Surveys to identify sources of blackleg-causing soft rot Pectobacteriaceae responsible for initial infection of potato crops

Abstract Surveys were conducted at two locations in the Netherlands during the growing seasons of 2022 and 2023, to identify sources of blackleg-causing soft rot Pectobacteriaceae (BL-SRP) responsible for initial infections of potato crops. For this, a TaqMan assay for a Pectobacterium brasiliense variant was developed, to allow detection of all BL-SRP variants prevalent in the Netherlands. Leaves of second and third generation prebasic (PB2, PB3) and ware potato crops located near PB1 (minituber) crops (cv. Agria) were analyzed, as well as soil, weeds, flying insects and rainwater. BL-SRP variants detected in these field samples were compared with those detected in PB1 crops using (enrichment) TaqMan assays. Depending on location and year, the first BL-SRP infections in PB2 and PB3 crops were found early in the growing season and were of the same BL-SRP variants later detected in the PB1 crops. Soil samples were negative for BL-SRP. One hundred weeds were analyzed per location, of which some were positive for BL-SRP, but only those later in the growing season. BL-SRP contamination of insects was already detected early in the growing season. The percentage of positive insects varied between 1.6 and 7.8% depending on location and year. Contaminated insects belonged predominantly to the families Chrysopidae and Anthomyiidae. Rainwater sampled at different locations in the Netherlands were all negative for BL-SRP. We hypothesize that later generation potato crops located nearby are the main source of initial infection of PB1 crops with BL-SRP. Insects may play a role in the transmission of BL-SRP.