- Key to Diseases of Oaks in the Landscape
- Key to Oak Diseases
- Oak Leaf Blister
- Tubakia Leaf Spot (Actinopelte Leaf Spot)
- Bacterial Leaf Scorch
- Oak Wilt
- Fusiform Rust
- Powdery Mildew
- Sooty Mold
- Crown Gall
- Hypoxylon Canker
- Heart or Canker Rot
- Bacterial Wetwood
- Armillaria Root Rot
- Root Rot
- Oak Diseases & Insect Pests
- Spanish Moss
- Insects & Related Pests
- Bur oak blight
- Common Causes of Oak Mortality
- Chronic oak dieback
- Disease overview
- Disease details
- Incidence and distribution
- Our research
- Related resources
- Common Oak Trees: Oak Tree Identification Guide For Gardeners
- Oak Tree Varieties
- Most Common Oak Trees
Key to Diseases of Oaks in the Landscape
Bulletin 1286 View PDF picture_as_pdf
Key to Oak Diseases
What is an “identification key?” It is a road map of sorts that aids in the identification of a disease or an insect specimen observed in the field. It is based on a series of questions that have as answers choices between two or more alternative pathways. The choices describe symptoms or morphological characteristics thought to be most distinctive of a particular insect or disease. In order to use these keys, you must first decide or make an educated guess as to whether the damage or symptom is caused by an INSECT or DISEASE. –From “Florida Tomato Scouting Guide”, University of Florida.
The following key will help you identify common diseases of oak trees.
|1. FOLIAGE AND/OR TWIGS AFFECTED||go to… 2|
|1. TRUNK OR BRANCES AFFECTED||go to… 10|
|1. ROOTS OR ROOT COLLAR AFFECTED||go to… 13|
|2. Leaf necrosis (dead spots or blights) present||go to… 3|
|2. Necrosis absent; leaves affected but remain green||go to… 6|
|3. Tiny to sprawling brown or gray spots along leaf margins and/or veins; no leaf blistering or puckering associated with spots||go to… 4|
|3. Tan, crusty spots associated with leaf puckering; seen in the fall (Oak Leaf Blister)||view info|
|3. Marginal, tip or interveinal leaf scorching; affectes all leaves on a branch||go to… 5|
|4. Spots seen in the spring; leaves are tattered or distorted; twigs killed (Anthracnose)||view info|
|4. Sprawling spots seen in the fall; often with yellow halo (Tubakia Leaf Spot)||view info|
|5. Scorching from leaf tip toward petiole; yellow to red line often precedes advancing scorch symptom on leaf (Bacterial Leaf Scorch)||view info|
|5. Yellow, brown or dull green scorching from leaf tip toward petiole or beginning at leaf petiole; affected branch often wilts (Oak Wilt)||view info|
|6. Leaves and/or twigs with mycelium on upper or lower surface||go to… 7|
|6. Light green, puckered areas in the spring; areas become necrotic in fall (Oak Leaf Blister)||view info|
|6. Small, yellow pin-point spots on upper leaf surface; brown, hair-like projections on leaf underside (Fusiform Rust)||view info|
|7. Upper or lower surfaces of leaf with white, powdery growth; pale green spots may also be present when growth is absent (Powdery Mildew)||view info|
|7. Black, sooty or powdery growth typically on upper leaf surface (Sooty Mold)||view info|
|10. Branches or trunk galled or swollen (Crown Gall)||view info|
|10. Branches show dieback; cankers on trunk; tree in decline||go to… 11|
|10. Branches show flagging, wilting; foliage yellows or browns||go to… 12|
|11. Black, crusty, small spots or vertical stripes on trunk or branches (Hypoxylon Canker)||view info|
|11. Conks or mushrooms protrude from trunk (Heart or Canker Rot)||view info|
|11. Sour- or yeasty-smelling sap oozes from trunk; trunk stains dark (Bacterial Wetwood)||view info|
|12. Vascular discoloration present (Oak Wilt)||view info|
|12. Vascular discoloration not present (Bacterial Leaf Scorch)||view info|
|13. Established tree in slow decline, white or thread-like material under bark (Armillaria Root Rot)||view info|
|13. Conks or mushrooms growing at root collar (Heart Rot or Wood Decay)||view info|
|13. Tree is newly transplanted; leaves turn brown, bark on lower stem may peel away (Root Rot)||view info|
Oak Leaf Blister
Oak leaves begin to show chlorotic, blister-like areas on the upper surface that can be as large as one half inch in diameter (Fig.1 & Fig.2). The lower surface has gray depressions that correspond to the raised blisters. As the disease progresses, the blisters turn brown and the leaf will curl as the blisters coalesce. Premature leaf drop also may occur. Trees are not severely damaged, but the appearance of the tree may be unsightly. All oak species are susceptible to this disease.
Figure 1. Chlorotic blister-like lesions. Figure 2. Puckered lesions of oak leaf blister.
Leaves become infected when buds are beginning to open in the spring. The pathogen can survive winter on plant twigs and bud scales. The disease develops during wet, humid, mild conditions in the spring. Spores causing oak leaf blister are spread by wind and rain.
Rake fallen leaves and debris and discard to reduce disease inoculum. No chemical control is necessary.
