Rhizosphaera needlecast on spruce trees

Originally published in the Kenosha News

By Horticulture Educator, Jeanne Hilinske-Christensen

Question: Several of the spruce trees in my yard seem to be dying. The inner needles on the lower branches turned brown, and many have dropped from the trees. The trees are growing in a row, and it appears that the problem is spreading from tree to tree. Is the culprit an insect or a disease? — S.S.

Answer: From what you describe, the issue may be Rhizosphaera needlecast, a disease caused by a fungus, Rhizosphaera kalkhoffii. Rhizosphaera needlecast symptoms include purplish-brown needles that eventually fall from the tree. Needles closest to the trunk of the tree are often affected while the needles at the tips of the branches remain green. This fungal disease usually starts on the bottom branches, then moves upward. While Colorado blue spruce is one of its most common hosts, other types of spruce can also be attacked by this fungus. Rows of black spots (fungal pycnidia) are visible when examining the affected needles with a hand lens. The black spots are fungal spores that have emerged through the pores on the needles.

Infection usually occurs in late spring/early summer. The fungus releases spores during wet weather and spreads by wind and splashing of rain drops. Infected trees don’t usually die due to this fungal infection, but the health and aesthetic value of the tree are diminished.

Increase air circulation around the trees by removing weeds or other obstructions to try to decrease the spread of the disease. Rake and remove infected needles that have dropped to the ground. Avoid overhead watering with sprinklers in the area of the affected trees. Improve tree health by fertilizing in late May or early June with an acidic fertilizer for evergreens.

In some cases, fungicide applications may be necessary and will need to be applied when new shoots are half elongated. Apply again when the shoots have just completely elongated. Remember, fungicides will manage the disease by protecting new needles from the fungal organisms but will not cure the tree of the disease. Read and follow all label directions when applying the fungicide.

Other types of fungal diseases, such as Stigmina needlecast and Cytospora canker, may cause symptoms similar to Rhizosphaera needlecast. However, the spores of Stigmina are not as uniformly circular as Rhizosphaera and have projections protruding from them. Cytospora canker usually emits a dried white sap on the bark of infected branches in addition to causing branches to die completely.

The University of Wisconsin Garden Fact Sheet XHT1006 on Rhizosphaera needlecast contains additional information and can be accessed at: http://hort.uwex.edu/links/