Rhizosphaera needle cast is a disease caused by the fungus Rhizosphaera kalkhoffii that commonly affects spruce trees growing outside of their native range. Rhizosphaera starts on the inner and lower growth and progresses upward through the tree. It can take up to 15 months for the needles of infected spruce trees to show visible symptoms after the initial infection. Young trees may be killed by this disease but usually branches of older trees die off after 3-4 consecutive years of defoliation, causing trees to look disfigured.
Symptoms of Rhizosphaera:
- Yellowish-brown to purple discoloration of needles beginning on the inner and lowest portions of the tree and progressing upwards
- Infected needles may show black dots in rows on their underside – these are the fruiting bodies of the fungus.
- Lower branch dieback over years of infection
- New infections begin during wet weather when spores from infected needles on the tree and those that have fallen to the ground are splashed onto uninfected needles.
- The yellowish brown to purple discoloration takes one year or more to develop after the initial infection.
- Spores overwinter on this year’s and last year’s needles.
- A spruce with a severe rhizosphaera needlecast infection may hold only the current year’s needles; whereas, a healthy spruce will retain needles 5 to 7 years.
Distinguish From Look-Alikes
- Cytospora, another common disease on spruce, forms cankers on branches and trunks and turn branches brown.
- Frost damage will cause browning of needles foliage on the tips, rather than not the inner parts, of the branches
- Sun scald will usually cause browning of needles foliage, usually on the south and west facing sides of plant.
Early identification of Rhizosphaera can prevent major damage to individual trees and prevent the spread to nearby trees. Protecting new growth as it emerges is very important. For the highest effectiveness, fungicides should be applied when the needles begin to emerge in spring. Heavily infected trees may require several years of fungicide applications.
The appearance of treated trees will begin to improve the season after the treatments were performed. Untreated trees will continue to lose needles and branches each year and will die from this disease if not addressed.
What You Can Do to Protect Your Spruce Tree
- Remove fallen needles by raking around tree, if possible.
- Ensure sprinkler heads are not wetting the needles.
- Water newly planted trees, especially during droughts.
- Prune out infected branches during dry weather.
- Plant resistant spruce species such as Norway spruce or white spruce.
- Properly space trees when planting to improve air flow.