Rhizosphaera Needle Cast in Colorado Blue Spruce Trees
Rhizosphaera needle cast is caused by a fungal pathogen. The Colorado blue spruce is highly susceptible to this foliar disease, followed by White and Black Hills spruce, which are moderately susceptible. Norway spruce has some degree of resistance.
Symptoms. Infected branches begin to thin out near the bottom of the tree as the inner needles turn purplish brown and fall from the tree. Symptoms begin with the loss of the inner needles, working from the inside of the branches out. Often, the only remaining living needles are found near the branch tips.
Trees of any age may become infected, especially those that are stressed and growing near one another where air movement and sunlight penetration are reduced. Prolonged wetness and humidity are required for infections to occur as the new needles are forming in the spring. Climate change has only made the disease more prevalent. The fungus overwinters in infected needles on the ground. Spores are released in the spring during wet weather and are dispersed by splashing rain to infect newly emerging needles.
This disease does not quickly kill the tree. Over the span of several years, the trees lower branches will die from the bottom up.
Management of Rhizosphaera Needle Cast.
Management aims to reduce the spread of the disease to healthy needles. This can be accomplished in several ways. Lower branches that are thinning and no longer attractive can be pruned from the tree. Fallen needles should be gathered up and removed from beneath the tree and disposed of offsite. This should be an annual practice every fall. Attempts to improve air circulation around the trees is also beneficial.
Once needles become infected, they do not heal. Fungicide applications can help manage the disease but are only protective in nature. They do not kill the fungus and must be applied several times each spring, on an annual basis. Precise timing is necessary for this method to work. Applications must be made as the new needles are forming in the spring. Products containing chlorothalonil as the active ingredient and are listed for control of Rhizosphaera needle cast, can be used according to label directions. Always follow label directions.
Eventually, on branches that have been repeatedly treated, the new needles that have been protected will help to fill in the bare spots over the course of several growing seasons.
