This plant disease is caused by a fungus (Cytospora kunzei) that is very destructive to Norway and Colorado blue spruce. The fungus enters the tree at a wound, killing the surrounding healthy tissue. A canker develops and expands through the wood in all directions. When the canker encircles a branch, the branch dies and the needles turn brown. Sap oozes from the dying branch. Eventually small black spore-producing bodies develop in the bark. Older (more than 15 years), weak, and injured trees are most susceptible to the disease.

The cooley spruce gall adelgid requires 2 years to complete its life cycle and may spend part of its life on both spruce and douglas fir. In late spring, the insect feeds, matures, and lays several hundred eggs at the base of needles near branch tips. After hatching, young insects move to growing needles. They suck juices from the needles, inducing the formation of galls that enclose them. The adelgids live and feed in chambers inside the galls. In July, the insects mature and emerge through openings in the dried galls. These adelgids develop wings and migrate to douglas fir or another spruce. The adelgids on douglas fir lay eggs, producing a generation of woolly aphids. (Adelgids are close relatives of aphids and are sometimes called aphids.) Some of these aphids grow wings and migrate back to spruce. Others are wingless and remain on douglas fir. Aphids that do not migrate to douglas fir lay eggs on spruce after emerging from the galls.

This disease is caused by a fungus (Rhizosphaera kalkhoffii) infecting only spruce trees. It is most common on Colorado blue spruce (Picea pungens) but also infects white spruce (P. glauca) and occasionally Norway spruce (P. abies). From late spring to fall, rain splashes spores from diseased needles-on the tree and the ground-onto young healthy needles or to stressed older needles. The fungus grows within the needles for almost a year with no symptoms; then needles turn yellow or purple-brown and die. The cycle begins anew the next spring, Spores infect leaves that are wet for at least 48 hours, so they are worse during wet springs. The fungus does not usually kill trees, but dead branches spoil the appearance.

Choristoneura species Spruce budworms are very destructive to ornamental spruce, fir, and douglas fir and may infest pine, larch, and hemlock. The budworm is cyclical. It comes and goes in epidemics 10 or more years apart. The moths are small (1/2 inch long) and grayish, with bands and spots of brown. The females lay pale green eggs in clusters on the needles in late July and August. The larvae that hatch from these eggs crawl to hiding places in the bark or in lichen mats, or they are blown by the wind to other trees, where they hide. The tiny larvae spin a silken case and hibernate there until spring. In May, when the weather warms, the caterpillars tunnel into needles. As they grow, they feed on opening buds; later they chew off needles and web them together. The larvae feed for about 5 weeks, pupate on twigs, and emerge as adults.

Endothenia albolineana The spruce needle miner is the larva of a small 1/2 inch), dark brown moth. The moth lays eggs in late spring to early summer on the undersides of old needles. The larvae that hatch from these eggs bore into the base of the needles, feeding on the interior. When the interior is consumed, the caterpillars cut off the needles at their base and web them together, forming a nest. The needle miners feed until the first frost and then enter a hollow needle, where they spend the winter. When the weather warms in spring, the larvae continue feeding until April or May. They pupate inside the webbed nest of needles and emerge as adults to lay more eggs. Several other types of needle miners may cause similar damage to spruce trees.

Oligonychus ununguis The spruce spider mite is one of the most damaging pests of spruces and many other conifers because it sucks sap from the undersides of needles. As a result of the mites'' feeding, chlorophyll disappears, causing a stippled appearance. This symptom may be mistaken for damage caused by air pollution. Spider mites first appear between April and June. In subtropical areas, mites may be active during warm periods in winter. They are most active in the cool temperatures of spring and fall, becoming dormant in hot weather (over 90°F). A complete generation takes only 17 days, so mites rapidly build up to tremendous numbers. Older trees may dieafter several years, with symptoms progressing from the lower branches upward. Young spruce trees may die the first season. Several other kinds of mites also infest spruce trees.