Monitoring of proteobacteria and phytoplasma in sugar beets naturally or experimentally affected by the disease syndrome ‘Basses richesses’

The disease syndrome ‘Basses richesses’ (SBR) affects sugar beet (Beta vulgaris) crops and causes important economic damage in eastern France. Up to now two phloem-restricted prokaryotes which cannot be cultivated, a stolbur phytoplasma and a γ-3 proteobacterium (called SBR proteobacterium), have been associated with the disease. The SBR proteobacterium is closely related to endosymbionts of Hemiptera in the genus Arsenophonus. Both the phytoplasma and the proteobacterium are transmitted by the insect vector Pentastiridius sp. (Hemiptera: Cixiidae). In the present work, we developed sensitive PCR tools for routine detection of SBR proteobacteria in sugar beets. The monitoring with PCR since 1997, of both SBR pathogen agents, showed the predominant aetiological role of SBR proteobacteria in SBR disease. Detection of SBR proteobacteria in sugar beet was correlated with development of SBR symptoms and reduction of sugar content in the taproot. Severity of symptoms and sugar content in experimentally inoculated sugar beet plants were a function of the number of Pentastiridius sp. used for transmission or the length of inoculation access period (IAP), suggesting a direct relationship between importance or precocity of populations of inoculative insects in fields and low sugar yield of crops.