Rhizosphaera needle cast is a fungal disease of spruce trees that causes spruce needles to brown and fall off.
What are the Risks?
White spruce, Norway spruce, and Colorado spruce can all be affected by needle cast. Damage will generally start on the lower branches and move up the tree. The fungus can survive the winter, and if needles are infected for 3-4 years in a row, the branch will die.
What to Look For
Typically, damage will cause thinning of the tree canopy as infected needles brown and fall off in late summer or fall. With needle cast, tiny black bumps can be seen on infected needles if looking through a magnifying glass.
Infected Colorado spruce Infected needles
Prevention
Here are some ways you can manage needle cast:
- Only plant spruce trees that are well-adapted to your growing area and growing conditions (buying at local nurseries can help ensure this);
- Plant trees in good conditions (moist, acidic, and well-drained soils with ample sun);
- Remove weeds under any trees and use wood chip mulch around the base of your trees to further prevent weed growth;
- Be careful to prevent sprinklers from spraying needles. If using a sprinkler irrigation system, direct the water away from the needles; and
- Certain fungicides can protect new needles from infection. Always be sure to confirm that your tree does have needle cast (since many other fungi can result in similar symptoms) before selecting a fungicide.