Answer: It's difficult to accurately diagnose the problem with your spruces without being able to carefully inspect the trees and the needles. Both insect infestations and disease pathogens can cause yellowing.
Spruces may be attacked by several needle cast diseases. One causes needles to turn yellow or brown and drop off. Another affects the lowest needles first then moves up the tree. Infected needles are a mottled yellow.
Several rust diseases attack Spruce. Infected needles turn yellow and drop off.
Mites are the worst problem, and in hot weather they can build to populations which require control. They can be a major problem in summer after hot dry weather, especially near concrete, buildings, and other surfaces which reflect heat. The small insects can?t be readily seen with the naked eye. The first noticeable symptoms are yellowing at the base of the oldest needles on infested branches. Close inspection with a magnifying glass will confirm the presence of the mites.
The Spruce needle miner makes a small hole in the base of a needle then mines out the center. Dead needles are webbed together and can be found on infested twigs. Hand pick these from the tree to reduce future damage.
Pine needle scale is a white, elongated scale found feeding on the needles only. Populations would have to be quite high to cause major damage.
Sawfly larvae may feed on the needles. One infestation will usually not kill the tree, but there may be two or more generations per year.
Now that you know all the different things that can cause yellowing, you may want to take some of the affected foliage to your local cooperative extension office for a positive diagnosis and control options.
Best wishes with your trees!
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