Symptoms vary with host, weather and time of infection. Shoot blight is one of the first symptoms seen in spring. Blighting causes leaves and shoots to brown and shrivel. Young leaves become cupped or distorted with necrotic lesions. Large lesions often follow leaf veins or are delimited by leaf veins (Fig. 3). Old lesions are papery and gray to white in color (Fig. 4). On the underside of an infected leaf, tiny brown fungal fruiting bodies may be visible on or near major veins. Premature leaf drop is common. Mature leaves are fairly resistant and the symptoms are simple necrotic spots. If infection is severe, branch cankers and twig dieback can occur during winter and early spring. The symptoms usually first appear on lower branches and then spread upward.
Figure 3. Anthracnose lesions following a leaf vein. Figure 4. Distorted leaves with papery lesions.
Anthracnose fungi overwinter in twigs and plant debris. If winters are mild, the pathogen is active causing cankers and dieback. Spores are spread by wind and rain during the spring and infect new shoots. This disease can have multiple cycles per year if the weather is moderately warm and wet.
Rake and destroy fallen leaves. Prune lower branches to increase air circulation and ensure proper tree fertility and irrigation. Chemical controls are rarely recommended unless trees are newly established.
Tubakia Leaf Spot (Actinopelte Leaf Spot)
The symptoms caused by Tubakia are brown or reddish brown blotches on the leaves. Premature leaf drop and twig cankers are common if the trees are severely infected. Spots are well-defined on young leaves and enlarge to necrotic blotches on older leaves (Fig. 5 & Fig. 6). Small fungal fruiting bodies can be seen within the lesions. Lesions may cause leaves to collapse if they are on veins and restrict water movement.
Figure 5. Newly developed circular lesions caused by Tubakia. Figure 6. Brown lesions on older leaves of Tubakia leaf spot.
The disease overwinters on twigs and plant debris. It favors wet, humid conditions and warm temperatures. Primary spore dissemination is by wind and rain. Red oaks are more susceptible than white oaks.
Rake and dispose of fallen leaves. Prune trees to increase air circulation and ensure proper fertility and irrigation.
Bacterial Leaf Scorch
Symptoms of bacterial leaf scorch are described as marginal leaf burn and are very similar to drought stress symptoms (Fig. 7). In addition to marginal leaf burn, there is a defined reddish or yellow border separating the necrosis from green tissue. Symptoms are more noticeable in late summer after hot, dry conditions, but symptoms can be expressed all year. Symptoms first appear on one branch or section of branches and on the oldest leaves. Each year symptoms will reoccur and progress to other parts of the tree (Fig. 8).
Figure 7. Marginal leaf burn due to bacterial leaf scorch. Figure 8. Branch dieback due to bacterial leaf scorch.
The disease is spread by leafhoppers, spittlebugs and through root contact with neighboring trees. Since the pathogen is harbored within insects, warm temperatures and high populations of leafhoppers and spittlebugs are conducive for bacterial leaf scorch.
Remove severely infected trees and replant using resistant species. Control weeds (to minimize insect populations) and ensure proper tree fertility and irrigation to maintain health and vigor.
Oak wilt is rare in Georgia. The disease has not been positively identified at the UGA Plant Disease Clinic. Oak wilt is first observed near the top of the tree. Browning and bronzing of the leaves from the margins toward the petiole are the first symptoms of oak wilt. Eventually the leaves will drop prematurely and the tree will die. White oaks are moderately resistant to oak wilt. Red oaks often die within four weeks of the first symptoms. It may take years before other oaks die. The most reliable way to diagnose oak wilt on live oak is to observe leaf veins (Fig. 9). Veins are chlorotic and eventually turn brown. Oak wilt on red oaks simply wilts young leaves and they turn pale green and brown. Mature leaves may have water soaking lesions or turn bronze on the leaf margins (Fig. 10). Only individual branches or a portion of the canopy show symptoms. The fungus may cause brown streaking of sapwood (Fig. 11). Red oaks may have fungal mats that break through the bark (Fig. 12).
Figure 9. Chlorotic veins on live oak. Figure 10. Bronzing of leaf margins on red oak.
Figure 11. Brown streaking in sapwood. Figure 12. Cracked bark due to fungal mats.
The disease is spread by oak bark beetles and root grafts. It is most active during moderate temperatures. Red oaks are more susceptible than white oaks. The pathogen cannot tolerate temperatures above 90 degrees F ; so often, the pathogen dies in leaves and twigs. However, it may remain in the trunk and in the roots. Oaks are most susceptible in spring as new wood is forming.
Remove infected trees. Trenching may be used to prevent root grafting. Trunk injections using a systemic fungicide have had some effect in reducing oak wilt spread within newly infected trees; however, this method is not recommended for trees in the southeastern United States because incidence is limited and of little value once disease is identified.
Lower leaf surfaces often have inconspicuous yellow to orange pustules and black hair-like clusters (Fig 13 & Fig. 14). However, the orange pustules can become very noticeable in summer and fall as rust infection continues on oak leaves. Leaves may have yellow spots or necrotic blotches and develop premature leaf drop. Symptoms on oak are not severe, but it is the alternate host to a more devastating disease in pines.
Figure 13. Yellow pustules. Figure 14. Black hair-like clusters.
