The Iowa Department of Natural Resources Forestry Bureau has received a number of disease questions about a fungus that has been causing second-year needles on blue spruce to turn a purple to brown color and fall from the tree.
Rhizosphaera needle cast is caused by a fungus that appears as small black growths (fruiting structures) in the small pores on the underside of infected needles. These fruiting bodies appear in rows along the length of the needle. A hand lens or magnifying glass is helpful in viewing these structures. Healthy, uninfected needles will have a line of white pores.
Symptoms typically start on the lower branches of the tree and work from the inside branches to the outside branches. Over time, the upper branches become infected and the needs turn purple to brown and die. The fungus can overwinter in infected needles.
In the spring spores are released during wet weather and infect newly emerging needle, which is why the state is seeing more of this needle cast disease.
If the disease is present, use fungicides in the spring to protect the newly emerging needles. The tree should be sprayed with a product such as chlorothalonil or a Bordeaux mixture during the last two weeks of May and again 4 to 6 weeks later. Good coverage and avoiding rain for two days after the applications are important. Be sure to read the label for application rates.
Other preventative measures include adequate tree spacing to promote good air circulation, improving tree vigor through mulching and watering when needed, inspecting trees for disease before planting, and avoiding planting susceptible trees next to infected trees.