains, potential mycoherbicides by a conventional microbiological approach that we can see it remains an unavoidable method, despite its limitations, to obtain fungal candidates with herbicidal action.