Rhizosphaera needle cast
"My spruces have lost most of the older needles from their lower branches" has been a common complaint from homeowners this spring. Upon asking the caller if they are blue spruce, the answer is usually yes. One of the most common diseases of ornamental spruce is Rhizosphaera needle cast, caused by the fungus Rhizosphaera kalkhoffi. Colorado blue spruce is usually the target of this fungal disease, but white and Norway spruce can also be infected. The loss of needles from the inner branches on the lower portion of the tree gives it a hollow appearance.
Repeated infections over several years may cause these lower branches to die. At this point the tree has lost its appeal as an ornamental. Infection rarely kills the tree, but it does not make for that stately appearance most people want. The extent of infection varies, depending on environmental factors that favor the spread of the spores, susceptibility of the tree, stress and years of infection. Unless the branch is dead the current year's needles, and last year's needles (already infected), will be live and green.
High moisture levels and poor air circulation favor the spread of this disease. Infection of new needles occurs in the spring when spores are released from the fruiting bodies of infected needles. These spores splash or drift onto uninfected needles where, under favorable conditions, they germinate and grow into the needles through their many stomates.
Treatment requires the use of a preventative fungicide called chlorothalonil. It is sold over the counter under such names as Multipurpose Fungicide, Daconil, Fertilome Fungicide or others. It should be applied when the new needles are half elongated, and two or three times later at 3 to 4 week intervals. Severe infections may require 2 or more years of treatment. It is important to note that the lost needles do not return. You are protecting future growth only. Planting resistant trees, maintaining proper soil and water conditions, mulching and providing adequate air circulation are important factors in controlling this disease.