July 31, 2015
Rhizosphaera kalkhoffii (fungus)
Colorado blue spruce is the primary host. Norway spruce is resistant. Other spruces and pines are infrequently attacked.
With a hand lens, you can see rows of black specks on the needles, where the pycnidia have emerged through the stomates of the needles. These black fruiting bodies are diagnostic of infection by Rhizosphaera. Healthy needles have white stomates.
How it’s spread
The fungus lives inside the needle tissue, producing black fruiting structures (pycnidia) that emerge through the stomates (pores) of the needle. Spores are released throughout the growing season during wet weather. Most infection occurs on mature needles in May and June during wet weather.
Don’t prune trees when foliage is wet. Disinfest tools after pruning diseased branches. Improve air circulation through pruning and proper plant spacing. Preventative fungicide applications when needles are half grown and again when fully grown may help if the disease isn’t too severe.
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