Fusiform rust is most damaging in slash and loblolly pines. During the spring, pine galls produce spores that are wind-blown to young oak leaves. The pustules on the bottom of oak leaves then release spores in late spring that are carried by the wind to branch tips and tender, green pine needles. Most pine infections occur in April and early May. One spore stage (urediniospores) produced on oak leaves can continue to re-infect new oak leaves throughout the summer during warm, moist, humid conditions. This stage of the disease does not affect pines.
Disease is usually insignificant to oaks, except in tree nurseries where the orange pustules can make leaves unsightly. Fungicides can reduce rust development on young newly planted oaks, but control is directed mostly towards saving pines.
Figure 15. White, powdery growth on the upper side of an oak leaf.
Powdery mildew is a white, powdery growth on the upper side of leaves (Fig. 15). Tiny black fruiting bodies are usually present in late summer or fall. Infection may also occur in the buds and shoots causing a witches’ broom as in the case of live oaks. Symptoms appear to be superficial.
Powdery mildew survives the winter on fallen leaves and in the spring, wind and water disseminate spores to new leaves. Cool, moist weather conditions in the spring are ideal for infection. Powdery mildew prefers warm days around 80 degrees F and cool nights of 60 degrees F. Temperatures above 90 degrees F will reduce powdery mildew growth. Powdery mildew also requires high humidity but dry leaves. This pathogen usually attacks a crowded canopy with poor air circulation.
Irrigate early in the morning, improve air circulation and remove fallen leaves. Witches’ brooming may be pruned from the tree. Fungicides are generally not recommended or needed as the disease is not lethal to the tree.
Figure 16. Black crust of sooty mold on foliage.
Sooty mold is a crust-like or powdery black growth on the foliage and stems of infested plants (Fig. 16). Sooty molds grow superficially and do not penetrate the leaves. Sooty mold fungi grow mainly on the excrement of sap-sucking insects such as aphids, scales, and whiteflies. Sooty molds do not attack the plant and it is mainly a cosmetic problem. Occasionally, severe build-ups can inhibit photosynthesis and reduce plant vigor. Sooty molds may persist long after the insects have left.
Sooty molds are present where there is a high insect population. Spores are dispersed by water.
Reduce insect population. No control is directed toward the sooty mold as its growth will stop when the insect population is controlled.
Roots, stems and crowns form large, rough galls (Fig. 17). Plants with large galls are often stunted and are the first to suffer from environmental stresses. Damage is greatest when galls encircle the root crown. Mature trees may survive, but severely stressed or diseased trees may be killed by secondary pathogens. Galls are initially spongy, becoming hard with age (Fig. 18). Dead surface tissue will decay and slough-off.
Figure 17. An extreme case of crown gall. Figure 18. Mature gall that is hard and rough.
A crown gall bacterium is dispersed in soil and water and enters through wounds on roots and crowns. Once inside, the bacterium transfers some of its genetic material into the trees cells. These cells form tumors and grow at a rapid rate. The bacteria may survive in the soil for two years and galls develop in temperatures near 72 degrees F. Temperatures above 86 degrees F stops normal cells from transforming into tumor cells but does not prevent galls after transformation.
Avoid introducing crown gall bacterium into an area. Use resistant varieties and avoid injuring tree roots and trunks. Control insects that feed on stems and roots. Disinfect tools when pruning infected trees.
Yellowing and wilting of leaves often related to physiological stress may be the first symptom of this disease. Fungal mats (stroma) will develop beneath the bark of infected trees. Bark will begin to slough-off due to pressure from the stroma beneath it. Stroma exposed by sloughing bark is hard, tan to silver gray on the outside, and black within (Fig.19 & Fig.20). There may be small patches to elongated strips. Old stroma eventually loses the gray color and appears black. The sapwood becomes tan to light brown and has a definite black border.
Figure 19. Gray and black patches. Figure 20. Silver gray patches.
Hypoxylon cankers are opportunists that attack trees weaken by other factors such as heat, drought, wound, root injury or other diseases. Oak species most commonly infected are black, blackjack, laurel, live, post, and white oaks. Hypoxylon can be present as a latent colonist in healthy trees and this may account for the rapid invasion of stressed trees. The fungus is favored by warm temperatures of 60 to 100 degrees F but the optimal temperature is near 86 degrees F. Spores are primarily wind dispersed.
Keep trees vigorous with proper fertility and irrigation. Prevent wounds from mechanical, weather and insect injury. Remove infected limbs or trees once they become hazardous to property or people, and to reduce disease spread to adjacent trees.
Heart or Canker Rot
Figure 21. Smooth, shelf-like conk.
The first symptom of this disease is a white rot of the heartwood and the eventual death of the cambium and sapwood. The decay removes lignin and the wood becomes spongy. The basidiocarps or conks of Ionotus andersonni exhibits dull brown peg like structures that may exceed 20 inches in length on the outside of the tree. Conks of I. hispidus form in the summer and early autumn. The conks are smooth and shelf-like and are fully grown within one to two weeks (Fig. 21). The top of the fungus is yellow to rusty red and the porous bottom is rust color. After three weeks, they dry and fall to the ground. Conks of I. dryadeus form at the base of infected trees among root flares. Conks are initially white or light-colored and turn black and crusty with age. Infected trees show symptoms of general tree decline including branch dieback, loss of leaves and yellowing or browning of leaves in summer. Trees weakened by drought stress, wounding or other injuries are most susceptible.
