Many aspen trees in Troms and Finnmark have been left almost without leaves, and the damage is so extensive that aspen is close to being wiped out in the north. After several years of searching, researchers have now discovered the cause.
Aspen canker disease has been occurring over a fifteen-year period in Northern Norway, but in recent years the damage has increased significantly, according to the annual Forest Health Report from NIBIO.
Until now the cause was unknown, but recently a research group at NIBIO has confirmed that the damage is related to a fungus called Cytospora chrysosperma.
Isabella Børja, Research Professor at NIBIO, describes how in the spring in Troms and Finnmark you can see many aspen groves with dead branches. The condition is also common in northern parts of Finland, and probably in neighbouring parts of Sweden and Russia as well. NIBIO researcher Venche Talgø has also discovered similar damage to aspen in elevated areas in Jotunheimen and along Gudbrandsdalen.
In the crown many of the branches are dead, and the leaves develop only sporadically on some branches. Upon closer inspection, you can see many small black balls sticking out through the bark on the dead or dying branches.
These black balls, which are smaller than pinheads, are the asexual stage of the canker-causing fungus, Cytospora chrysosperma. Finding the sexual stage (Valsa) is rarer.
The researchers have isolated several fungi from cankered aspen trees.
“We have been able to confirm that Cytospora has been isolated from several of our aspen samples,” says Børja. “Our observations indicate that the fungus can probably live in healthy branches and buds. It doesn’t cause damage until the trees become weakened or stressed.”
Because the damage is so prominent and extensive that it threatens to wipe out aspen trees in the north, the researchers have started working on several systematic collections and analyses of plant materials.