The fungus Rhizosphaera kalkhoffii causes needle cast of spruce. It mostly occurs on spruces that are not being grown in their native habitat. The Colorado blue spruce is the most susceptible spruce to this disease. White spruce is moderately resistant and Norway mostly resistant. The other spruces fall somewhere in between susceptible and resistant. Other conifers may also be hosts including some pines such as Austrian pine.
Infection occurs in the spring usually on lower branches and works upward around the tree. Some times the disease may start higher and then work downward. Any size tree may be attacked but small trees are more susceptible and are more likely to be killed by serious yearly infection. Infected foliage usually turns a mottled yellow in late summer on current year needles. During the late winter and early spring, the needles turn brown (purplish brown on Colorado spruces). These discolored needles then fall off during the summer and fall leaving current year needles.
Basically the brown to purplish color takes 12 or more months from the time of infection. Under the right conditions, fruiting bodies (pycnidia) can be seen on all sides of the needles. The pycnidia are in rows since they emerge from the stomata that are in rows on the needles. The pycnidia can usually be found on the dead needles but sometimes the pycnidia can be found on infected green needles.
Plant spruces that are most resistant. Provide good air circulation. Avoid overhead watering and watering at night. Get positive identification of the disease. There are other non-infectious diseases that may mimic needle cast symptoms. Use a fungicide when necessary. Fungicide treatments need to be done for two years in a row. Remove all infected needles and destroy.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud , Extension Specialist, Pesticide Applicator Training and Plant Pathology, University of Illinois at Urbana-Champaign.