Invasive Phytophthora species affecting broadleaved tree species in urban and landscape settings in Southern Sweden

Abstract: Phytophthora translates to ‘plant destroyer’ and are fungal-like, hemibiotrophic plant pathogens. Phytophthora was discovered in the mid 19th century, as the causal agent of the widespread collapse of potato crops throughout Europe. Today, more than 150 Phytophthora species have been identified and they continue to cause massive problems, both in agriculture and forest ecosystems. There are many Phytophthoras species responsible for dramatic forest diebacks. In Europe, Phytophthoras are casual agents of oak decline, ink disease of chestnut, beech decline and dieback of alders. The aim of this study was to examine which forest Phytophthora species are present in southern Sweden. Symptomatic trees were identified in five urban forest settings in Southern Skåne. Stem tissue and soil surrounding symptomatic trees were collected. Phytophthora isolates were cultured on artificial media and their DNA sequenced by sanger-­sequencing. The obtained sequences were compared with the known reference sequences using the basic local alignment search tool (BLAST) at NCBI. Five Phytophthora species were identified: P. cactorum, P. cambivora, P. gonapodyides, P. plurivora and P. syringae. The result includes the first findings of P. gonapodyides causing stem lesions on trees in Sweden. Confirmation of pathogenicity of P. gonapodyides in an inoculation trial using obtained P. gonapodyides isolates confirmed that they were able to infect healthy beech seedlings, resulting in stem lesions. Many common tree species in Southern Sweden are susceptible to the Phytophthora species found in this study. Trade with nursery seedlings is an important pathway of dispersal and establishment of Phytophthoras in natural environments. There is no efficient treatment and it is practically impossible to eradicate Phytophthora once it has become established in a site. If the identified Phytophthora species become widely established in Sweden, they may cause severe damages to forest ecosystems as well as for the Swedish forestry sector.