There are several reasons why a spruce trees needles might turn brown and drop. Before action is taken on saving the tree, it is important to find out why the needles are dropping as this will determine the necessary course of action, if any. The most common cause for needle drop is simply natural causes. Needles will stay on a spruce tree for 2-3 years before dropping, typically from the inside of the tree and on older branches. Environmental factors can influence this cycle as well; for example, a tree that is under stress from things like drought, soil compaction or excessive moisture will enhance needle drop. Trees that were under water during last year’s wet spring/early summer may be exhibiting stress in the form of browning/needle drop for the first time this spring.
If needles are browning at the tips of the branches followed by lower branches dying, you may be dealing with a fungal disease known as cytospora canker, which is the most common unnatural cause for needle drop on Colorado blue spruce. Cytospora canker is expressed through dead areas, known as cankers, which form at the base of the branches. The cankers are often hard to see however they will produce a white resin that runs down the tree. Unfortunately, fungicides will not control this disease however, if it is detected early the tree can be saved by pruning (prune when the tree is dry to avoid the spreading of spores on pruning tools) and maintaining tree vigor through a fertilizer schedule and watering during dry periods.
Another common spruce tree disease is rhizosphaera needle cast. With needle cast, needles may look yellow by mid to late summer and turn brown or purplish brown by late winter or early spring. The best evidence for needle cast are tiny black dots arranged in rows on infected needles. These black dots are fungal spores and can easily be seen with a magnifying glass. Needle cast will rarely kill a tree however if a tree is infected for 3-4 years severe needle loss can occur. Needle cast is best treated by maintaining tree vigor. A fungicide can be sprayed, but should only be used once the presence of rhizosphaera is confirmed by an expert, such as Purdue’s Plant & Pest Diagnostic Laboratory.
If spruce needles are turning yellow and dropping it should also be inspected for spider mites. These can be detected by looking for a fine webbing between needles, and by holding a white piece of paper under a branch and shaking vigorously. If mites are present you will see small black specks moving on the paper. For small spider mite infestations, simply spraying the tree with a garden hose may suffice. For a larger infestation chemical control may be required.