Figure 22. Cracked bark due to fungal mats.
Oaks in the red and black group are most often infected. Branch stubs within 16 feet of the ground are the most common infection sites, but entry also occurs through other injuries (Fig.22). Optimal temperature for growth is 95 degrees F. Old conks or remnants may continue to fruit for up to five years.
Avoid unnecessary injury or stress to trees. Remove hazardous trees to protect property and people. Time pruning of infected branches to minimize exposure of susceptible tissues and when spores are not disseminated (late winter or spring).
A sour odor is often associated with wetwood as water-soaked wood with large numbers of dead bacteria begin to break down. The build-up of bacterial populations within the tree causes fermentation resulting in internal gas pressure of up to 60 pounds per square inch. Foliage sometimes wilts and branches may dieback. However, most of the time, wetwood is a minor problem that leaves a vertical streak on the bark where pressurized liquid escaped out of wounds (Fig. 23 & Fig. 24). Many times, secondary fungi and bacteria infect the surface liquid and create a slimy texture on the bark.
Figure 23. Staining of bark. Figure 24. Bark stain due to wetwood.
Bacteria that cause wetwood tolerate low oxygen and are often found in soils and on plant surfaces. Bacteria enter through assorted wounds above and below the soil line. The bacteria may lay dormant during the greatest periods of growth and become active in mature or older tissues.
There are no known controls for bacterial wetwood. A 10 percent bleach solution may be used cosmetically to clean stains off the bark.
Armillaria Root Rot
There are several general symptoms that accompany Armillaria root disease, including crown dieback, growth reduction, premature leaf drop or death of the tree. Because these fungi commonly inhabit roots, their detection is difficult unless characteristic mushrooms are produced at the base of the tree. Removing the bark will expose the characteristic, white mycelial rhizomorphs that grow between the wood and the bark (Fig. 25). Short-lived mushrooms may be found growing in clusters around the bases of infected trees. They are honey brown to reddish in color (Fig. 26). The fungus breaks down lignin and cellulose causing the wood to become spongy.
Figure 25. White rhizomorphs of Armillaria. Figure 26. Honey-colored Armillaria mushrooms.
Spread occurs when rhizomorphs contact uninfected roots. Rhizomorphs can grow for distances of up to 10 feet and penetrate the roots by a combination of mechanical pressure and enzymatic action. Mushrooms are produced in late summer or autumn, and are most abundant during moist periods.
Vigorously growing trees often confine the fungi to localized lesions and limit their spread up the roots by secreting resin and rapidly forming callus tissues. But when infected trees are in a weakened condition, Armillaria spreads rapidly through the roots.
Because these fungi are indigenous to many areas and live on a wide variety of plants and woody material, their eradication is not feasible. Management should be directed toward increasing tree vigor through proper irrigation and fertility.
Above-ground symptoms vary, but generally include reduced tree vigor and growth, yellowing or chlorosis of leaves and eventual collapse or death of the tree. Infected trees may decline slowly over one or more years, or they may collapse and die rapidly after resuming growth in the spring. Rapid death of trees usually occurs following excessively wet periods. On trees that decline slowly, leaves will yellow or brown while leaves on healthy trees remain green. To observe below ground symptoms, you need to remove several inches of soil around the base of the declining tree. A diagnostic reddish-brown discoloration of the inner bark and wood can be observed after cutting away the outer bark layer (Fig. 27). Similar symptoms can be found on roots, but it is generally difficult to see root symptoms without removing the tree (Fig. 28).
Figure 27. Reddish brown wood discoloration. Figure 28. Brown discoloration of roots.
Phytophthora root and crown rots are caused by several Phytophthora species. While some species are much more destructive than others, all species require extremely wet or saturated soils in order to infect and cause significant damage. These fungi over-winter and persist in soil as mycelium in infected wood and can remain viable in the soil for years. When soils are wet, the fungus germinates and spreads to susceptible plant tissue where they infect. They may also spread to the soil surface and move over longer distances in runoff water. Trees appear to be most susceptible during spring and autumn, which are also the times of year when soil temperatures are most conducive to fungus growth and zoospore production. Fungal activity is low in the winter when trees are dormant.
Control of Phytophthora root rot is most successful using an integrated program of cultural practices including good soil drainage and proper irrigation. Avoid planting trees too deep as this also contributes to disease development and decline.
Forestry Images — www.forestryimages.org
Purdue University — www.ppdl.perdue.edu/PPDL
Texas A&M University — http://plantclinic.tamu.edu
University of Minnesota — http://pdc.umn.edu
University of Natural Resources and Applied Life Sciences
West Virginia University — http://www.davis.wvu.edu
Status and Revision History
Published on Aug 15, 2005
Published on Feb 10, 2009
Published on May 05, 2009
Published with Full Review on Jun 22, 2012
Oak Diseases & Insect Pests
Although oak trees (Quercus species) are well adapted to South Carolina, they can be commonly affected by many pests and diseases. Maintaining healthy oak trees, by following recommended cultural practices, is the first line of defense in preventing most of these problems. For more information see HGIC 1017, Oak.
