Invasive Phytophtora Species

SNS-121: Assessing the role of climate factors in association with spread of invasive Phytophtora species in forests and from urban landscapes

Coordinator: Michelle Cleary, SLU, SE

Funding: 313.000 SEK / Year

Duration: 2016 – 2019

Read about the project’s kick-off meeting HERE!

Forestry is one of the major economic engines of Nordic countries as well as a sociocultural icon. However, climate change and increasing international trade of plants and plant products are presenting major challenges to the health of forests that potentially threaten the forest sector’s sustainability. Over the last 200 years, the number of invasive forest pathogens introduced to Europe has increased exponentially (Santini et al. 2013). Outbreaks of forest disease caused by both native and introduced forest pathogens are predicted to become even more frequent and intense with climate change as drought and other abiotic stressors are amplified under the predicted climate scenarios. However, uncertainty pervades these predictions about the future impacts of forest diseases, in part because the effects of climate change on host-pathogen interactions are complex.

Several species within the genus Phytophthora belong to a group of microscopic pathogens responsible for major plant disease in many parts of the world. By destroying the trees’ fine roots, these pathogens disturb nutrient and water uptake, thereby reducing the fundamental vitality of trees, leading to instability and premature death of trees. Some devastating Phytophthora spp. are also airborne (e.g. P. ramorum). In contrast to most forest diseases that impose a threat to one specific host species (e.g. ash decline), Phytophthora-pathogens are a potential threat against a broad range of tree species, including both conifers and broadleaved trees.

Over the last 15 years, the prominence of Phytophthora tree pathogens in Northern Europe has increased dramatically. This is believed to reflect a marked increase in the introduction and spread of invasive Phytophthora spp. via imported planting stock. However, the situation is probably not due to the increasing global trade of plants alone, but also connected to changes in climate conditions over the last 60 years, i.e. increased mean winter temperatures, seasonal precipitation shifts from summer into winter, and a tendency for heavy rain; all factors favoring infection by several species of Phytophthora.


The impact of climate change on the virulence and spread of Phytophthora species found in Nordic and Baltic countries is unknown. Research elsewhere in the world with different Phytophthora pathosystems suggest that these pathogens may benefit from changes in environmental conditions due to climate change (Brasier 2003), because anticipated warmer winters with winter rain and more regular summer drought conditions with higher temperatures will favour pathogen activity. A critical climate change component to the virulence of Phytophthora species is temperature. Changes in temperature (both air and soil) may cause the disease to develop also in areas at higher altitudes and more northern latitudes than present; that is, forest types currently considered too cool for disease development. In Europe, some research already predicts that the increasing temperatures associated with climate change will lead to a potential range expansion of some Phytophthora species of up to a few hundred kilometres (Bergot et al. 2004). In addition, it is predicted that in areas where rainfall will exceed 600 mm per year, there is likely to be an increase in microorganisms’ activity.

Nevertheless, limiting factors might also appear in areas of drought because Phytophthora spp. rely on flagellate zoospores swimming in free water, and therefore requires adequate soil moisture and spaces between the soil particles filled with water, to spread (Desprez-Loustau et al. 2006, Fitzpatrick et al. 2008). In an effort to understand the potential impacts of Phytophthora species in Nordic and Baltic countries in the face of climate change more knowledge is needed on how host, pathogen and soil interactions, will play out under changing climatic conditions.