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
- 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.
- 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.
- 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