Oak leaf blister on water oak.
Andrew J. Boone, South Carolina Forestry Commission, www.forestryimages.org
Oak Leaf Blister: Oak leaf blister is a disease caused by the fungus Taphrina caerulescens. Most oak species are susceptible, but the red and black oak group are especially so. Minor infections cause little harm, but midsummer defoliation can occur when infections are severe. Blister-like patches appear on the leaves. They are often lighter green than the surrounding tissue and later turn brown. The blister-like patches result from overgrowth of infected leaf tissue, which is caused by substances secreted by the fungus.
The fungus survives the winter as spores in buds. The spores germinate in the spring to infect the leaves. Infected leaf tissue grows much faster than uninfected tissue, resulting in the distorted blisters. The fungus grows and produces spores within the leaf, until the cuticle (surface wax layer) is ruptured by the mass of fungal tissue. Spore dispersal occurs in the fall.
Prevention & Treatment: Leaf blister is rarely severe enough to require control measures. Once infection has occurred, fungicide treatments usually are ineffective. If infection is very heavy and if the tree is small enough to obtain good coverage, a fungicide could be applied in the spring just before bud break.
Browning of edges of leaves caused by bacterial leaf scorch.
Edward L. Barnard, Florida Department of Agriculture and Consumer Services, www.forestryimages.org
Bacterial Leaf Scorch: This disease is caused by the bacterium Xylella fastidiosa and is characterized by a disruption of water movement, decline of vigor, marginal reddening or yellowing followed by browning of margins of leaves, leaf drop, dieback, and eventual death. The symptoms usually first appear in early fall, following summer drought. Because trees infected with bacterial leaf scorch will decline gradually, it may take five to ten years before a tree may need to be removed. This disease may be observed more commonly in pin oak, red oak, white oak, bur oak, shingle oak and sycamore. It is occasionally found in red, silver and sugar maples, hackberry, elm and sweetgum.
Prevention & Treatment: Remove infected trees and replant with several different species. Avoid planting all the same species close together. The bacterium is spread by leafhoppers and spittlebugs. Control weeds and wild plants, which support these insects. Extra care to fertilize and irrigate may prolong the life of an infected tree, but trees with extensive leaf scorch and dieback should be removed. Currently, spray treatments are not available. However, certified arborists can perform annual root flare injections of antibiotic treatments, using oxytetracycline (such as Bacastat), which can reduce symptoms by suppressing the pathogen.
Actinopelte Leaf Spot: This fungal disease may be a serious problem in wet weather. It is caused by the fungus Tubakia dryina (formerly called Actinopelte dryina). The symptoms are circular, dark to reddish brown leaf spots with a diameter of ¼ to ½ inch. Spots may run together to form irregular blotches. Tiny black specks in rings are visible in the spots and blotches. Severe infections cause the trees to loose their leaves prematurely. Trees of low vigor, that are repeatedly defoliated, may die.
Prevention & Treatment: Destruction of all infected plant material will reduce the spread of the fungus. Small trees defoliated several years in a row may need spraying. Apply mancozeb, chlorothalonil, thiophanate-methyl or a copper fungicide according to the instructions on the labels. Complete coverage is necessary for control. See Table 1 for examples of specific products.
White coating on leaf caused by powdery mildew.
Petr Kapitola, State Phytosanitary Administration, www.forestryimages.org
Powdery Mildew: This disease is caused by several fungi (Erysiphe trina, Microsphaera alni, Phyllactinia corylea and/or Sphaerotheca lanestris). The symptoms consist of a white, powdery growth on both leaf surfaces. The foliage may be malformed, dropping prematurely or drying out and shriveling. Sometimes the grayish-white fungal growth changes to tan and then brown with age. Tiny black dots (fruiting bodies) may be seen in the brown felt, abundantly in some years, rarely in others.
Prevention & Treatment: Control is usually not practical nor warranted. Myclobutanil, triadimefon or thiophanate-methyl may be used to control powdery mildew in severe cases. Follow the directions on the label. See Table 1 for examples of specific products.
Armillaria root rot mushrooms near infected oak tree.
Joey Williamson, ©2013 HGIC, Clemson Extension.
Armillaria Root Rot: This disease, caused by the fungus Armillaria mellea, is widespread on oak. The symptoms are a slow decline of the tree. Mushroom fruiting structures on or near the root collar are prevalent in late summer and fall, especially during wet weather.
Prevention & Treatment: Remove diseased trees and as much of the root system as possible. Do not replant the site with a susceptible host. Trees which are resistant or tolerant to Armillaria root rot include baldcypress (Taxodium distichum), Chinese elm (Ulmus parvifolia), dawn redwood (Metasequoia glyptostroboides), eucalyptus, ginkgo (Ginkgo biloba), Japanese cedar (Cryptomeria japonica), Leyland cypress (Cupressocyparis leylandii), maple (Acer spp.), sourwood (Oxydendrum arboreum) and sweetgum (Liquidambar styraciflua).
Asexual brown fungal stroma and black sexual stage of Hypoxylon on oak.
