Vanilla is a high value cash crop that is continuously demanded by the agri-food and cosmetics industries for its incomparable flavor. Most of vanilla comes from the cured fruits of V. planifolia G. Jackson, a hemi-epiphytic climbing orchid cultivated in the humid tropics. In all the countries were it is cultivated, the vanilla vines suffer from a root and stem rot (RSR) caused by the soil borne fungus Fusarium oxysporum which dramatically reduces plant production and the durability of plantations. No efficient control method is currently available for this disease. Sources of genetic resistance to RSR exist in few vanilla relatives, but so far no commercial resistant variety has been produced. The purpose of this thesis was to better describe the diversity and histopathology of the causal agent of RSR and to evaluate the potential sources of genetic resistance that could be used in breeding programs. In a first step, a collection of 377 single-spored Fusarium isolates recovered from rotten roots and stems during surveys conducted in 52 vanilla plots from Reunion Island, Madagascar and French Polynesia were characterised. Representative subsets of isolates were genotyped using the Elongation Factor 1a and Inter genic Spacer gene sequences. Their pathogenicity was assayed by root dip inoculation on the susceptible V. planifolia accession pla0001. Results showed that F. oxysporum was the principal species responsible for the disease in the field, although a few F. solani isolates showing slight pathogenicity were also isolated. Fusarium oxysporum isolates were highly polyphyletic regardless of geographic origin or pathogenicity. Remarkably, their pathogenicity varied in gradient between non-pathogenic (about 42% of isolates) to highly pathogenic (14%). I n a second step, 254 vanilla accessions comprising 18 species and six types of hybrids were assessed for resistance to RSR in the field (natural inoculum) and in the lab ( in-vitro plants inoculated with Fo072). The s