Several samples have come into the Plant Disease Clinic lately, showing symptoms and signs typical of Rhizosphaera needle cast. This disease is caused by the fungus Rhizosphaera kalkhoffii and is most common on Blue spruce; however, Black Hills spruce is also susceptible.
Symptoms associated with Rhizosphaera needle cast can occur on any age of tree and usually progress from lower to upper branches typically on trees that are stressed. This disease generally causes second-year needles to turn a purplish-brown and fall from the tree. Once a spruce is infected with the Rhizosphaera fungus, it usually takes up to 15 months for needle loss to occur. Repeated needle loss can result in branch death after 3 to 4 years and eventually tree death.
The best way to distinguish Rhizosphaera needle cast from other problematic symptoms is to look for black fruiting bodies on the needles. Small black fruiting bodies appear in rows on both green and brown needles, and can be seen using a hand lens. These fruiting bodies release spores during wet conditions in late spring, which spreads the fungus to healthy trees.
Several methods are recommended to help prevent infection. When planting new trees, examine needles for fruiting bodies and only plant disease free plants. If infected trees are present, do not plant healthy susceptible species next to them. In addition, provide adequate spacing when planting new trees to promote good air circulation. One important way to reduce tree stress is by mulching and watering when needed. Finally, do not trim tree branches during wet conditions because this encourages fungal spread.
Rhizosphaera needle cast can also be managed with two fungicide sprays in the spring, one applied in the last two weeks of May and the other four to six weeks later. Fungicides effective against this disease include Bordeaux mixture, other copper fungicides, Daconil, Terranil, Manicure, and Twosome.
Small black spots (fruiting structures of the Rhizosphaera fungus) appear in rows on infected needles.
Needles infected with Rhizosphaera needle cast turn a purplish-brown and will eventually fall from the tree.
Rhizosphaera needle cast on blue spruce showing needle loss on lower branches.