Pectinolytic bacteria such as Dickeya spp. and Pectobacterium spp. represent a significant threat for seed potato production in Europe. Dickeya spp. induces various symptoms such as plant wilting, stem rot (usually referred to as blackleg) and tuber soft rot. Expression of wilting and blackleg symptoms depends on a range of factors: the susceptibility of the cultivar, the aggressiveness of the Dickeya strain and environmental factors such as soil moisture, temperature and plant evapotranspiration. Using the flow of sap driven by plant transpiration, these bacteria can colonize the entire plant vascular system. However, it is not known whether this vascular colonization differs according to the nature of the inoculum (soil-carried or tuber-borne) and whether this has an impact on the expression of symptoms. To better characterize the consequences of colonization of the xylem vessels by Dickeya, greenhouse and field trials were carried out with plants infected via mother tuber inoculation or root inoculation. The development of symptoms was tracked, as were changes in plants’ transpiration rate, and plant transpiration was used as an index of the early stages of symptom expression. Our trials revealed that seed- and soil- carried inocula can both induce wilting and blackleg symptoms in potato plants. It was also found that blackleg symptoms appearing during the first three weeks after plant emergence are caused mainly by a tuber-borne inoculum. Finally, it was established that the presence of a latent inoculum in the xylem vessels does not alter the plant transpiration process, as long as there are no wilting symptoms on rotting lesions.
