Rhizosphaera needle cast
- This disease is caused by the fungus Rhizosphaera kalkhoffii and is probably the most common needle disease in Minnesota.
- Trees stressed from drought, poor planting practices or other factors are more likely to suffer from Rhizosphaera needle cast.
- Several other fungi result in symptoms very similar to Rhizosphaera and correct identification is important.
Trees affected by Rhizosphaera needle cast
- Colorado blue spruce, Picea pungens, is highly susceptible to this disease.
- White spruce (including Black Hills spruce), P. glauca, is intermediate in susceptibility.
- Norway spruce, P. abies, is relatively resistant.
Rhizosphaera needle cast symptoms
- 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.
- Tiny black spots arranged in neat rows on infected needles can be seen with a magnifying glass.
- These are pycnidia, fungal spore producing structures.
- These may be confused with pycnidia of Stigmina lautti, which look similar.
- Infected needles typically fall off in the summer, 12 to 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 lives through the winter 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.
How to manage Rhizosphaera needle cast
- 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 and mulching the soil around the tree.
- 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 to 4 weeks later or as prescribed on the fungicide label.
- 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.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Reviewed in 2018