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2019

Sammendrag

Invasive alien species and new plant pests are introduced into new regions at an accelerating rate, due to increasing international trade with soil, plants and plant products. Exotic, plant pathogenic oomycetes in soil from the root zone of imported plants pose a great threat to endemic ecosystems and horticultural production. Detecting them via baiting and isolation, with subsequent identification of the isolated cultures by Sanger sequencing, is labour intensive and may introduce bias due to the selective baiting process. We used metabarcoding to detect and identify oomycetes present in soil samples from imported plants from six different countries. We compared metabarcoding directly from soil both before and after baiting to a traditional approach using Sanger-based barcoding of cultures after baiting. For this, we developed a standardized analysis workflow for Illumina paired-end oomycete ITS metabarcodes that is applicable to future surveillance efforts. In total, 73 soil samples from the rhizosphere of woody plants from 33 genera, in addition to three samples from transport debris, were analysed by metabarcoding the ITS1 region with primers optimized for oomycetes. We detected various Phytophthora and Pythium species, with Pythium spp. being highly abundant in all samples. We also found that the baiting procedure, which included submerging the soil samples in water, resulted in the enrichment of organisms other than oomycetes, compared to non-baited soil samples.

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Sammendrag

Potato soft rot Pectobacteriaceae (SRP) cause large yield losses and are persistent in seed lots once established. In Norway, different Pectobacterium species are the predominant cause of soft rot and blackleg disease. This work aimed to evaluate the potential of real-time PCR for quantification of SRP in seed tubers, as well as investigating the status of potato seed health with respect to SRP in Norway. A total of 34 seed potato lots, including certified seeds, was grown and monitored over three consecutive years. All seed lots contained a quantifiable amount of SRP after enrichment, with very few subsamples being free of the pathogens. A high SRP prevalence based on a qPCR assay, as well as a high symptom incidence in certified seeds were observed, suggesting that current criteria for seed certification are insufficient to determine tuber health and predict field outcomes. Pectobacterium atrosepticum was the most abundant species in the examined seed lots and present in all lots. Consistently good performance of first generation seed lots with respect to blackleg and soft rot incidence, as well as low quantity of SRP in these seed lots demonstrated the importance of clean seed potatoes. Weather conditions during the growing season seemed to govern disease incidence and SRP prevalence more than seed grade. The impact of temperature, potato cultivar and Pectobacterium species on tuber soft rot development were further examined in tuber infection experiments, which showed that temperature was the most important factor in nearly all cultivars. Large-scale quantification of latent infection and predictive models that include contributing factors like weather, infecting bacterial species and cultivar are needed to reduce soft rot and blackleg.