Plant Diseases of Oak Trees

With a reputation for strength and durability, the mighty oak tree thrives on little or no care and has a lifespan of 200 to 400 years. Despite this, it is susceptible to a number of diseases that undermine its natural hardiness. Adapted to a specific environment where summer drought is common, most diseases effect oak trees because of unsuitable growing conditions, particularly over-watering.

Crown Rot

Crown rot (Phytophthora and Pythium spp) is a common oak diseases. Typified by wilted appearance and overall lack of vigor, the tree shows diminished growth and stunted foliage with yellowish (chlorotic) leaves. A dark fluid may leak from wounds on the bark. Die-back of twigs is common. Caused by a fungus, crown rot is exacerbated by excess moisture, poor soil drainage and over-fertilization.

Oak Root Fungus

Most mushrooms at the base of an oak tree are beneficial, but oak root fungus (Armillaria melea) is not one of them. Branch die-back and tan-colored mushrooms around the base of the tree usually indicate the disease. Often a white, fan-shaped growth protrudes between the bark and wood. Armillaria exists on most oak trees. Under normal circumstances, where a summer drought is present, the disease is prevented from spreading. Causes include frequent irrigation, impeded drainage, over pruning, drought, root loss and soil compaction. Symptoms usually appear at a late stage, preventing significant intervention.

Heart Rot

Heart rot is a disease that acts on the heartwood of the tree. Though the heartwood is not technically "active," if it is significantly weakened, the tree may need structural support. Pruning and bracing may lighten the load on afflicted areas.

Diplodia Canker

This disease effects the twigs and small branches of the Coast Live Oak and Valley Oak. Caused by a fungus, trees that are stressed are most often susceptible. Trees show leaf loss, leaf browning and an overall weak appearance. Poor landscape practices such as soil compaction around the root zone and improper transplanting contribute to this disease.


Warm, wet weather perpetuates this disease. The fungus (Discula platani) is spread throughout the tree when rain or other water sources splash from one leaf to another. Typically, the disease follows the veins of infected leaves on the lower part of the tree upward-eventually covering the entire tree. This disease can quickly cause a healthy tree to become brown but tends to clear when the weather is less humid.

Sudden Oak Death

Caused by a water mold (Phytophthora ramorum), sudden oak death is so named because of the speed of which it occurs. Browning of leaves happens within two to four weeks. Death of the tree occurs in about two years. Not all oaks are susceptible. White oak species and younger saplings seem to be immune. A canker on the trunk of the tree is the most visible symptom. The canker is dark in color and generally oozes a brownish sap. Once effected by sudden oak death, insects tend to invade, hastening the inevitable decline of the tree.

Powdery Mildew

Powdery mildew is most often found in coastal areas where humidity and cool temperatures are the norm. Affecting mainly new growth, the leaves take on a "witches broom" appearance and do not develop properly. A tree with powdery mildew can become quite dull and defoliated in appearance. Trees grown in turfgrass are particularly afflicted because constant exposure to moisture does not allow the tree to dry out. Because new growth is effected, the tree may weaken significantly and eventually die.

Drippy Nut Disease

More a nuisance than a significant problem for the oak tree, drippy nut disease, caused by the bacterium Erwiniaquercina effects acorns of the coast live oak and the interior of the live oak. The bacteria gains access through insect holes in the acorns. Warmer weather and bacteria growth leads to a sticky substance that drips below the tree.

Keywords: oak tree diseases, oak tree death, oak root rot, oak root fungus, anthracnose, heart rot

About this Author

Andrea Peck has been writing since 2006. Her work has appeared in "The Rogue Voice," "Information Press" and "The Tribune." Her writing focuses on topics about gardening and the environment. Peck holds a Bachelor of Arts in mathematics and a minor in biology from San Diego State University.