Spruce needle rust
Photo by MNDNR Bugwood.org
This disease is caused by fungi from several species of the genus Chrysomyxa.
Colorado blue spruce, Picea pungens, white spruce, P. glauca, and black spruce, P. mariana, are all susceptible to the disease. Norway spruce, P. abies, is only occasionally infected.
Photo by Gaston Laflamme, Natural Resources
Canada, Canadian Forest Service.
Photo by J. O'Brien
USDA Forest Service
Spore producing structures of Stigmina lautti on a spruce needle.
- Needles at the tips of the branches (current year needles) turn yellow
- Pale orange to white tube-like projections appear on infected needles in July or August and release powdery orange spores
- Severely infected trees may have a tan to pinkish cast
- Infected needles fall off in September
- Some species of rust can cause witches' brooms, a clump of small weak branches arising from one point on a large branch
The spruce needle rust fungi live half of their life cycle on spruce needles and the other half on an alternate host in the Ericaceae family. In Minnesota labrador tea (Ledum groenlandicum), leather leaf (Chamaedaphne calyculata), and bearberry (Arctostaphylos uva-ursi) can all serve as alternate hosts for spruce needle rusts. In the spring, spores are released from the alternate host plant and infect young growing spruce needles if cool wet weather is present. In the summer months, these infected needles develop spore producing structures that release orange powdery spores. These spores can only infect the alternate host plant and do not reinfect the spruce. Infected needles often turn yellow and in most cases, fall off at the end of the growing season. The fungus that causes witches' brooms in addition to needle infection can overwinter within the infected spruce branches. All other spruce needle rust fungi overwinter in the alternate host plant.
- In most cases spruce needle rust is a cosmetic problem and no management is needed.
- Reduce moisture on needles by redirecting lawn sprinklers away from spruce trees and spacing spruce trees to allow good air circulation around the tree.
- If witches' brooms are present these can be pruned out and destroyed.
- Removal of all alternate host plants within 1000 ft will reduce disease on spruce but is often impractical.