SNS-109: Decline of Fraxinus excelsior in northern Europe (2010-2012)

Project leader: Rimvydas Vasaitis, SLU, Sweden, rimvys.vasaitis@mykopat.slu.se

Financing: 50.000 Euro/year for 3 years

The current spread of ash (Fraxinus excelsior L.) dieback is an alarming forest health problem, threatening the existence of the tree species in large parts of Europe. A new Nordic-Baltic project, supported by SNS, will increase our understanding of the disease and develop strategies for reducing its impact.

The first symptoms of ash decline were observed in the southern Baltic States in the mid-1990s. In 2002 the disease had been observed only locally in southern Sweden. However, in the summer of 2004 it spread throughout the south, towards western and central parts of the country. In 2003-2004, dieback was first noticed in Denmark, where it spread rapidly in 2005-2008. In 2007-2008, the disease had also emerged in Norway and Finland. Consequently, the epidemic is now in differing stages in different areas of northern Europe: an initial phase in Finland and Norway, peaking in Sweden and Denmark, and a post-decline (or chronic) phase in Latvia and Lithuania.

The fungus Chalara fraxinea has been demonstrated to be the causal agent of the dieback. It has recently been identified as a form of a common, decomposing fungus (Hymenoschyphus albidus), which is native and widespread in Europe, and known to decompose ash in the forest litter. It is not known why it has emerged, or become so aggressive. However, the pathogen could be an invasive hybrid, or an indigenous species that has become more virulent recently, possibly because the trees have been weakened by stresses, such as those associated with drought, changes in temperature or frost.

In a new project, a team of researchers from Sweden, Denmark, Finland, Norway, Lithuania and Latvia will:

• Check the identity of the dieback-causing fungus Chalara fraxina, and assess its potential invasiveness
• Investigate if climatic factors affect the development of the disease
• Study the genetic structure, biology and epidemiology of the fungus
• Suggest silvicultural management regimes for declining ash stands
• Evaluate long-term phytosanitary consequences in regions devastated by the disease
• Assess the degree of genetic variation in susceptibility of the Nordic ash population, in order to guide breeding programmes to counter C. fraxinea