Ecology and Characterization of Pectinolytic Bacterial Pathogens in Potato

Abstract

Blackleg and soft rot (BSR) diseases remain persistent threats to potato production in the upper Midwest, yet the full diversity of their bacterial causes is not well understood. In Wisconsin, routine diagnostics have primarily focused on Pectobacterium and Dickeya species; however, blackleg symptoms occasionally appear in field samples that test negative for these pathogens, raising questions about the completeness of current detection methods. To investigate this, we characterized the culturable pectinolytic bacterial community from frozen blackleg-symptomatic potato tissues collected between 2016 and 2020. Using selective media, tuber and stem pathogenicity assays, and 16S rRNA gene sequencing, we isolated and evaluated 87 bacterial strains from both PCR-positive and PCR-negative samples.

Twenty of these isolates caused soft rot symptoms in potato tubers, and half of them also induced lesions on potato stems. Most notably, Pectobacterium parmentieri was the predominant species among the pathogenic isolates, even in samples that had previously tested negative via qPCR, suggesting that earlier diagnostic protocols may have missed this species due to its former taxonomic ambiguity. In addition to established pathogens, we recovered several non-traditional genera, including Chryseobacterium, Bacillus, and Acinetobacter, from symptomatic tissues. Some of these, such as Chryseobacterium mulctrae and Bacillus pumilus, demonstrated clear pectinolytic activity and caused disease symptoms, highlighting their potential roles as opportunistic or emerging pathogens within the blackleg disease complex.

The inability to culture Dickeya spp. from PCR-positive samples likely reflects both biological characteristics, such as slower growth rates, and technical challenges associated with long-term frozen storage. Furthermore, the frequent co-isolation of multiple pectinolytic species from single samples underscores the likelihood of polymicrobial interactions contributing to disease development. Together, these findings reveal a broader spectrum of bacterial contributors to BSR symptoms than is typically captured by conventional diagnostics. This study emphasizes the importance of integrating culture-based, molecular, and ecological approaches to improve our understanding of pathogen diversity and inform more effective disease management strategies in seed potato systems.