Common Diseases Affecting North County Trees

Three things are required for a disease to develop:

  • A pathogen (the disease-causing agent)
  • A plant 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 the majority of plant problems in urban areas, can be caused by such factors 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 proper treatment options.

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

Tree diseases commonly found in North County San Diego

Passalora Needle Blight on Leyland

Symptoms usually appear during summer months and typically the disease only affects one year old foliage.

Common symptoms include 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 spreads 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 a group of diseases on hard woods 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. Common hosts include oak, maple, sycamore, ash, walnut, and dogwood.

Symptoms vary with species, but in general the most obvious 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 then 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 normal leaf drop in the fall (e.g. oak). In sycamore, the fungus is able to grow out of leaves into adjacent twigs where it causes small cankers, shoot dieback, and witches brooms or deformed twigs.

Powdery Mildew

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

Fusiform Rust

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 important alternate hosts for this disease, but do not sustain any damage.

On pine, the most obvious symptom of infection is the formation of a spindle-shaped gall on a branch or main stem. The gall may be pitch soaked and occasionally exude sap. During cool spring months, bright orange spores are often produced on the gall surface. 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 key diagnostic characteristic on oaks are the bright orange spores (urediospores) produced on the underside of the leaf.

Fire Blight

Fire blight is a disease caused by the bacteria Erwinia amylovora. There are more than 100 species of plants known to be susceptible to fire blight; all are in the Roseaceae 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 tips of infected shoots will also droop and turn black and bend over into a shepherd’s crook shape. If many shoots are infected, trees will appear to be burned or scorched, hence the name “fire blight”.

Cankers may form when branches and stems are infected by the bacteria. Bark on cankers may appear raised and slightly blistered, especially 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.

Root Rot

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. In general they are pathogens of the roots and lower stems of both hardwoods and conifers 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. Growth reduction, chlorotic or scorched leaves, early fall coloration and/or premature leaf drop, branch dieback, wind-throw, and tree death are common above-ground symptoms. 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. Cankers may expand slowly and eventually kill large roots; entire stems are not usually completely girdled, but large lesions may cause dieback or death. After a tree dies, the fungus colonizes and decays sapwood.


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 they are stressed by other factors.

The presence of green stems and thick oval shaped mistletoe leaves is the only reliable sign of an infestation. Identification of an infestation is easier to observe in the winter when all the leaves are off the host plant. Leafy mistletoe has opposite evergreen leaves, rounded growth habit, and reaches approximately 2 feet in width. Inconspicuous flowers produce small, sticky, whitish berries in the fall.

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For more information about disease control or our other Complete Tree Care Services, please feel free to contact us directly at: 760.745.7871.

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