Fungal disease raises questions about the growth potential of Douglas fir

We now need more knowledge about the distribution of the needle diseases and the factors that determine which Douglas fir trees are most resistant to it.

Douglas fir has attracted growing interest as forestry seeks to adapt to a changing climate. With its rapid growth, high-quality timber and potential to create greater diversity into forests dominated by spruce and pine, Douglas fir is seen as a promising alternative.

Recently, research trials and operational plantations at several locations show discoloured needles, needle loss and thinned crowns in Douglas fir. Following field visits and DNA analyses, researchers have established that symptoms in several locations are associated with the fungus Nothopheaocryptopus gaeumannii, which causes Swiss needle cast, affecting the needles and reduces their function, which can have a negative impact on the trees’ growth. However, other fungi are also involved including Rhizosphaera, Sydowia polyspora and possibly Rhabdocline pseudotsugae, all of which contribute to premature needle cast.

The severity of the damage to the trees varies considerably. In some stands, affected trees retain only one or two years of needles, whilst in other stands there are trees with significantly denser crowns and retain needles from several years. Although the overall extent of the problem in Sweden is yet unknown, these observations suggest that the prevalence and impact of needle diseases in Douglas fir may be more widespread and consequential than previously recognized.

‒ Determining the distribution of the diseases, the susceptibility of different provenances and the environmental factors that influence disease development is important knowledge for assessing the future risk to Douglas-fir in Swedish forestry, says Michelle Cleary, senior lecturer in forest pathology at SLU.

Why do some trees cope better?

An interesting discovery made in North America shows that trees react very differently to these needle diseases. Some trees appear to be able to restrict fungal infection, whereas others retain their needles and continue to grow despite the presence of the fungus. Why some trees cope better than others is therefore an important question to investigate. 

‒ In western North America, we have repeatedly seen that Douglas fir provenances planted on unsuitable sites, can be heavily affected by needle-cast diseases. Matching the genetic material to the local climate can be just as important as maximising growth potential. The same question must now be asked regarding Douglas-fir in Sweden, says Michelle.

Swedish trials may provide important answers

In Sweden, there is a large network of research trials where Douglas-fir from different seed sources has been planted in various environments, from the coast to inland areas. Together, these trials enable researchers to compare trees of different origins, at different ages and under varying climatic conditions.

This makes it possible to investigate how susceptibility to needle diseases varies among provenances and environments and whether current recommendations for the selection and use of Douglas-fir sources need to be revised.

Many remaining questions 

Although Nothophaeocryptopus gaeumannii has been found in several locations in Sweden, we still know relatively little about the fungus and its impact here. Among other things, it is unknown how and when it arrived in Sweden, whether it first entered the country on one or more occasions, and whether different genetic lineages of the fungus result in symptoms of varying severity.

Greater knowledge of the fungus’s origin, genetic diversity and distribution could help researchers understand why the disease causes greater damage in some places than others.

There is also limited knowledge of how Swedish climatic conditions – such as temperature, precipitation and humidity – affect the development of the disease. This is particularly important as the Douglas-fir is being promoted as a species that can contribute to the adaptation to climate change. To be able to make well-informed decisions, we therefore need a better understanding of where, and under what conditions, these needle diseases pose a significant risk.

Future research needs to provide answers to these questions so that we can better assess the role Douglas-fir can play in the future of Swedish forestry.