The syndrome “basses richesses” (SBR) of sugar beet in France is associated with two phloem-restricted uncultivable bacteria: a stolbur phytoplasma and a y-3 proteobacteria. The known vector of proteobacteria is a cixiid planthopper, Pentastiridius leporinus (Hemiptera Cixiidae), formerly shown to transmit both the prokaryotes. The role of P. leporinus and of two other planthopper species, Cixius wagneri and Hyalesthes obsoletus, in spreading the two pathogens to sugar beet were compared and quantified. Because of its abundance and high infection rates with proteobacterium, P. leporinus was confirmed to be the economic vector of SBR disease. P. leporinus and C. wagneri were infected by and transmitted proteobacterium, neither was infected by the phytoplasma. Neither of two populations of H. obsoletus living near sugar beet fields on bindweed and nettle respectively, carried proteobacterium but they were highly infected with two RFLP-differentiable stolbur types. Only the bindweed stolbur type was transmitted and pathogenic to sugar beets. Symptoms associated with the two prokaryotes were similar, but stolbur caused a stronger reduction on taproot biomass and sugar content than proteobacteria. This work underlines possible confusion in aetiology of diseases associated with phytoplasmas or phloem-restricted proteobacteria and the increasing importance of the latter plant pathogens.