Oak Tree Diseases: How to Treat Them

Yellow oak leaves in a grouping, next to a few acorns in the grass

You pull on your gloves, carefully place safety glasses atop your nose, and grab your tools. It’s time to do surgery — well, at least some minor cosmetic work — on your landscaping.

But sometimes, when you head out to trim and prune, you don’t know where to start. You may see something that looks off. Some discoloration. Unusual shapes. Maybe even an odd scent. Some are harmless; others can signal a disease.

It’s important to recognize what you’re dealing with, so you can remedy the ailment properly. With countless pests and fungi waiting to wreak havoc on any of the U.S.’s 90 varieties of oak trees, there’s a lot to consider when it comes to caring for these arbors.

Prevention Is the Best Medicine

As is true with most things, proper care is the best defense against any illness or setback. Oak trees, like all plants, benefit from adequate water, sunlight, and good soil conditions. To keep trees healthy and stress-free, ensure the following:

  • Apply about 3 inches of organic mulch from the trunk out to the dripline; do not pile mulch against the trunk.
  • Do not apply weed killers near the tree.
  • Use slow-release fertilizers in early April if necessary.
  • Water trees during droughts, providing about 1 inch of water weekly.
  • Protect trees and root systems from damage during construction in the area.
  • Promptly remove and dispose of infected trees, and grind stumps to prevent disease spread.

Disease Identification for Oak Trees

Many diseases present in oak trees can be identified with only a little knowledge. By inspecting leaves and trunks, and looking for dead branches, you can figure out what ails your oaks. Fortunately, many diseases require no treatment. Others, though, indicate immediate removal. Read on for a primer in disease identification and treatment.

Actinopelte leaf spot

Symptoms: Circular, dark to reddish-brown spots on the leaves; can run together creating blotches. Severe infections cause premature leaf drop.

Causes: Wet weather increases the vigor of this complex fungus.

Season: The disease can survive winter in infected twigs and leaves that do not drop, with signs emerging following bud break.

Treatment: Remove all infected plant material to slow spread. Spraying may be indicated for small trees that have lost their foliage several years in a row.

Anthracnose

Symptoms: Dead areas appear between leaf veins, mostly on lower branches. Brown spots form on the lower leaf surface and on dead twigs.

Causes: Cool, wet weather helps to incubate this fungus that is easily spread.

Season and risk level: Springtime and cool summers are ideal for this illness to take hold. All oaks, as well as many plants and vegetables, are prone to this infection.

Treatment: In general, pruning dead twigs and branches during dormancy is the best treatment. For further protection, apply an appropriate fungicide to protect new growth.

Armillaria root rot

Symptoms: Honey-colored mushrooms form annually at the base of the tree. These growths appear in large groups and are followed by a white fan of fungal growth under the bark at the base.

Causes: Borne in soil, wet weather, and improper drainage increase the likelihood of infection.

Season and risk level: All varieties of oak are susceptible, and damage will appear in late summer and fall.

Treatment: This root rot leads to a slow decline. Remove infected trees and protect healthy ones from stress.

Bacterial leaf scorch

Symptoms: Edges of oldest leaves turn brown, beginning on the inner and lower portions of the tree. A reddish-colored band sometimes develops between the brown and green on the leaf and branches begin to die.

Causes: Leafhoppers and spittlebugs carry the xylella fastidiosa bacteria, which causes a disruption of water movement throughout the tree.

Season and risk level: Symptoms appear late summer to fall, typically following a summer drought. Most common in pin, red, white, bur, and single oak varieties.

Treatment: Oxytetracycline injections by a professional can calm symptoms but will not cure the disease. Eventually, trees will need to be removed. If replanting, consider proper spacing to prevent movement of fungi-carrying bugs.

Bacterial wetwood, aka slime flux

Symptoms: Dark streaks of sap oozing from the bark, which typically carries an unpleasant odor.

Causes: Many different bacterias cause this as they enter the tree through wounded bark.

Risk level: All oaks can develop this, but pin oaks are especially prone.

Treatment: Avoid wounding the bark to prevent further stress. Care for sick trees as normal and minimize stress as possible.

Ganoderma root rot

Symptoms: Shelf-like structure forms on the wood near the soil line; usually brown to reddish. With time tree growth slows and dying branches produce small, yellowed leaves.

