Spruce trees are coniferous evergreens. Desirable for their naturally pyramidal shape and graceful form, they are often used as Christmas trees and are the state tree of four western states. Although generally hardy, spruce trees can suffer from some diseases, most of which affect the needles of the tree. These diseases render the tree less valuable commercially and decreases its landscape usefulness, especially in the case of trees planted for screening purposes, according to North Dakota State University.
Cytospora canker is a fungal disease. The fungus (Cytospora kunzii) travels on rain, pruning tools, or animals and enters the tree through a wound in the wood. Any crack in the bark can create an opening for this fungus. Cuts made by pruning shears can also cause the fungus to enter the tree. The fungus causes the wood of the tree to die, creating cankers, or black, dead areas of wood. Often, the lowest branches of the tree die first. The most noticeable symptom of Cytospora canker is that the outer needles of infected branches will turn brown and drop off.
Cytospora canker most commonly affects older, blue and white spruces, according to North Dakota State University. Removal of infected branches can slow the process of the disease. The best way to prevent the disease is to make sure you plant a healthy tree and use pruning tools that are sterilized with bleach.
Rhizospaera Needle Cast
Rhizospaera kalhkoffii is the pathogen that causes this fungal disease. Spread by splashing or blowing rain, this fungus infects the needles of the tree as the tainted water sits on them. Very wet spring weather can increase the chances of an infection developing. The Colorado blue spruce is the species most affected by this disease.
Infected needles turn brown and appear to be speckled with tiny black spots, which are the fruiting bodies of the fungus. In severe cases, the needles will fall off. Needles close to the trunk of the tree will be affected first. Often, these turn brown while the needles at the ends of the branches remain green. The disease usually works its way up the tree from the lower branches.
Rhizospaera needle cast is hard to prevent. Plant spruce species that are less likely to get the disease, such as the Sitka spruce. Provide plenty of air circulation around your spruce so that water left on the needles will quickly dry. Prune off infected branches to slow the spread of the disease.
Sooty mold is a fungus that infects and coats the needles and branches. The fungus appears black and powdery, giving rise to the common name. The fungus gets its nutrients from honeydew, which is excreted by insects. For this reason, spruce trees infested with insects are most likely to get sooty mold disease. If the build-up of fungus is very heavy, it can interfere with the process of photosynthesis.
The best way to prevent sooty mold is to monitor and control insect infestations. Young spruce trees can be sprayed with an insecticide if an infestation seems severe. In most cases, however, the trees are too large to properly cover. In that case, try to rinse off as much of the sooty mold as you can with a strong stream of water.