Damage, symptoms and biology
The fungus causes fir needles to wilt and drop prematurely. In severe infections of young trees, the disease can cause a severe reduction in growth. The fungus lives on two hosts in alternation, balsam fir and fireweed Epilobium sp., a plant with purple, lilac coloured flowers. While living on its fireweed host, the fungus occasionally infects other ornamental plants in the same family. In spring, small white blisters develop on the lower surface of balsam fir needles. The yellow-orange spores discharged from these blisters end up infecting the young shoots of fireweed plants.
Fireweed is a beautiful wild plant but it should not be naturalized in our gardens because it is highly invasive and serves as the primary vector for the disease. In Christmas tree plantations, it is important to remove the alternate host to prevent the disease from appearing in the crop trees.