Rhizosphaera needle cast is the most common disease on spruce trees, including Colorado blue spruce, white spruce, Oriental spruce, and occasionally the Norway spruce and red spruce. A healthy spruce tree will retain its needles for five to seven years; however, a spruce tree infected with Rhizosphaera needle cast may only hold the current year’s needs. Mild temperatures and prolonged exposure to moisture favor disease development. Needles on interior branches in the lower canopy are most commonly infected first, as airflow is limited, and shade is more prevalent.
When infected, needles on the tree turn purple to brown and begin to drop. When the disease becomes well established in the lower canopy of the tree, the infection will then spread upwards in successive years, killing off the branches.
When branches in the lower canopy decline or die, it is important to prune out the infected branches. Also, should a chemical control of the disease be required, an application should be made in the spring, as the disease infects newly emerging spruce needles.