Evaluation of Pathological Risks Associated with Use of Leguminous Cover crop and Living mulch species

In the past 10 to 15 years, there has been a tendency in Europe to switch from cereal based cropping systems, with winter and summer fallow, to cropping systems which combine greater crop diversity with maximum soil cover. Increasing the opportunities of including legumes into crop rotations to be grown as cover crops, intercropped or under-sown with cereals can provide multiple beneficial services to agro-ecosystems. A major concern is however, that many of the leguminous species of interest share important soil-borne pathogens among each other and with some important main crops. This thesis is concerned with the species of the genus Fusarium as they are characterized by considerable ecological plasticity which enables them to efficiently spread along crop rotations, and often infect a wide range of plants under diverse environmental conditions. Three different studies involving pathology, morphology and molecular phylogenetics of Fusarium species were carried out. The first study aimed at determining frequency, distribution and aggressiveness towards peas of Fusarium spp. associated with the roots of subterranean clover, white clover, winter and summer vetch grown in rotation or association with wheat. Although, the samples originated from five different sites within temperate and Mediterranean Europe, there was no strong separation in Fusarium community structure among the sites with F. oxysporum and F. avenaceum being the most frequently isolated species from all hosts and in all environments. Subsequent studies performed under controlled conditions using pea as a model legume cash crop revealed high pathogenic potential of F. avenaceum suggesting that this species could emerge as potential risk in intensive legume cropping systems. In the second study, a collection of 79 F. solani isolates was characterized by molecular genotyping and greenhouse aggressiveness bioassays on pea. This study also aimed to clarify the host range of F. solani f. sp. pisi by inoculating 62 accessions of 10 legume genera with single selected strain of the species. The majority of the isolates belonged to two lineages: F. solani f. sp. pisi mainly accommodating German and Swiss isolates, and the Fusisporium (Fusarium) solani lineage accommodating mainly Italian isolates. This study revealed that (i) pathogenic and non-pathogenic isolates of F. solani f. sp. pisi, the fungus named by its specific pathogenicity to pea, can be found in a variety of habitats under diverse agro-ecological conditions, (ii) the fungus has a broad host range and (iii) that the concept of formae speciales needs a revision. In the third study, using robust greenhouse and laboratory methodology, three endophytic F. equiseti isolates found during the European survey in the first study were shown to have potential to promote pea growth and to alleviate detrimental effects of the necrotrophic soil borne pathogens, F. avenaceum and Peyronellaea pinodella was demonstrated. This work provides new insights into diversity, geographical distribution, prevalence, aggressiveness and ecology of Fusarium spp. associated with the roots of studied hosts grown across a broad range of ago-climatic conditions in Europe. Some of the major pathogens of various leguminous and non-leguminous crops are characterized by high ecological plasticity, have the ability to endophytically colonize the hosts studied and the potential to cause yield losses of subsequent main legume grain crops such as pea. Furthermore, this work points to the problem of the fomae speciales concept in Fusarium which is often misleading and needs to be reconsidered. The results also highlight endophytic potential of F. equiseti and its ability to sustain stable populations and function within diverse ecosystem conditions. Taken together, the detection of Fusarium species in the plant tissue is not proof of ongoing disease infection; often, additional screening tests are needed to confirm pathogenicity of recovered isolates. Therefore, to fully understand the ecological significance of the Fusarium species associated with a given host often a multi-focal approach is needed.