This disease mostly occurs on spruces that are not being grown in their native habitat.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. Sometimes the disease may start higher and then work downward. Any size tree may be attacked; however, trees over 20 years of age are more likely to be infected. Current year needles will appear healthy, even though they may be infected. These needles usually turn a mottled yellow by late summer. During the late winter and early spring, the needles turn brown (purplish brown on Colorado spruces) and fall off during the summer and fall.
Look closely at the shapes of the Rhizosphaera and Stigmina fruiting bodies because they may look similar if you look too quickly. Stigmina makes the needles look dirtier than Rhizosphaera infected needles. Also, the Stigmina fruiting bodies look like "little spiders" while the Rhizosphaera fruiting bodies look like "smooth bowling balls" especially when seen especially under magnification.
Basically, the brown to purplish color takes 12 or more months to develop after infection. Under the right conditions, fruiting bodies can be seen on all sides of the dead (and occasionally on green) needles. The fruiting bodies are aligned in rows because they emerge from the stomata that are in rows on the needles.
Plant spruces that are considered 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. Remove all infected needles and destroy. Chemical controls are effective if the disease is not too severe. Such controls are preventive and applied when new needles are half elongated and again when they reach full length. Because this fungus requires twelve to eighteen months for symptom expression, at least two years of fungicide sprays are usually required. To reduce the risk of reinfection of older trees, continuted treatments may be required.