Phytophthora in Finnish nurseries

International trade and travel have facilitated the spread of harmful organisms around the world. Human-mediated movement of plants and plant products is now generally accepted to be the primary mode of introduction of pathogens and pests. Species of Phytophthora, a genus of plant pathogens, are commonly spread in this way and have caused severe epidemics in silviculture, horticulture and natural systems all over the world. The objective of the study was to gather information on the occurrence of Phytophthora spp. in Finnish nurseries. Furthermore, the aim was to produce information for risk assessments for these Phytophthora spp. and establish molecular means for their detection. Phytophthora cactorum was found to persist in natural waterbodies and results suggest that irrigation water might be a source of inoculum for the disease in nurseries. In addition to P. cactorum, isolates from ornamental Rhododendron in nurseries yielded three species new to Finland: P. ramorum, P. plurivora and P. pini. The only species with quarantine status, P. ramorum, was able to persist in the nursery in spite of an annual European Commission sanitation protocol. Phytophthora plurivora and the closely related P. pini had more hosts among Nordic tree species and other plants abundant in Finnish nurseries and forest ecosystems. They were also found to have higher infectivity rates compared to P. ramorum and P. cactorum. All four species survived two weeks in -5 °C , and thus soil survival of these Phytophthoras in Finland is likely under current climatic conditions. The most common tree species in Finnish nurseries, Picea abies, was highly susceptible to P. plurivora and P. pini in pathogenicity trials. In a histological examination of P. abies shoot tissues inoculated with P. plurivora zoospores, fast necrotrophic growth was observed in nearly all tissues of the fresh shoot. The production of propagules in P. abies shoot tissue was only weakly indicated. In this study, a PCR–DGGE technique was developed for simultaneous detection and identification of Phytophthora spp. It reliably detected Phytophthora in plant tissues and could discriminate most test species as well as indicate instances of multiple-species infections. It proved to be a useful detection and identification tool either applied alone or in concert with traditional isolation culture techniques. The limitations of the method are also discussed. In conclusion, all of the introduced species of Phytophthora had properties that promote a high risk of establishment and spread in Finland. The efficient transport of Phytophthora via commercial traffic and favorable conditions due to expected climate change will increase the incidence and establishment of Phytophthora in new areas. Thus, it is probable that pathogens of this genus will be introduced and become established in Finland and other Nordic countries unless efficient phytosanitary control becomes standard practice in the international plant trade.