The relation between dose and infection incidence determined for Pectobacterium brasiliense spray-inoculated on potato haulms

Abstract In field experiments in the Netherlands, risks were assessed for haulm and tuber infections of potato plants with a blackleg-causing strain of Pectobacterium brasiliense after spray-inoculation of haulms with the pathogen. For this, young potato plants (cv. Agria) grown from minitubers were inoculated with different densities (0, 10 1 , 10 3 , 10 5 or 10 8 cfu/ml) of a rifampicin resistant mutant of P. brasiliense . During inoculation, contamination of the soil with inoculum was avoided as much as possible. In an additional treatment, soil was inoculated directly around the plants with 10 8 cfu/ml. In 2021, a relatively rainy growing season, an inoculum density of 10 3 cfu/ml produced a significantly higher disease-incidence in haulms than found in the water-treated control. However, also in the haulms of the negative control, infections were found, probably due to dissemination of the pathogen with water in soil. All plants yielded infected tubers, also plants of the negative control. At an inoculum density of minimally 10 5 cfu/ml symptomatic plants were found. In 2022, a year with less rainfall during the growing season, only an inoculum density of 10 8 cfu/ml resulted in a significantly higher infection-incidence in the haulms than found in the negative control. In contrast, an inoculation density of 10 cfu/ml resulted in a significantly higher infection incidence of tubers than found in the water control. In 2022, symptomatic plants were only observed after inoculation of soil with 10 8 cfu/ml.