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Common Forest Diseases

While more trees are killed each year by forest insects than by any other cause, losses in annual timber production due to disease make insect-caused losses pale in comparison. In fact, the volume of timber lost each year due to reduced growth and mortality caused by tree diseases is larger than all other causes (including insects, fire, weather, and others) combined. Diseases are caused by pathogenic organisms (pathogens) that are infectious and transmissible, meaning that pathogens infect (enter) the tree, cause a physiological disruption, and then can spread to other trees. Pathogens include microorganisms such as viruses, bacteria, and nematodes; but most diseases are caused by fungi.

It should be noted that most microorganisms are not pathogens; rather most play beneficial roles in the forest such as in decomposition, nutrient cycling, and even participate in symbiotic relationships that benefit plants. Without these microorganisms our forests would not exist. Even pathogens play beneficial roles in our forest ecosystems by culling weakened or stressed trees, promoting natural succession, and producing habitat for wildlife species. However, when the conditions are just right, pathogens can have devastating consequences for our tress and other woodland plants.

In forestry, effective disease management begins with using good forest management practices to promote vigorous, healthy trees. Most pathogens are opportunistic, and only cause disease on stressed or otherwise unhealthy trees. However, some pathogens cause diseases that can kill healthy trees, reduce timber values, or even wipe-out an entire tree species if left unchecked. Other diseases may not cause any serious harm to trees, but may make trees unsightly susceptible to attack by other pathogens or insects. Tree diseases can be difficult to control, and therefore prevention is the preferred management option. Please seek assistance from your county forest ranger if you suspect a disease is plaguing your woodlands.

Foliage/Leaf Disease

Some of the most common diseases observed in our forests are those of foliage, including broadleaves on hardwoods and needles on conifers. Leaves are green tissues that are more susceptible to attack by pathogens than are the woody parts of a tree, and the effects of foliage diseases are often very noticeable because of their dramatic appearance. Most leaf diseases are relatively harmless, causing little if any long term damage, and disappear when leaves are shed in the fall. However, there are some pathogens which infect leaf tissues, but are then able to spread into other parts of a tree causing serious disease and even tree death. Leaf diseases tend to be weather dependent and are often most severe in years with cool wet springs. Look for leaf spots (lesions), discoloration of foliage, and early leaf drop (defoliation).

Branch and Stem Diseases

Branches and stems are the trees main structural components above ground, and allow trees to grow above other plants in the forest to capture light used for photosynthesis. Branches and stems need to be very strong to support the massive weight of these organisms at such terrific heights. In addition, branches and stems house the trees vascular system. Therefore, diseases that affect branches and stems can have a significant impact on overall tree health. The stem is also the most valuable part of a tree in terms of timber production, and some branch and stem diseases cause defects that can destroy long term investments by making standing timber unmarketable or worthless. These diseases are often untreatable, and therefore proper forest management is critical. Look for galls and abnormal growths, cankers, weeping wounds or cracks, cavities, and signs of fungi including colorful spores and fruiting bodies (such as brackets, conks, and shelf fungi).

Root Diseases

Trees have enormous root systems whose main function is to deliver water and nutrients from the soil into the canopy where photosynthesis occurs. Roots also anchor trees, allowing them to grown to amazing heights and making them stable in high winds. The soil with which the roots have contact is teaming with microorganisms; some of which are beneficial to the tree. Mycorrhizal fungi engage in important interactions with tree roots that allow improved water and nutrient uptake. Yet, some microorganisms are pathogens of tree roots. Some pathogens feed on the fine roots and root hairs that are responsible for drawing water and nutrients out of the soil, while other pathogens attack the larger structural roots that stabilize trees. Root diseases are some of the most difficult to diagnose and treat. Proper forest management and maintaining tree health is often the only option. Trees that are stressed because they are planted in the wrong place, do not receive enough water, or are growing in nutrient poor soils are often very susceptible. Look for an overall decline in tree health. Symptoms may mimic those of drought or nutrient deficiencies such as yellow leaves, early color change and leaf drop, leaf browning, and branch dieback. Trees with structural root problems may exhibit similar symptoms, and are more prone to wind-throw and leaning.

Vascular Diseases

Trees have a vascular system (just like we do) that transports water, nutrients, and carbohydrates throughout the tree. The xylem transports water and nutrients from the soil through the roots and up into the leaves. The phloem delivers carbohydrates produced in the leaves during photosynthesis down into the branches, stem, and roots to maintain growth and normal physiological functions. A number of pathogens are capable of entering a tree's vascular system (often with the assistance of insects) and causing disease. Vascular diseases are some of the most severe in our forests and can cause catastrophic losses; full-grown mature trees can die within weeks to months in some cases. Fortunately, there are a variety of proven methods for preventing vascular diseases, and even some treatments to save trees that are infected. Look for browning or wilting leaves, crown dieback, and early leaf drop.


Some diseases are not caused by a single pathogen, but rather by a combination of predisposing stress factors and opportunistic insects and diseases. Foresters refer to these diseases as "declines". The underlying cause of declines is poor tree health, so proper forest management and tree care is critical for prevention and treatment. Declines degrade tree health and eventually kill over many years. Often the initial symptoms go unnoticed, and it is not until the later stages of decline that the problem is recognized. Predisposing factors include trees planted out of range or off-site, drought, poor soil conditions, age, root damage, and many minor insects and disease problems. Opportunistic insects and diseases, which could not normally attack a healthy tree, and then able to move in and make the problem worse. Look for early color change and leaf drop, branch dieback beginning high in the crown, smaller than normal leaves, leaf yellowing, crown thinning, greater than normal fruit or nut production, and unusual insects or diseases.

Abiotic Diseases (Disorders)

Some diseases are not caused by living pathogens at all, but rather are the result of non-living stress agents such as nutrient deficiencies, fire, lightning, floods, injuries, and chemicals. Therefore, these problems are often referred to as disorders rather than diseases because they are not infectious and cannot be transmitted from one tree to another. Many of these disorders are the result of actions (or inactions) by people, and therefore are the most easily prevented of all diseases. Symptoms can be wide-ranging, and vary greatly with cause. Look for problems that affect more than one trees species. Diseases caused by pathogens are usually species-specific, whereas abiotic disorders can and usually do effect many tree species in an area. Knowledge of the site's history and recent activities is often critical in determining the cause and available treatment options.

This page updated: Tuesday, January 17, 2017 12:10

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