One of the most common diseases of ornamental spruce in Minnesota is Rhizosphaera needle cast, caused by the fungus Rhizosphaera kalkhoffii. In Minnesota, Colorado blue spruce (Picea pungens) is highly susceptible and can sustain severe damage. White spruce (P. glauca) and its variant Black Hills spruce are intermediate in resistance, while Norway spruce (P. abies) is relatively resistant. Sometimes young trees are killed by the pathogen, but more commonly branches lose their needles resulting in misshapen trees (Figure 1).

Infected needles on the tree and those which have fallen to the ground produce spores during wet weather in the spring (late May to early June). These spores are then rain splashed to uninfected needles, starting new infections.

Infection usually begins on the inner portions of the lowest branches and gradually progresses upward and outward, leaving the tree with a hollow or barren appearance near the base of the tree. Occasionally, infection begins higher up in the tree and spreads downward and outward.

Figure 2. Photo: U of MN Department of Plant Pathology

The first sign of infection occurs in late fall or in the spring one year after infection. At that time, the fruiting bodies (pycnidia) of the fungus emerge from the stomata or "breathing" pores of infected needles. These may be observed by examining needles with a hand lens. The fruiting bodies resemble tiny black dots in neat, even rows (Figure 2). The second summer after infection, symptoms appear as yellow needles which later turn purplish-brown and drop from the tree. A few of these infected needles may persist on the tree over the winter and drop off the following spring.

Because of the long delay between infection in spring and needle drop the following summer, the ends of infected branches appear green and healthy. Branches appear to lose their needles from the trunk outward. Those branches which repeatedly lose needles for three or four years may die.

A positive diagnosis is based on the pattern of defoliation and the presence of black fruiting structures on needles. Early identification of the disease can prevent extensive damage to individual trees and prevent the spread to adjacent trees. A protective fungicide with the active ingredient chlorothalonil (sold as Multi- Purpose Fungicide, Daconil 2787, and others) can prevent new growth from becoming infected. It is important to protect new growth as it emerges, therefore fungicides should be applied in a timely manner when the new needles are half elongated (late May or early June) and again three to four weeks later. Rhizosphaera needle cast may be controlled in one year if fungicides are applied correctly. However, severely infected trees usually require two or more years of fungicide applications. Even though fungicide application will effectively control this disease, reinfection may occur in subsequent years. Application to large trees requires special equipment to ensure adequate coverage. Read fungicide labels carefully and apply only as directed.

When planting new trees, consider planting Norway or white spruce which are more resistant to Rhizosphaera. Properly spacing spruce trees will help reduce disease incidence. Spruce trees grow best in moderately moist, well-drained soils but can be planted in other soils if adequate moisture is available. Water newly planted trees and water during drought periods to help maintain tree vigor. Stressed trees should also be mulched and fertilized as needed. Properly prune dead or severely infected branches during dry weather. If trees are severely infected, the lower whorl of branches may also be removed to help increase air circulation.

Reference

Skilling, D.D. and J.A. Walla. 1986. Rhizosphaera Needle Cast of Spruce, pages 124-125 In: Diseases of Trees in the Great Plains. USDA Forest Service, General Technical Report RM-129.