Spruce Sirococcus Blight

Host Trees

Sirrococcus blight affects several species of pine trees and western hemlock. Colorado blue, black, red, white, and Norway spruce are also preferred hosts of this fungus. Symptoms in spruce trees typically appear from June to August and if it is not eradicated, the fungus will remain throughout the winter in the dead branches and produce spores the following spring, when it reinfects the host species and vulnerable trees around it.

Sirococcus blight infects young needles and then grows to the new branches, where it forms cankers. The fungus rarely extends into older wood, but will spread into the tissue of new and emerging shoots. As the fungus begins, you may observe small black spots called pycnidia on dead needles and shoots. These are fruiting bodies that produce transparent spores, called conidia. The spores move to new hosts by splashed water droplets from rain or irrigations systems. They germinate when the humidity is high and the temperature and light is low.

Spruce needles and branches with sirococcus blight will curl, turning brown and then wilting. This is commonly found first in lower branches, but it may also be observed in higher parts of the tree. During early summer months, browning is seen at the base of the needles. The discoloration spreads until the entire needle is brown. Drops of liquid may also be observed coming from the needles and the tree's new branches show small black dots, which are the fungus' fruiting bodies. Infected branches appear hooked and the needles eventually fall off. These defoliated shoots can remain on the tree for several years. Cankers appear on the current year's branches as well. These cankers are purplish in color. Repeated infections with sirrococcus blight can be fatal to young trees. Mature trees that are otherwise healthy rarely die from this infection but often will become misshapen with lower branches dying off completely.

Control sirococcus blight by removing infected branches and destroying them. This pruning should be done regularly to reduce the risk of the fungus infecting nearby trees. In cases of severe infection, the entire tree may have to be removed and destroyed. Prevent infection in other trees by ensuring that trees have enough space between them to allow for adequate aeration. This reduces the moist, shady conditions in which the fungus thrives. Fungicides are available to treat sirococcus blight as well, but they must be applied in late spring, or when the spores are first spread to healthy trees. Typically fungicides should be applied to young trees with new branches or needles and repeated in three to four week intervals. If high moisture conditions are present or if trees exhibit rapid new growth, increase the frequency of treatment.