Rhizosphaera Needle Cast May Lead to Skimpy Spruce

Rhizosphaera needle cast is often to blame for brown or thin spruce in the landscape. In Kentucky, Rhizosphaera needle cast is the most common disease of spruce; it also affects some pine species. This disease causes needle drop on lower branches, resulting in a distinct thinned appearance. Management options include reduction of plant stress, good sanitation practices, and timely use of fungicides.

Rhizosphaera Needle Cast Facts

Figure 1. Needles infected with Rhizosphaera turn purplish brown during summer. (Photo: USDA Forest Service Archive, USDA Forest Service, Bugwood.org)

• Becomes evident in summer when needles on lower branches turn purplish or brown (Figure 1).
• Needles fall within a few weeks and lower limbs are left bare (Figure 2).
• Small, dark fruiting bodies (called pycnidia) form in stomata (pores in needles) and can be used to confirm diagnoses (Figures 3 & 4). Pycnidia are most easily recognized with a hand lens, but are also visible with the naked eye.
• Caused by the fungus Rhizosphaera kalkhoffii.
• Spread by rain; moisture is needed for infection.
• If defoliation occurs over 3 to 4 consecutive years, branch death is likely.

Figure 2. Needle drop and thinning of lower canopy are classic symptoms of Rhizosphaera needle cast in spruce. (Photo: Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources, Bugwood.org)

Figure 3. Fungal pycnidia are often visible without a hand lens. (Photo: Paul Bachi, UK)

Figure 4: Fungal pycnidia protrude from stomata. (Photo: Paul Bachi, University of Kentucky)

Management Options

• Stressed trees are more susceptible to infection than healthy plants, so steps should be taken to maintain plant vigor.
• Properly space plants to improve air circulation, thereby encouraging rapid drying of needles.
• Practice good sanitation habits.
• Homeowners can apply fungicides that contain chlorothalonil, copper, or mancozeb during needle emergence (mid-April). During rainy seasons or in plantings with a history of disease, fungicides may be applied two consecutive years during spring when fungi are most active.

Additional Information

• Homeowner’s Guide to Fungicides (PPFS-GEN-07)
• Landscape Sanitation (PPFS-GEN-04)
• Common Diseases of Spruce in Kentucky (PPFS-OR-W-24)

By Nicole Ward Gauthier, Extension Plant Pathologist, and Kimberly Leonberger, Extension Associate