Report: Resistance of potato tubers against soft rot Pectobacteriaceae (SRP): Disease resistance in potato tubers against soft rot Pectobacteriaceae

Soft rot Pectobacteriaceae (SRP) continue to cause blackleg disease and tuber soft rot during cultivation of seed tubers. Currently, there are no chemical compounds that are effective against SRP and no resistant cultivars. Past studies have indicated differences in disease incidence between tuber lots of the same cultivar. The goal of the present study was to determine if there are indeed differences in disease incidence between different lots inoculated with the currently most frequently occurring SRP species Dickeya solani and Pectobacterium brasiliense. Subsequently, we assessed if these differences were associated with differences in the tuber microbiome (bacteria and fungi) and/or the set of secondary compounds in the tuber.In two consecutive growing seasons, different lots of the cultivars Kondor and Spunta, originating from different locations, were planted in two common fields with an equal load of the two pathogens. Differences in disease incidence were assessed and in both years three lots with a high disease incidence and three lots with a low disease incidence were selected for microbiome and metabolome analysis.These lots belonged to the cultivar Kondor, as disease incidence in cultivar Spunta was generally low and no large differences could be detected. In addition, a pot experiment was conducted, in which the different lots were infected with the Rhizoctonia solani and Colletotrichum coccodes in order to determine if potential suppressiveness in the lots would also be effective against these fungal pathogens. In the second year, microbial community composition was additionally measured in the different soils that the lots originated from to compare taxa present in the tuber and in the soil. Abiotic soil parameters were measured as well to determine their potential influence on suppressiveness.In both years, differences between lots in blackleg disease incidence were found, although differences were larger in year 1 than in year 2 and disease incidence varied with location and pathogen. No differences were found in the disease incidences with R. solani and C. coccodes. Both the bacterial and the fungal community composition differed between lots with a low and a high disease incidence. In 2018, the bacterial taxa Pseudomonas sp., Curtobacterium sp., Pantoea sp. and Rhodococcus sp., as well as the fungal taxa Vishniacozyma heimaeyensis, Penicillium brevicompactum, Debaryomyceshansenii, and Rhodotorula babjevae were among the taxa that were significantly increased in tubers with a lower disease incidence, i.e. higher suppressiveness. In 2019, the bacterial taxa Staphylococcusspp., Pseudarthrobacter sp., Glutamicibacter sp., Paenarthrobacter sp., Brevibacterium sp., CandidatusUdaeobacter, and members of the Bacillacae, and the fungal taxa Fusarium oxysporum, Debaryomyceshansenii, Plectosphaerella niemejerarum, Vishniacozyma heimaeyensis, and Mycosphaerella tassianashowed most association with a low disease incidence. In soil, taxa from the genera Bacillaceae and Planoccocaceae, and the genera Candidatus Udaeobacter and Bradyrhizobium, as well as the fungi Cladosporium cladosporides, Saitozyma podzolica, Fusarium oxysporum and two species of the genus Solicoccozyma were correlated with a low disease incidence in the tubers originating from this soil. Several of these taxa have previously been described as being involved in plant growth promotion and disease suppressiveness, indicating that they might have showed antagonism against SRP. Moreover,there was a high variation between the two years and between individual lots in microbial community composition. This indicates that different taxa can increase resistance against the SRPs rather than a specific set of species. It was also found that most of the taxa that were associated with suppressivenessin the tuber were also present in the soil of origin, indicating that these taxa might have colonized the plants from the soil as endophytes. However, abundance in soil was not correlated with abundance in the tuber, meaning that also soils with a low abundance of the respective taxa can yield tubers with a high abundance and vice versa. Which factors influence recruitment and final abundance in the tubers,is poorly understood and should be subject to further investigation. In addition, no clear correlation was found between abiotic soil parameters and disease suppressiveness, indicating that suppressiveness is not dependent on soil type or mineral status within the range of soils that was tested in this study.The results of this study suggest that bacterial and fungal taxa in the soil can colonize tubers growing in the respective soil and contribute to disease suppressiveness against SRP in the following field generation. In order confirm this hypothesis, the respective taxa will have to be added to infected tubers to prove an effect in disease incidence