Spruce, firs, pines, and other evergreens add value to landscape.
The much-needed green throughout the winter reminds us of what is to come with the spring. Often, however, evergreens in this region struggle for many reasons. Our clay soil holds water too long in the spring and winter while during the summer, it becomes hard and difficult for roots to penetrate. There are a couple of pests that we should watch out for.
Many species of evergreens are susceptible to needle cast disease.
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Colorado blue spruce is highly susceptible to Rhizospaera needle cast, but Norway spruce has shown some resistance. Trees stressed from drought and other factors are much more likely to suffer. Needle cast disease often starts at the bottom of the tree and on the innermost needles. The needles turn brown and drop relatively fast. The disease moves up the tree and eventually can kill the tree, especially if the tree has been affected for several years.
There are several types of needle cast, but Rhizospaera needle cast is a very common form of this disease in our area. In the spring and early summer, you can watch for this disease by examining both green and brown needles under a small magnifying glass. Tiny black bumps can be seen in straight, neat rows on the infected needles. The fungus survives over winter in the discarded needles under the tree and in the spring, will re-infect the tree via rain drops splashing on the soil surface.
Controlling Rhizospaera needle cast is difficult because the disease cycle is two-plus years. Spraying a fungicide containing Chlorothanlonil as you see new growth in the spring and repeating the spray 3-4 weeks later will help get the disease under control. You must repeat these sprays for 2-3 years to be effective.
Several insects also can cause serious damage to our evergreens. Spider mites are one pest that we see often on spruces. Spider mites are piercing, sucking insects and feed on the chlorophyll in the needles. As they feed and empty the needle of chlorophyll, a tiny cream or yellowish spot occurs. This spot is called stippling and if severe enough, can cause the needles to die and fall off the tree.
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Spider mites are a cool-season pest and typically will be active in the spring and fall. You can check for mites by holding a piece of white paper under a branch and tapping the branch against the paper. If mites are present, you will see small, dust-like mites moving on the paper.
Control is fairly easy and best done with a preventative spray of dormant oil in the spring and fall. Dormant oil should not be used if temperatures are above 75 degrees as it will damage the plant with higher temperatures. Spray the dormant oil throughout the plant with emphasis on the lower and inner branches. During the early summer, you can use a water hose to wash mites off the needles.
These are just a couple of common pests that attack spruces and other evergreens but can be controlled.
P. Andrew Rideout is the UK Extension Agent for Horticulture and can be reached at firstname.lastname@example.org.