The Blue Spruce belongs to the genus Picea. This genus is plagued by a litany of pests and diseases. Fortunately, most of these are easily identified by the damage they cause. The damage caused by insects is often easier to determine than damage caused by diseases. Sometimes just looking at the plant from a distance will reveal to the trained eye the general cause of damage. Closer examination may be required to pin down the exact cause, though in some cases even this will not yield a clear result.
Identifying the probable cause is the first step in control, if it is possible. Severe damage by either insects or disease often cannot be reversed, ending in the death of the plant. Remember, evergreen plants take longer to show signs of distress than do deciduous species. That being said, how do we determine what caused damage to the plant? Insect damage can often be mistaken for a disease, so close inspection is essential.
Look for tell-tale signs such as: 1. Has new growth turned brown and died, while the rest of the branch remains healthy? If so, are the dead new growths only at the top of the tree, mainly the central leader, or throughout the tree? 2. Are parts of the branches dead or only partly dead and are they throughout the tree? 3. Is the tree dropping needles that to the naked eye appear normal? The above three conditions are likely caused by insects and require different approaches for control than would disease.
Condition 1 is probably spruce budworm damage, caused by the larvae of a moth. Close examination at the base of a dead growth will reveal a small round hole, the exit hole of the insect. It is too late for control of the current year’s damage, but application of a systemic pesticide will reduce next year’s damage. Condition 2 is probably caused by the spruce gall aphid. Aphids are insects that suck sap through a specialized mouth part, in the process, transferring a fungus to the damaged area, resulting in a cankerous growth. This growth seals off the supply of water and nutrients, leading to the death of that part of the branch. If the outbreak is severe, whole branches will be infected, causing devastating loss and ultimate death of the plant. Contact sprays usually help, especially if used in tandem with systemic treatments.
Condition 3 could be caused by spider mites. These are not really insects, but tiny arachnids, closely related to standard spiders. They also have special mouth parts and suck the sap from the needles of spruce trees. They remove the chlorophyl from the foliage, denying the plant necessary photosynthesis, leading to tissue death. The needles fall from the tree, sometimes having turned a whitish grey, sometimes appearing normal color but simply dropping. Needles may fall for other reasons, such as drought, so to determine if the plant has mite damage, place a sheet of paper under a branch and gently tap the branch. If the needles drop readily onto the paper, remove it, gently tip the paper so the needles to fall off, then gently rub the edge of your palm over the paper where the needles were. If you see reddish streaks on the paper, then it is clear there are mites present. These pests thrive in hot, dry conditions. They don’t like it wet. Mites are easily controlled by a miticide spray.
As far as diseases are concerned, Colorado spruce are especially susceptible to cytospora canker, which attacks the lower branches, causing defoliation and death of the branch. They are also prone to attack by Phomopis occulta, which causes branch dieback and attacks branches at random throughout the tree. These diseases are difficult to control as they are both spread by water droplets falling through the tree. The plant is most likely to die.
Identification of pests and diseases is not always easy, and rarely completely certain. Knowing where to look gives you a basic idea of the potential cause. Remember, your professional landscape company, garden center, and tree care company are there to advise you.