Increasing Resistance to Root Disease in Douglas-Fir

December 2011

Mushrooms of the armillaria root disease fungus

In Canada, armillaria root disease affects trees from coast to coast. This slow-growing fungus can reduce growth and increase a tree’s vulnerability to stresses such as drought and insects. The disease can eventually be fatal, with between one half to one percent of conifer trees succumbing to the fungus annually.

The fungus increases its presence by colonizing tree stumps and using them as a food base. Almost 50 percent of trees in Canada’s southern boreal forest can become infected by root disease by the age of 100 years. In southern British Columbia the problem is especially bad, with up to 80 percent of trees being infected by the age of 80.

Armillaria root disease is a serious issue for the forestry industry, since it can reduce wood value by decreasing timber quality. “This is a major issue upstream in the value chain, where buyers rely on consistent wood quality,” says Mike Cruickshank, a research scientist with the Canadian Wood Fibre Centre at Natural Resources Canada (NRCan).

Douglas-fir trees killed by armillaria root disease

To reduce the impacts of the disease, NRCan researchers are developing resistant strains of Douglas-fir trees. Douglas-fir is the current focus of research because it is both economically important and highly susceptible to root disease. Through the selection of seeds from trees that effectively counter the disease, economically desirable trees can be grown to harvest.

Identifying Natural Resistance

To identify trees with a natural resistance to root disease, Mike and his team have partnered with the BC Ministry Forests, Lands and Natural Resource Operations. Several field studies as well as a greenhouse study were conducted to analyze the effects of the disease on survivorship, tree growth and wood properties.

The results showed that the best-surviving trees have good growth before infection, but suffered the highest growth impact after infection. This finding suggests that trees able to halt the fungus do so at a cost in growth. That cost is significant, since growth rates determine tree ring width, which affects both the quality and quantity of wood products.

The research continues. The team is now working to identify trees that are both good survivors and growers, having desirable shape and wood properties even when diseased.

Distorted tree stem ring growth caused by root disease

“We believe that a combination of disease-tolerance or -resistance traits could maximize growth after infection, reduce wood heterogeneity and, ultimately, improve the value of timber products,” says Mike. The ability to withstand root damage caused by disease will also help tree stands cope with other co-occurring stresses such as drought.

What does this mean for forest managers? Mike recommends that besides looking at commonly studied traits like height growth and tree size, disease tolerance and resistance should also be considered in multiple trait selection. This may help to prevent timber harvest losses and declines in wood quality and quantity over time.

To find out more about the armillaria root disease, visit the Canadian Forest Service’s “armillaria root rot” Web page.

If you found this article interesting, you may want to read “Forest Fungi Research: Fighting a Hidden Enemy”.

To read about related articles, see Forest Disturbances

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