Causes: The fungi-carrying spores spread through the wind, leading to wood decay. Drought and physical injury increase the risk of infection.

Risk level:  All species of oak are susceptible, but proper care reduces risk.

Treatment: This may take years to kill the tree, but in the meantime, it will be susceptible to significant wind damage. Trees located near structures should be removed for safety purposes.

Hypoxylon canker

Symptoms: This white-rot fungal disease leads to the death of branches and leaves at the tree’s crown. There will be eventual dieback, and outer bark will slough off.

Causes: Reduced water uptake by trees, usually caused by some form of damage to the root zone, including drought and construction damage.

Risk level: Most species are susceptible. Post, water, southern red, white, and blackjack varieties are most likely to develop this disease.

Treatment: There is no treatment, but removing damaged branches can help slow the progression of the illness.

Inonotus root rot

Symptoms: Branch dieback and fewer leaves, which are usually yellowed. Illness begins far out in the root structure, and the tree may topple before visible signs are noticed. Visible signs also include conks at the base of the tree.

Causes: Fungi enter through wounds in the bark, including pruning injuries.

Season: Infection occurs at any time, and severe damage becomes noticeable in summer and early autumn.

Treatment: Sadly, this disease is terminal. The only option is to immediately remove the tree.

Laetiporus root rot

Symptoms: Large groups of yellow to pink, shelf-like fruiting structures turn white with age; the bark where these form becomes depressed and cracked.

Causes: Fungal infections and poor drainage from the roots.

Season: Growths form in summer and autumn, and fall off in winter.

Treatment: By the time fruiting structures grow, significant damage is already done and the tree has become susceptible to wind damage and toppling. The tree should be removed at the first sign of infection.

Leaf spot

Symptoms: Irregular, brown spots from between leaf veins. Spots will turn reddish-brown, and a yellow halo may appear with time.

Causes: Iron chlorosis takes hold during wet, humid, and mild conditions, typically as buds break.

Season and risk level: Spring and mid- to late summer; spores causing this illness can spread in the wind.

Treatment: Because little damage results from this, there’s no need to treat it. If you’re very bothered, you can apply a fungicide at bud break.

Oak leaf blister

Symptoms: Small spots that turn light green as the leaf grows; the center of the spot raises like a blister due to rapid cell growth.

Causes: Fungi secretions lead to overgrowth of leaf tissue, and tend to occur during especially damp spring seasons.

Risk level: Red and black oaks are most susceptible.

Treatment: Landscaped trees don’t require any treatment as spotting is usually minor and leaves do not drop prematurely.

Powdery mildew

Symptoms: A white growth on the surface of the leaves, both top and bottom. Severe infections may cause foliage to be malformed and can lead to early leaf drop.

Causes: This fungus prefers young foliage in shade, as there is a likelihood of moisture in which the disease thrives.

Season: Spring and autumn, when weather is cooler, provide conditions for this disease to thrive.

Treatment: The late onset of this disease means there is no significant damage, and therefore no treatment is required. If the issue is especially bad, a fungicide can be applied when the tree is dormant.

Oak wilt

Symptoms: Leaves at the top of the tree turn brown around the edges and wilt; damage progresses down the tree. Eventually, branches and twigs die altogether. Extensive leaf drop occurs by mid-summer.

Causes: The pathogen blocks water and nutrients from moving properly throughout the tree; usually affects closely planted trees.

Season and risk level: Damage appears in spring and early summer. Red oaks are most susceptible, while white oaks are somewhat resistant.

Treatment: Trees typically die within one year of infection. Remove infected trees quickly and inject neighboring oaks with a fungicide to inhibit spread.

Call the Doctor

There’s a time and place for professionals, and severely sick oak trees often require the help of licensed arborists. If you haven’t been able to remedy the issue in one or two growing seasons, and the disease is one that will ultimately kill the tree, it’s time to bring in support.

For severely damaged arbors, particularly those that are prone to windthrow and breakage, it is always best to seek out a licensed, insured, and experienced tree care expert. With some varieties reaching heights of 100-feet, oaks need to be handled carefully and skillfully.

Main Photo Credit: Pexels

Alison Hoover

Alison Hoover

Alison is a Midwesterner through and through, and loves to spend her time baking and reading. Always at home in the dirt, as a kid, Alison raised a vegetable garden with her dad, and flower gardens with her mom.