Rhizosphaera needle cast
This disease is caused by the fungus Rhizosphaera kalkhoffii and is probably the most common needle disease in Minnesota.
Colorado blue spruce, Picea pungens, is highly susceptible to this disease. White spruce (including Black Hills spruce), P. glauca, is intermediate in susceptibility and Norway spruce, P. abies, is relatively resistant. Trees that are stressed from drought, poor planting practices, or other factors are more likely to suffer from Rhizosphaera needle cast.
- Infected needles may look yellow and mottled by mid to late summer.
- Infected needles turn brown or purplish brown by late winter or early spring.
- Newly growing needles in the spring do not show symptoms. Needles closest to the trunk of the tree (the older needles) are often discolored while the needles at the tips of the branches remain green.
- With a magnifying glass, look for tiny black spots arranged in neat rows on infected needles. These are pycnidia, fungal spore producing structures. These may be confused with pycnidia of Stigmina lautti, which are very similar in appearance.
- Infected needles typically fall off in the summer, 12-15 months after the initial infection. Infected trees have thin canopies.
- Damage typically starts on the lower branches and moves up the tree.
- After 3 to 4 years of severe infection the lowest branches may begin to die.
The pathogen overwinters on living and recently killed needles. Spores, called conidia, are dispersed by splashing water spring through early autumn. New needles on the lower branches are most commonly infected but if conditions are very favorable for infection (extended periods of moisture on the needles at 77° F) any needles can become infected.
- Plant Norway or Black Hills spruce instead of Colorado blue spruce or Engelmann (P. engelmannii) spruce. Whenever possible plant spruce trees grown from local seed sources as these plants are likely to be best adapted to the local conditions.
- Avoid planting young spruce near old spruce trees that may be harboring fungal pathogens.
- Reduce stress on spruce trees by watering during periods of drought, mulching the soil around the tree, etc.
- Do not allow lawn sprinklers to spray the spruce needles.
- Space spruce trees to allow good air circulation around the trees.
- Do not shear spruce as shearing creates a dense, compact growth that stays wet longer.
- Chlorothalonil can be sprayed twice in the spring to protect new needles. The first spray should be applied when needles are half the length of the mature needles. A second spray should be applied 3-4 weeks later or as prescribed on the fungicide label. Read and follow all instructions on the label when applying a fungicide!
- Before spraying fungicide, confirm that Rhizosphaera is the fungal pathogen
causing damage by sending a lab sample to the University of Minnesota plant disease diagnostic clinic.
Several other fungi result in symptoms very similar to Rhizosphaera.