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Healthy Trees!

Common Tree Diseases

Common Tree Diseases

Trees can become infected with diseases just like people. There are many different bacterial and fungal diseases that can cause damage to the health of your trees. Diseases are often more damaging because they sometimes affect the water and nutrient functions of the tree, which can cause further damage to the surrounding landscape.

Three things are required for a disease to develop:

  • A pathogen (the disease-causing agent)
  • A plant’s susceptibility to that particular pathogen
  • An environment suitable for disease development

Plants vary in susceptibility to pathogens. Many disease-prevention programs focus on the use of pathogen-resistant plant varieties. 

Diseases can be classified into two broad categories:

  • Infectious: transmittable diseases caused by microscopic living agents
  • Non-infectious: non-transmittable diseases that are inherited or the result of non-living agents

Examples of infectious agents include fungi, protozoa, viruses, and bacteria. Non-infectious diseases, which account for most plant problems in urban areas, can be caused by factors such as compacted soil, nutrient deficiencies, temperature extremes, vandalism, pollutants, and fluctuations in moisture. Non-infectious disorders often produce symptoms similar to those caused by infectious diseases; therefore, it is essential for a Certified Arborist to distinguish between the two to determine appropriate treatment options.

Only an ISA Certified Arborist should assess a tree for disease; many look similar, and treatment should only be done by a professional.

Tree diseases commonly found in North County San Diego

Symptoms usually appear during the summer months, and typically the disease only affects one-year-old foliage. Common symptoms include the browning of needles and eventual needle drop. Passalora needle blight symptoms start on lower branches near the trunk and then spread outward toward branch tips. The loss of foliage extends upward and outward from year to year. In severe cases, the entire tree will turn brown except for the new growth at the tips of the branches. Brownish spores develop during late spring to summer on infected foliage.

Anthracnose is a general term for hardwood diseases that cause lesions on leaves, twigs, and fruits. The fungi responsible for anthracnose are Gloeosporium spp., Gnomonia spp., and Apiognomonia spp. Hosts include a wide variety of hardwoods. Shared hosts include oak, maple, sycamore, ash, walnut, and dogwood. Symptoms vary with species, but in general, the most apparent symptoms are the leaf lesions produced in the spring and expanding throughout the summer. Lesions often begin as pale green or greenish-grey blotches but turn yellow, tan, reddish-brown, or brown. Some trees respond to infection by prematurely shedding leaves (e.g., sycamore and ash), but others retain their leaves until regular leaf drop in the fall (e.g., oak). In sycamore, the fungus can grow out of leaves into adjacent twigs where it causes small cankers, shoot dieback, and witches brooms or deformed twigs.

Powdery mildew is caused by numerous fungi (Erysiphe spp., Phyllactinia spp., and others). Powdery mildew has an extensive host range; trees most commonly affected include crape myrtle (Lagerstroemia indica) and flowering dogwood (Cornus florida). Symptoms usually appear late in the growing season during high relative humidity. The injury commonly seen on infected plants includes stunting and distortion of leaves, buds, growing tips, and fruit.

This fungal disease attacks several southern pine species but is most damaging on slash pine (Pinus elliottii) and loblolly pine (Pinus taeda). Oaks, such as water oak (Quercus nigra), willow oak (Q. phellos), and southern red oak (Q. falcata), serve as critical alternate hosts for this disease but do not sustain any damage. The most apparent infection symptom on pines is the formation of a spindle-shaped gall on a branch or main stem. The gall may be pitch-soaked and occasionally exude sap. Bright orange spores are often produced on the gall surface during the cool spring months. These aeciospores are blown off by the wind and serve to infect oak leaves. On oak, symptoms are limited to small leaf spots that may be chlorotic or necrotic. A critical diagnostic characteristic of oaks is the bright orange spores (urediospores) produced on the underside of the leaf.

Fire blight is a disease caused by the bacteria Erwinia amylovora. More than 100 species of plants are susceptible to fire blight; all are in the Rosaceae family. Trees susceptible include apples, hawthorns, pears, and mountain ash. The disease can also occur (but is far less common) in the stone fruits: peach, cherry, and plum. Blighting flowers is usually the first symptom to appear in spring. Flowers will darken, droop, shrivel, and turn black. The infected shoots’ tips will also droop, turn black, and bend over into a shepherd’s crook shape. If many shoots are infected, trees appear burned or scorched, hence the name “fire blight.” Cankers may form when the bacteria infect branches and stems. Bark on cankers may appear raised and slightly blistered, significantly when cankers are actively expanding during the growing season. Cankers can eventually girdle and kill branches or the entire tree. Bacteria may be visible oozing from or near infected plant parts during warm, humid weather.

Armillaria root rot is a general name for a group of diseases caused by fungi of the genus Armillaria, most commonly A. mellea and A. ostoyae. They are pathogens of hardwoods and conifers’ roots and lower stems and are important decomposers of dead trees.

The symptoms of Armillaria root rot often resemble many other diseases and disorders of trees, such as drought, decline, Hypoxylon canker, Annosus root rot, and Phytophthora root rot. Common above-ground symptoms include growth reduction, chlorotic or scorched leaves, early fall coloration, premature leaf drop, branch dieback, wind throw, and tree death. Conifers may produce large crops of undersized cones during decline. Trees are often affected in groups. Armillaria causes cankers (lesions) on the inner bark and outer sapwood on the root crown and lower stem. After a tree dies, the fungus colonizes and decays sapwood. Cankers may expand slowly and eventually kill large roots; entire branches are not usually wholly girdled, but large lesions may cause dieback or death.

Mistletoe is a perennial, broad-leafed, evergreen plant that parasitizes many species of hardwood trees, most commonly oaks and hickories. Heavily-infested trees may be reduced in vigor, stunted, or even killed, especially if other factors stress them. Leafy mistletoe has opposite evergreen leaves, rounded growth habit, and reaches approximately 2 feet in width. The presence of green stems and thick oval-shaped mistletoe leaves is the only reliable sign of an infestation. Identifying an infestation is easier to observe in the winter when all the leaves are off the host plant. Inconspicuous flowers produce small, sticky, whitish berries in the fall.