Prevention and Control of Common Conifer Tree Diseases

Dutch Elm Disease
Dutch Elm Disease. USFS/FIDL

Like any kind of tree, the conifer is susceptible to a number of diseases that can damage or destroy it. Sometimes, these diseases strike trees in the forest; other times, only urban or suburban trees are stricken. Dead and dying trees are unsightly but they're also a potential safety hazard.

In populated areas, rot can cause limbs to drop or entire trees to collapse, especially during storms. In forested areas, dead trees can dry out, creating fuel for potential forest fires. By learning how to recognize different conifer diseases, you can improve the health of trees on your property and preserve the integrity of the local ecosystem.

Types of Conifer Disease

Softwood or coniferous trees can be harmed or killed by disease-causing organisms called pathogens. The most common tree diseases are caused by fungi, though some diseases are caused by bacteria or viruses. Fungi lack chlorophyll and derive nourishment by feeding on (parasitizing) trees. Many fungi are microscopic but some are visible in the form of mushrooms or conks. Other factors affecting tree disease include climate and where the tree or trees are planted.

Not all parts of a tree may be affected or exhibit symptoms. Disease may strike the needles, stem, trunk, roots, or some combination thereof. In some instances, trees can be saved by applying pesticides, trimming the diseased portions, or removing neighboring trees to provide more room. In other cases, the only solution is to remove the tree entirely.

Needle Cast

Needle cast is a group of tree diseases that cause conifers to shed needles. The symptoms of needle cast tree disease first appear on needles as light green to yellow spots, which eventually turn red or brown. Tiny black fruiting bodies form on the surface of the needles before or after the infected needles are shed. If left untreated, fungal growth can kill the entire needle. Treatment options include applying fungicides, removing diseased needles at first sign of infection, and trimming neighboring greenery to prevent overcrowding.

Needle Blight

This group of needle blight tree diseases, including Diplodia, Dothistroma and brown spot, attack conifers at the needles and on twig tips. Infected needles often fall from the tree, creating a denuded look. Blight can result in dramatic browning of the foliage, beginning on the lower branches. Repeated annual cycles of infection can result in dead limbs and eventual loss of any meaningful ornamental value. The most effective treatment option is copper fungicide spray, but you may have to spray repeatedly in order to break the life cycle of the fungi that causes blight.

Canker, Rust, and Blister

The term "canker" is used to describe a dead or blistered area in the bark, branch, trunk of an infected tree. Dozens of species of fungi cause canker diseases.  Cankers often appear as waxy discharge on the bark. Blisters or galls appear on branches and look like cysts or tumors on the surface of the bark and may also occasionally produce a waxy or yellowish discharge. Often, lower branches will be the first to show symptoms. Treatment options include pruning affected areas and applying a fungicide.

Wilts and Root Diseases

These are wood-decay diseases. They may get in through wounds in the lower part of the tree or penetrate roots directly. They involve the roots and in some cases the butt also. These fungi travel from tree to tree either through the air or soil.  Symptoms include die-off of needles on entire branches or limbs, peeling bark, and dropped branches. As rot progresses, the underlying root structure decays, making the tree unstable. Treatment options are few; in many cases, the entire tree must be removed.

If you plan to treat a diseased tree yourself, remember to follow all product directions if using fungicide. Make sure you are properly equipped and wearing goggles, gloves, and other safety gear if you plan to remove part or all of a tree. When in doubt, call a professional tree service.