Report: effects of antagonists on resistance against soft rot Pectobacteriaceae in potato

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Soft rot Pectobacteriaceae (SRP), causing blackleg and soft rot in potatoes, are still a major problem in seed potato farming. Since no resistant cultivars are known yet and no chemical compounds are available with effectivity against SRP, there is increased interest in the use of biocontrol agents. In the presented study we investigated the effectivity of potential bacterial antagonists on resistance against black leg. Bacterial isolates were obtained from potato tissue or the surrounding soil as these strains are adapted to living on and in the potato ecosystem. An informed choice was made from the panel of bacterial strains, using several selection criteria: in-vitro inhibition of SRP, the ability to macerate tuber tissue, presence in high densities in tuber tissue, belonging to risk group 1, and a high taxonomic diversity. The selected antagonists were tested in a 4-year field trial. In all 4 years potato tubers of the cultivars Agria (2018), Kondor (2019) en Kondor and Mozart (2020 & 2021) were vacuum inoculated with either Pectobacterium brasiliense or Dickeya solani. Subsequently, the tubers were spray-inoculated with the selected isolates either as single strains or in combinations of three strains. In 2020 and 2021, several combinations from the previous years were used as well as new combinations. Also combinations of 2 or 5 strains were tested in addition to talc formulations of the isolates, application of the bacteria directly into the planting hole and inoculation on naturally infected tubers. Tubers were planted in the field, in 2018 and 2019 in one location and in 2020 and 2021 in two locations (sandy soil and clay soil) and scored every 2 weeks for blackleg symptoms. In 2018, plant emergence was very low and therefore only little differences could be detected between the treatments and control. In 2019, in contrast, several treatments consisting of three strain mixes were highly effective against P. brasiliense, reducing disease incidence to as low as 0%. However, when these treatments were applied again in 2020, this effect could not be replicated. In general, the effect of antagonist treatments was highly variable between the two cultivars, locations and replicates. Also in 2021, the treatment effects differed from the previous years and between locations, cultivars and replicates. Talc formulation or plant hole inoculation did not reduce disease incidence. In both 2020 and 2021, disease incidence was higher in cultivar Kondor than in cultivar Mozart and higher in sandy soil than in clay soil. In order to characterize the antagonists used in this study in more detail, the whole genomes of 78 of the used isolates were sequenced. An analysis of the presence of genes for secondary metabolite production showed that most isolates had the potential to produce antimicrobial compounds. It was also tested if the isolates were able to survive in storage for two weeks from inoculation to planting. 22 of the 25 tested strains could still be detected at relative high densities after 15 days. Moreover, plant stems from three treatments and controls were harvested at the end of the growing season in 2021 and tested for the presence of the used isolates. In only one stem of 28 stems, one of the isolates could be detected. The results indicate that the isolates had the potential to suppress SRP in potato tubers. However, it is possible that inoculation on the surface of the tuber did not lead to an effective colonization of the plants. The antagonists were exposed to the indigenous soil and tuber microflora under low temperature conditions. On the other hand, the pathogens were inoculated by vacuum inoculation and were therefore already present inside the tuber periderm in high concentrations. The effect of the antagonists may be improved if they are vacuum infiltrated into the tubers. As endophytic bacteria have been demonstrated to mostly originate from soil and colonize the plant through the roots, increase of plant resistance could be achieved through improvement of the soil microbiome and stimulating plant colonization of antagonistic bacteria. However, both soil microbiome manipulation as factors determining plant colonization are still poorly understood. More knowledge should be generated on the soil microbiome manipulation and plant colonization of biocontrol agents.
Original languageEnglish
Place of PublicationWageningen
PublisherWageningen Plant Research
Number of pages57
Publication statusPublished - Jan 2021

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NameReport / Stichting Wageningen Research, Wageningen Plant Research, Business Unit Biointeractions and Plant Health


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