Meg Williamson, Plant and Pest Diagnostic Clinic, Clemson University
Hypoxylon canker: This is a white-rot fungal disease primarily of oaks, hickories and pecans in South Carolina, and is caused by Biscogniauxia atropunctata var. atropunctata (syn. Hypoxylon atropunctatum). Many species of oaks are susceptible to this disease, but post oak, water oak, southern red oak, white oak and blackjack oak are most often affected. This opportunistic pathogen is a common inhabitant of bark of hardwood trees, but it is only of consequence when the trees are under severe stress. Environmental stress caused by drought, as well as by root injury during construction, utility trenching in the root zone, soil grade changes, soil compaction and root diseases, all can play a role in weakening the trees and in the subsequent infection by B. atropunctata var. atropunctata. Any root injury will reduce water uptake by trees, and drought stress appears to be the most significant factor in infection.
As the fungus spreads and forms cankers, the first symptom that may be observed is the dying back of the crown (top) of the infected tree. However, other tree problems may also result in dieback. Subsequently, the outer bark begins to slough off in areas of infection, and pieces of bark can be seen at the base of the tree. This bark loss exposes the first sign of the fungus, which is a brownish fungal stroma where conidia (or asexual spores) of the pathogen are produced. This area may be several inches to several feet long on limbs and trunks. These conidia are wind-disseminated and can cause new infections on other trees.
As the infection continues to develop, the exposed area of fungal stroma changes to a gray or silver color, and finally to black as a second type of spore is produced. This is the sexual stage of the fungus, and these spores, which are also infectious, are spread by splashing rain or insects to nearby trees.
Prevention & Treatment: There are no controls for Hypoxylon canker on these hardwood trees once infection has begun on the trunk. If infection is observed on branches, these may be removed and burned, but there may be other infection sites that are not yet apparent on the tree.
Stress reduction is the key to prevent infection. Keep the trees as healthy as possible.
- Protect trees from damage during home construction and utility repairs. For more information, see HGIC 1002, Protecting Trees During Construction.
- Water trees during periods of summer drought with 1 inch of irrigation water per week. For more information on proper irrigation, see HGIC 1056, Watering Shrubs & Trees.
- Trees should be mulched with a 3-inch layer of organic mulch from the trunk to the dripline, but don’t pile mulch against the trunk. For more information on mulching trees, see HGIC 1604, Mulch.
- Do not apply weed killers near the tree, especially beneath the limb canopy.
- Fertilize trees with slow-release tree and shrub fertilizer during early April. For more information on proper fertilization, see HGIC 1000, Fertilizing Trees & Shrubs.
- Remove and burn or dispose of any infected trees, and cut the remaining stumps flush with the soil.
Pine-oak gall rust spore containing pustules on an oak leaf.
Robert L. Anderson, USDA Forest Service, www.forestryimages.org
Pine-Oak Gall Rust (Eastern Gall Rust): The fungus Cronartium quercuum causes gall rust on approximately 25 to 30 species of pine and oak. Infection of oak causes small brown or yellowing areas on the leaves. On the underside of the leaves, yellow to orange powder (spores) is visible.
Prevention & Treatment: All fungal spores, which infect both pine and oak, are primarily windborne. High humidity increases the incidence of infection. Chemical control is usually not warranted. The fungicide myclobutanil may be applied to oaks according to the directions on the label. Follow the directions on the label. See Table 1 for examples of specific products.
Mistletoe infestation becomes obvious during winter.
Randy Cyr, GREENTREE Technologies, www.forestryimages.org
If, after all the leaves have dropped in the fall, your trees still have clumps of green in the top, your trees are parasitized by mistletoe (Phoradendron species), a parasitic plant. Although mistletoe does obtain water and minerals from the tree, it does not depend totally on the tree for food. The green leaves of this plant contain chlorophyll and are capable of making their own food. Mistletoe produces small white berries, which are extremely toxic to humans. The stems and leaves are also toxic and are reported to cause skin irritation on contact in some people.
Prevention & Treatment: Mistletoe can be controlled by cutting out infected limbs 1 to 2 feet below the point of attachment. In a few instances, breaking out the tops of the mistletoe has proven an effective means of control. Ethephon (Florel Brand Fruit Eliminator) is labeled for mistletoe control. Large infestations may be difficult to control with a single spray application, and retreatment may be required. Make applications after fall leaf drop through mid-winter.
A lichen is an unusual organism composed of a fungus and an alga living together symbiotically. The alga converts sunlight and carbon dioxide in air to food. The fungus surrounds the alga, protecting it from drying, and lives off the food it provides. Lichens appear as green to gray-green leafy or crusty growths on the trunk and branches of trees in poor health. They are totally harmless and are in no way responsible for the poor health of the tree. The reason they are associated with declining plants is that as woody plants loose vigor and decline, the number and size of leaves decreases. This allows more sunlight, which lichens need to grow, to reach the trunk and branches.
Prevention & Treatment: If plant health is restored by correcting the real cause of decline, leaves will increase in size and number; less sunlight will get to the trunk and limbs; and lichens will gradually disappear.
Heavy growth of Spanish moss weighting live oak limbs in rain.
