A new pathogen arose in Finland – our research anticipates its potential effects

Preparing for climate change risks is a solid part of our research at Natural Resources Institute Finland (Luke). Climate change not only increases the risks caused by native pests but is also forecast to bring new pests to Finland. We therefore strive to monitor the various pests that threaten the Finnish forests, conduct research on them, and anticipate the potential risks they may cause.

The summers of 2021 and 2022 were hot and dry. Trees stressed by the drought were in trouble, as they became too weak to defend themselves effectively against pests and pathogens. The conditions were especially serious in rocky and barren sites, where the effects of drought are most severe.

Because of the drought, the fungus Diplodia sapinea emerged as a pathogen along the south-west coast of Finland. The previously symptomless symbiont of pines had transformed into a deadly pathogen in the drought-stricken trees and began to kill both seedlings and mature pines.

“Diplodia sapinea is already a common pest in Central Europe, but in Finland it is still often confused with symptoms caused by other pathogens during drought”, says Dr. Eeva Terhonen, who is responsible for studying the disease at Luke.

The research started by determining how widely the disease had spread. Information was gathered by means of citizen science. The aim was to obtain suspected samples of D. sapinea from citizens from all over Finland. To date, 1,200 samples have been tested, one third of which have been positive for D. sapinea.

Currently, D. sapinea has spread across the south-west coast and some inland regions of Southern Finland. Genetic analyses are currently being performed on the collected strains, which will enable us to assess the probable distribution history of the fungus. The research was funded by Luke and Alfred Kordelin Foundation.

However, as D. sapinea also lives in pine trees without causing symptoms, its true distribution is yet to be determined. How far north can the disease kill trees if droughts become more common? Which insect species spread D. sapinea, and which other plants can serve as its hosts? The research will continue by addressing these questions, among others.

According to climate change scenarios, hot and dry periods during the summer will become more probable. This will also increase the risk of the disease caused by D. sapinea especially in dry and barren sites, which are typically places where only Scots pine can thrive. This is one of the challenges of this pathogen: it strikes in places where replacing pine with other tree species is difficult or even impossible. Pine is also an important urban and park tree, especially in the coastal region.

Under climate change, the health of forests can be threatened not only by pests and pathogens that we already are familiar with, but also by usually harmless fungal partners such as the D. sapinea. They can become opportunistic and deadly pathogens when the changing climate favours them, by weakening their host trees.