Diseases are common plant problems in the landscape. Often, it can be difficult to identify diseases because the causing organisms are rarely seen with the naked eye, and they spread by microscopic growth and spores. However, the signs and symptoms on the tree become obvious as the disease takes hold.
Tree diseases can affect any part of the tree, or the entire tree. Named for the type of damage they cause, we have leaf spot, leaf blotch, scab and blister, defoliation, needle cast, and yellowing or chlorosis as named symptoms. Stem canker and galls, trunk and root rot are also very prominent tree disorders.
Before you take corrective action, you have to determine what is causing the problem. A correct diagnosis is the first and most important step in developing and applying a correct treatment, which allows you to address the problem rather than simply treat the symptom. This is when a diagnosis from a professional arborist may prove critical.
Here are some of the most virulent tree diseases that might affect your area:
Combinations of crown dieback, discolored or loss of foliage and a generally unhealthy appearance.
Trees can suffer phytophthora for years before death, if the decline starts to spread from the root system, as pictured. If the crown or basal stem is attacked, however, the tree may be killed within a single season.
Young trees are especially vulnerable to phytophthora, due to their underdeveloped root systems and crowns. Good soil drainage is recommended to guard against phytophthora, as the disease thrives in warm, moist soil.
Anthracnose refers to a symptom rather than a specific fungus. Many fungi produced these symptoms on specific host plants.
The most common symptom of this group of diseases is irregular dead areas or blotches on the leaves. Sometimes whole leaves are engulfed. Cankers may girdle twigs and small branches causing them to die. The resulting regrowth from lateral buds can give the tree a gnarled or crooked appearance.
Symptoms on most trees are confined to the leaves the disease is considered more lethal if the fungi invades the twigs and branches and can prove fatal as in the flowering dogwood. Note that many plant diseases and adverse weather conditions can cause similar symptoms.
This disease spreads to oak trees in two ways: carried by insects and by root graft infections (when roots of different trees meet each other in the soil and form a connection). Sap feeding beetles spread this fungal disease from infected trees to wounds on healthy trees. Oak wilt disease symptoms progress differently in red oaks, white oaks, and live oak. However, they have one critical symptom in common: leaf drop. This is important because most other oak disorders do NOT cause leaf drop. When leaf drop is combined with one or more other oak wilt symptoms, the disease may be reliably identified.
On red oaks, the symptoms include: leaf drop; partially brown and partially green leaves, starting at the margins and working toward petiole (the leaf’s stem). There is a rapid progression of symptoms from top down and the tree dies quickly, resulting in dark streaking that may appear under bark and spore mats underneath the bark (this appears ONLY in the red oak family). Surrounding red oaks may also wilt and die.
On white oaks the symptoms include: leaf drop; partially brown and partially green leaves that seem to discolor from the tip down in a more solid pattern than red oak group; may be olive drab in color and appear dry. There is a rapid progression of symptoms from tip of branches inwards. The branches die one at a time over a period of time; the tree death may take months or years and dark streaking may appear under bark.
The live oak family (Texas oak) symptoms include: leaf drop and interveinal chlorosis (yellowing between the leaf veins). Symptoms appear throughout the tree, the tree dies within one to six months, dark streaking may appear under bark. Surrounding live oaks may also wilt and die.
Any plant infested with large numbers of sucking insects or growing beneath one of these plants may be affected by sooty mold.
Sooty mold fungus is a general term for several species of fungi or molds that use the honeydew secreted by sucking insects as food to develop and grow.
It generally does not cause damage to the plant, but can be a considerable nuisance to homeowners as it affects anything beneath the trees including hardscape, cars and structures.
Fire blight is a common bacterial infection that results in twig dieback.
The classic symptom of a shepherd’s crook with a burned appearance to the leaves and twig is a sure sign of this destructive disease. The disease can destroy limbs and even entire shrubs or trees.
Phytophthora Root & Crown Rot: Andrej Kunca, National Forest Center (Slovakia)
Anthracnose: R. J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company
Sooty mold: Joseph O’Brien, USDA Forest Service
Oak wilt: William M. Ciesla, Forest Health Management International
Fire blight: John Hartman, University of Kentucky
All images catalogued on www.bugwoord.org.
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