Why is my spruce losing its needles?
There are many pests and pathogens that affect our landscape evergreens. Today, I will be discussing one of the more common spruce diseases that we see here in Minnesota. Rhizosphaera Needle Cast is a fungal disease that can cause the needles of the spruce to turn brown and fall off and if branches are infected for 3-4 years dieback can occur. This disease primarily affects Colorado Blue Spruce and Black Hills spruce. With these being popular varieties we see planted here in Minnesota it is helpful to know what we can do to mitigate the spread and impact of Rhizosphaera on our spruce.
How do you know if your tree is potentially infected?
Source: MN Department of Natural Resources
Some common ways to identify if your spruce has Rhizosphaera Needle Cast are if in spring to early summer the older needles are turning brown/purple while the new needles are still green and if using a magnifying glass, you see little black dots arranged in neat rows on infected needles. The discolored needles tend to fall off in the late summer early fall leaving the tree to look thin and see through.
Now that we know some of the signs and symptoms, how do we manage Rhizosphaera Needle Cast?
One of the best ways to help mitigate the disease is to maintain good air circulation around the trees. If the fungi does not have a suitable environment to grow in it will struggle to infect the plant. Typically this can be done by removing the lowest branches up to about 2-3’ to improve air circulation through the canopy. Another way to manage Rhizosphaera is with the use of Fungicides. Treatment by a licensed professional can be applied in the spring/early summer to the new needles once they have grown to about half their mature length followed by a second treatment in about 3 to 4 weeks . This treatment will help protect new needles from being infected and can improve plant health overtime.
If you suspect your tree may be suffering from Rhizosphaera Needle Cast or any other potential plant problem, have your trees inspected by one of Shadywood Tree Experts International Society of Arboriculture Certified Arborists.