Karen Russ, ©2008 HGIC, Clemson Extension
Spanish moss (Tillandsia usneoides) is an epiphyte. An epiphyte is an organism that lives upon a plant, using only the plant for support and protection. Spanish moss does not feed directly on the tree but obtains its water and nutrients from the air and rain. Spanish moss is limited to warm, humid areas of the southern and coastal regions of the state. Each bundle of moss is made up of a mass of long, gray-green filaments, which are its stems and leaves. Since the leaves of Spanish moss require sunlight to produce their own food, it is usually found in trees that are in a state of decline. Heavy infestations of Spanish moss can lead to further tree decline by shading out lower leaves and the weight of large masses of wet Spanish moss can lead to limb breakage.
Prevention & Treatment: Increasing tree vigor through proper watering and fertilization is one way to restrict the growth of Spanish moss. Removal by hand may also be necessary to rid the tree completely.
Insects & Related Pests
Large oak apple gall, one of many types of oak galls.
James Solomon, USDA Forest Service, www.forestryimages.org
Galls: There are at least 750 different galls that have been identified on oak. In fact, more galls occur on oak than on any other kind of plant. Galls are defined as irregular growths or swellings. They vary greatly in size, shape and their location on the plant. Gall development is a reaction by the plant tissue to feeding or egg laying by various mites and insects. While most galls do not seriously harm oak trees, most are unsightly and detract from the beauty of the tree. Twig galls may kill individual limbs and sometimes the whole tree.
Control: Many gall-producing insects and mites are parasitized by other insects and are fed upon by various birds and animals. Simple removal and destruction of fallen leaves with galls will help to reduce the number of emerging adults that will produce the next generation. Where possible, all twig galls should be pruned out while green or before emergence holes appear. In most cases, chemical control is not practical or effective. This is especially true in the case of large trees.
Orangestriped oakworm in mid to late summer.
USDA Forest Service – Northeastern Area Archive, USDA Forest Service, www.forestryimages.org
Oakworms: There are three closely related moths (Anisota senatoria, A. stigma and A. virginiensis) that occur in South Carolina. Their larvae (or caterpillars) are pests of oak, feeding heavily on the leaves. The orangestriped oakworm (Anisota senatoria) is the most commonly occurring. At maturity, it is about 2 3/16 inches (5.5 cm) in length and has two long slender black ‘horns’ that project from the second segment behind the head. In the fall, it crawls to the ground and burrows into the soil. It overwinters (survives the winter) in the soil and matures to an adult (moth). The moths appear in June and July. The female moths lay clusters of eggs on the undersurfaces of oak leaves. When the larvae hatch, they are small and greenish yellow. When small, the caterpillars typically feed in groups and eat all of the leaf except a lacy network of veins. Orangestriped caterpillars mature and reach their full size by early fall. Older caterpillars are black with yellow or orange stripes running the length of their bodies. Older caterpillars tend to be solitary eaters. They eat all of the leaf but the main vein.
As a result of the caterpillar feeding, small trees may lose all their leaves by midsummer. While healthy trees can tolerate feeding by oakworms, young trees may be weakened if they lose all their leaves several years in a row. Mature trees may lose enough leaves to suffer twig dieback as a result of sunscald damage.
Control: With a light infestation on young trees, caterpillars can be handpicked and destroyed. On large trees, control is more difficult. Where possible, rely on birds, parasites and diseases to control their numbers naturally. If chemical control becomes necessary, the following insecticides are recommended: permethrin, cyfluthrin, lambda cyhalothrin, bifenthrin, acephate, and spinosad. Bacillus thuringiensis (B.t.) is a microbial insecticide that contains spores of this bacterium. It is effective against young larvae, and is a safer alternative when spraying up into a tree. Follow the directions on the label. See Table 1 for examples of specific products.
Scale: Various kinds of scale are pests of oak. Scales are unusual insects in appearance. As adults, they are small and immobile, with no visible legs. They vary in appearance depending on age, sex and species. Scales feed on sap by piercing the leaf or stem with their mouthparts and sucking. As they feed on plant sap, some scale insects (soft scale) excrete a sugary substance called honeydew. The sooty mold fungus feeds on the honeydew, resulting in unsightly, dark fungal growth.
Oak lecanium scale on small twigs.
James Solomon, USDA Forest Service, www.forestryimages.org
Oak lecanium scale (Parthenolecanium quercifex) is a common pest on oaks. The adult females are 1/16 to ¼ inch (2 to 6 mm) in diameter. They are round and reddish brown. Males have wings and are brown. The female lays eggs in April and May. Crawlers (immature scale insects) hatch from the eggs. The crawlers also suck sap from leaves. Serious scale infestations may result in stunted plant growth, small flowers, yellowing of leaves and early leaf drop. In addition, the presence of the honeydew results in dark splotches on the surface of the leaves.
Control: A combination of various natural enemies, including ladybird beetles (ladybugs) and parasitic wasps, usually keep scales under control. In small trees with light infestation, scale can be scraped off or infested branches can be removed and destroyed. In a large tree, controlling scale chemically is not always practical. The size of the tree, the need for specialized equipment and the cost may prohibit this solution.
Scales are not easily controlled with chemical insecticides. The adults are protected by their waxy covering. For heavy infestations of scale insects, spray with horticultural oil in the spring and fall to kill adults, crawlers and eggs by smothering them. Be sure to thoroughly coat the trunk and all of the branches.
