What Diseases Can Kill Evergreens in My Landscape?
Spruce, fir, pine, and other evergreen trees add value to the landscape.
Much-needed green throughout the winter reminds us of what’s to come with spring. However, the evergreens in this region often struggle for many reasons. Our clay soil retains water for a very long time in the spring and winter while during the summer, it is difficult for the roots to penetrate. There are two types of pests that we must watch out for.
Many types of evergreens are susceptible to needle sickness.
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Colorado blue spruce is highly susceptible to Rhizospaera needle, but Norway spruce has shown some resistance. Trees stressed from drought and other factors are more likely to suffer. Needle castration often starts at the bottom of the tree and on the innermost needles. The needles turn brown and drop off relatively quickly. The disease travels up the tree and can eventually kill the tree, especially if the tree has been affected for several years.
There are several types of cast needle, but Rhizospaera needle cast is a very common form of this disease in our area. In spring and early summer, you can keep an eye on this disease by examining both green and brown needles under a small magnifying glass. Small black bumps can be seen in straight, neat rows on infected needles. The fungus lives over the winter in discarded needles under the tree and in the spring, it will re-infect the tree via raindrops falling on the soil surface.
Control of Rhizospaera needle splint is difficult because the disease course is over two years. Spraying a fungicide containing chloroethanlonil as you see new growth in the spring and repeating the spray after 3-4 weeks will help control the disease. You have to repeat these sprays for 2-3 years for them to be effective.
Many insects can also cause serious damage to our evergreens. One of the pests that we often see on spruces is a spider mite. Spider mites pierce insects, sucking them in and feeding on the chlorophyll in the needles. As they feed and empty the chlorophyll needle, a small or yellow spot occurs. This spot is called stippling and, if severe enough, can cause needles to die and fall from the tree.
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Spider mites are a cool season pest and are usually active in the spring and fall. You can check for mites by holding a piece of white paper under a branch and pressing the branch against the leaf. If mites are present, you will see tiny, dust-like mites moving around on the paper.
Control is fairly easy and is best done with a dormant oil preventive spray in the spring and fall. Dormant oil should not be used if temperatures are above 75 degrees because it will damage the plant with higher temperatures. Spray the dormant oil all over the plant, concentrating on the lower and inner branches. During early summer, you can use a water hose to wash the mites off the needles.
These are just a group of common pests that attack spruces and other evergreens but can be controlled.
Andrew Rideout is UK Horticultural Agent and can be reached at firstname.lastname@example.org.