Transmission tests of ‘Candidatus Liberibacter solanacearum’ by carrot seeds

Following the recent detections of 'Candidatus Liberibacter solanacearum' on carrots in Europe and particularly in fields used for seed production in France in 2012, experiments to study the transmission of this pathogen by seeds were conducted. Three batches of carrot seeds harvested in three infected carrot fields in France and one batch of commercial carrot seeds, as negative control, were used. Except for the commercial seed lot, 'Ca. L. solanacearum' was detected in each batch of seeds at a level close to 100%. The first three months, the experiments were conducted in greenhouse. Then, seedlings were placed in overwintering conditions for 4 months. Each month, during 9 months and for each batch, 100 seedlings were analyzed individually by real-time PCR for the presence of the bacterium. At the end of the first three months, no positive result was obtained. After 9 months, no seedling was found positive. These results are not consistent with those obtained in a Spanish laboratory and suggesting that 'Ca. L. solanacearum' should be considered as a seed-borne pathogen. Indeed, the bacterium was detected in seedlings after 3 months. After 5 months, between 15 and 42% of the seedlings from positive seed lots were tested positive. The overwintering conditions applied on the French carrot seedlings may have an impact on the transmissibility of the bacterium. Anyway, these results indicate the need to duplicate tests to conclude on seed transmission of 'Ca. L. solanacearum' by carrot seeds.