Unit 5 - Health of the Forest Ecosystem

Lesson 5: Disease

Disease-causing organisms infect and reproduce within the tree. In a healthy forest, diseases can actually play an important role since they serve to remove old or weakened trees.

Usually disease is a natural part of the ecosystem, but sometimes the activity of disease-causing organisms can become extreme.
As is the case with insect pests, diseases from another region or country can often have severe effects for a forest, given that the trees have no natural immunity.

Two common tree diseases affecting Alberta forests include Armillaria root disease and dwarf mistletoe.


Armillaria root disease can infect all tree species in the  province.

It is caused by a fungus that slowly grows up into the root, eventually into the wood underneath the bark and into the trunk.
The infection then spreads and induces sapwood decay in the affected areas; this eventually kills the tree.

For more information on Armillaria root rot, click here.







Dwarf mistletoe affects lodgepole pines. It is a parasite that depends on the tree for nutrients and interferes with the normal growth and function of the tree's trunk and branches.

Trees affected by the dwarf mistletoe are deformed. Their branches tangle together in a cluster that resembles a witch's broom.

Dwarf mistletoe causes damage and eventual death to lodgepole pine.

For more information on dwarf mistletoe, click here.