October 2014 Plant Disease Update

Rhizosphaera needle cast, caused by the fungus Rhizosphaera kalkhoffii, is a common problem in landscape and Christmas tree plantings of blue spruce, but it is also a problem on other spruce, such as Norway spruce. Although symptoms appear in the fall, the fungus that causes the disease infects needles in the spring. 

Symptoms of Rhizosphaera needle cast
Symptoms of Rhizosphaera needle cast typically begin at the bottom of the tree and gradually move upward.

Rhizosphaera needle cast is best managed with spring applications of protectant fungicides; applications in the fall will not be effective. Sprays should begin when new growth is 1/2" to 2" long and be repeated at 2 to 4-week intervals or at shorter intervals during heavy rainfall. New infections can occur throughout the summer, but sprays are most effective during the early part of the summer when conditions are typically wetter. 

Rhizhosphaera Needle Cast on Spruce full image
The fungus infects needles in the spring, but symptoms of browning and needle drop do not appear until fall.

When trees have suffered significant defoliation it can take several years of fungicide treatment before the tree regains full foliage. Stigmina needle cast is another fungal needle cast disease of spruce with similar symptoms and control recommendations to Rhizosphaera. Spruce mite infestation can also cause spruce needles to brown and drop, and, like Rhizosphaera needle cast, the problem starts at the bottom of the tree and appears to move upward.