Role of co- infection by Pectobacterium and Verticillium dahliae in the development of early dying and aerial stem rot of Russet Burbank potato

Potato early dying (PED) is a disease complex primarily caused by the fungus Verticillium dahliae. Pectolytic bacteria in the genus Pectobacterium can also cause PED symptoms as well as aerial stem rot (ASR) of potato. Both pathogens can be present in potato production settings, but it is not entirely clear if additive or synergistic interactions occur during co-infection of potato. The objective of this study was to determine if co-infection by V.dahliae and Pectobacterium results in greater PED or ASR severity using a greenhouse assay and quantitative real-time PCR to quantify pathogen levels in planta. PED symptoms caused by Pectobacterium carotovorum subsp. carotovorum isolate Ec101 or V.dahliae isolate 653 alone included wilt, chlorosis and senescence and were nearly indistinguishable. Pectobacterium wasabiae isolate PwO405 caused ASR symptoms including water-soaked lesions and necrosis. Greater Pectobacterium levels were detected in plants inoculated with PwO405 compared to Ec101, suggesting that ASR can result in high Pectobacterium populations in potato stems. Significant additive or synergistic effects were not observed following co-inoculation with these strains of V.dahliae and Pectobacterium. However, infection coefficients of V.dahliae and Ec101 were higher and premature senescence was greater in plants co-inoculated with both pathogens compared to either pathogen alone in both trials, and V.dahliae levels were greater in basal stems of plants co-inoculated with either Pectobacterium isolate. Overall, these results indicate that although co-infection by Pectobacterium and V.dahliae does not always result in significant additive or synergistic interactions in potato, co-infection can increase PED severity.