The role of haulm infections in the epidemiology of soft rot Enterobacteriaceae

Potato seed production in Europe starts with the use of mini-tubers or clonal selection material free from plant pathogens, including Pectobacterium and Dickeya spp., the causative agents of blackleg and slow wilt. However, already after harvest of the first field generation the first symptomless infections are found in progeny tubers. Several potential sources have been identified responsible for initial infections, including machines, in particular harvesters, irrigation water, rain water, aerosols, insects and carry-over from movement of animals and men. The aim of our research was to determine the relative importance of haulm infections in the epidemiology of blackleg. In two-years surveys, we determined the rate of contamination of in total 4000 leaves collected from 40 seed potato crops grown from mini-tubers at the end of the growing season. Leaf washings and leaf extracts were analyzed with enrichment TaqMan assays. We found contaminations with P. carotovorum subsp. carotovorum but not with blackleg or slow wilt causing organisms. We also determined the fate of Dickeya solani and Pectobacterium wasabiae in potato plants after spray-inoculation of (damaged) leaves using GFP-tagged strains. No symptoms were recorded, but in six weeks, population densities increased in both damaged and undamaged leaves, and the pathogens were translocated from leaves via stems into stolons and, incidentally, tubers. The infection incidences were dependent on the initial inoculation densities, higher in plants with damaged than undamaged leaves, and higher for D. solani than for P. wasabiae. In conclusion, the incidences of haulm infections in a first generation crop seem to be low, but if contamination occurs the risks for systemic colonization of plants is high.