Sugar beet (Beta vulgaris) is threatened by several viral diseases amongst which the Rhizomania is the most devastating. This pathology is caused by Beet necrotic yellow vein virus (BNYVV), a multipartite RNA virus that is transmitted by the soil-borne plasmodiophorid Polymyxa betae. In diseased plants, BNYVV is frequently found in co-infection with other viruses such as Beet soil-borne mosaic virus, Beet soil-borne virus (BSBV), Beet virus Q and Beet black scorch virus (BBSV) as well as its associated satellite RNA (satRNA). These viruses have been relatively less studied in comparison with BNYVV, raising concerns regarding their impact on sugar beet. In particular, this thesis aims to shed light on the pathogenicity of two of those viruses: BSBV and BBSV. To this end, agroinfectious clones (shortly agroclones) were constructed for both viruses, allowing rapid plant inoculation via delivery by Agrobacterium tumefaciens. Such agroclones for both pathogens were efficiently used to demonstrate viral infections of two model plant species, namely Beta macrocarpa and Nicotiana benthamiana. In addition, infection was evidenced on sugar beet following mechanical inoculation by agroinfected material, allowing for the first time to show a symptomatic response from this host for viral cDNA clones of BSBV and BBSV. Eventually, transmission assays via zoosporic vectors were successfully carried out in order to validate a complete viral cycle. Indeed, BBSV and its satRNA were efficiently transmitted by zoospores of the fungus Olpidium virulentus that was originally recovered from infected soils. As far as BSBV is concerned, proof of transmission was given using resting spores of the P. betae strain A-2641. Altogether, the obtained results confirm that the developed agroclones are efficient tools for reverse genetics studies, paving the way for exploring the pathosystems of BSBV and BBSV, their sugar beet host and their respective vectors.