Answer: What you describe sounds like Rhizosphaera needle cast, a fungus disease caused by Rhizosphaera kalkhoffii, affecting blue spruce (Picea pungens). Premature needle drop is the primary damage, causing the tree to become unsightly. However, branches that are defoliated for four or five years may die. If left uncontrolled, the disease can eventually kill the tree.
The disease affects blue spruce of all ages. Infection takes place in the spring, but symptoms are not visible until the following spring and summer. The fungus can sometimes be seen with a hand lens (10X) as tiny brown to black spots called pycnidia, or fruiting bodies, emerging from stomata (pores) on the needles. Many of the affected needles fall off in the late summer of their second growing season.
Some needles stay on the tree over winter and the following spring produce spores, which spread the disease.
The disease is spread primarily by rain water splashing the spores from infected needles to newly emerging needles in the spring. Pycnidia emerge from these newly infected needles the following spring to start a new disease cycle.
Fungicides can be used as part of a management program. Infected trees can be sprayed with bordeaux mixture 8-8-100 (8 lb hydrated lime, 8 lb copper sulfate, 100 gal water) or chlorothalonil fungicide. Fungicides should be available at local nurseries, garden supply stores, or feed stores. Fungicides provide protection against infection and prevent spread of the disease within the tree. They should be applied to the tree in the spring when the new needles are half developed and again when they are full length.
Two years of treatment usually restores moderately affected trees to full foliage. Severely affected trees may require more years of treatment. If the trees are very large, you may have to hire an arborist or pest control person to inspect the trees and spray if appropriate.
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