Crawlers are susceptible to conventional insecticides, however. Monitor the crawler emergence with sticky cards, double-faced tape wrapped around a branch, or by putting an infested shoot or leaf into a baggie and watching for crawler movement. Insecticides labeled for use by homeowners against scale crawlers on oaks include cyfluthrin, permethrin, bifenthrin, lambda cyhalothrin, and malathion. Apply one of these materials when crawlers appear and repeat in 10 days. As with all pesticides, read and follow all label instructions and precautions. See Table 1 for examples of specific products.
Note: Chemical control of diseases and insects on large trees is usually not feasible since adequate coverage of the foliage with a pesticide cannot be achieved. The use of horticultural oil is a safer alternative to insecticides for spraying upward into a large tree.
Table 1. Insecticides & Fungicides to Control Oak Insect Pests & Diseases.
|Insecticides & Fungicides||Examples of Brand Names & Products|
|Acephate||Bonide Systemic Insect Control Concentrate|
|Bacillus thuringiensis (B.t.)||Bonide Thuricide Bt Concentrate
Natural Guard Caterpillar Killer Spray with Bt Conc.
Safer Caterpillar Killer with Bt Concentrate
Southern Ag Thuricide Bt Caterpillar Control Concentrate
Tiger Brand Worm Killer Concentrate
|Bifenthrin||Bifen I/T Concentrate
Ferti-lome Broad Spectrum Insecticide Concentrate
Hi-Yield Bug Blaster Bifenthrin 2.4 Concentrate
Ortho Bug-B-Gon Insect Killer for Lawns & Gardens Concentrate
TalStar P Concentrate
Up-Star Gold Insecticide Concentrate
|Chlorothalonil||Bonide Fung-onil Multi-Purpose Fungicide
Ferti-lome Broad Spectrum Landscape & Garden Fungicide
GardenTech Daconil Fungicide Concentrate
Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide
Monterey Fruit Tree, Vegetable & Ornamental Fungicide
Ortho Max Garden Disease Control Concentrate
Southern Ag Liquid Ornamental & Vegetable Fungicide
Tiger Brand Daconil
|Copper-based Fungicides||Bonide Liquid Copper Concentrate
Camelot O Fungicide/ Bactericide Concentrate
Monterey Liqui-Cop Concentrate; & RTS1
Southern Ag Liquid Copper Fungicide
Natural Guard Copper Soap Liquid Fungicide Concentrate
|Cyfluthrin||Bayer BioAdvanced Vegetable & Garden Insect Spray Conc.; & RTS1|
|Horticultural Oil||Bonide All Seasons Spray Oil Concentrate
Ferti-lome Horticultural Oil Spray Concentrate
Monterey Horticultural Oil Concentrate
Southern Ag Parafine Horticultural Oil
Summit Year Round Spray Oil Concentrate
|Lambda Cyhalothrin||Spectracide Triazicide Insect Killer for Lawns & Landscapes Concentrate; & RTS1
Martin’s Cyonara Lawn & Garden Concentrate
|Malathion||Bonide Malathion Concentrate
Hi-Yield 55% Malathion Insect Spray
Gordon’s Malathion 50% Spray Concentrate
Martin’s Malathion 57% Concentrate
Ortho Malathion Plus Insect Spray Concentrate
Southern Ag Malathion 50% EC
Spectracide Malathion Insect Spray Concentrate
Tiger Brand 50% Malathion Concentrate
|Mancozeb||Bonide Mancozeb Flowable with Zinc Concentrate
Southern Ag Dithane M-45
|Myclobutanil||Spectracide Immunox Multi-Purpose Fungicide Concentrate
Ferti-lome F-Stop Lawn & Garden Fungicide
|Permethrin||Bonide Eight Insect Control Vegetable Fruit & Flower Concentrate
Bonide TOTAL Pest Control – Outdoor Concentrate
Hi-Yield Indoor/ Outdoor Broad Use Insecticide Concentrate
Tiger Brand Super 10 Concentrate
|Propiconazole||Banner Maxx Fungicide
Bonide Infuse Systemic Disease Control Concentrate
Bonide Eight Yard & Garden Ready to Spray (RTS1)
Ferti-lome Liquid Systemic Fungicide II Concentrate
|Spinosad||Bonide Captain Jack’s Deadbug Brew Concentrate; & RTS1
Bonide Colorado Potato Beetle Beater Concentrate
Ferti-lome Borer, Bagworm & Leafminer Spray Concentrate
Monterey Garden Insect Spray Concentrate
Natural Guard Spinosad Landscape & Garden Insecticide RTS1
Southern AG Conserve Naturalyte Insect Control Concentrate
Dow Conserve SC Turf & Ornamental Concentrate
Ortho Insect Killer Tree & Shrub Concentrate
|Thiophanate-methyl||Cleary’s 3336-WP Turf & Ornamental Fungicide
Southern Ag Thiomyl Systemic Fungicide
|1RTS = Ready to spray (hose-end applicator)|
Bur oak blight
Bur oak blight (BOB) is a leaf blight caused by the fungus Tubakia iowensis. It is a disease of concern to anyone who has bur oak trees on their property. BOB is most prevalent on Quercus macrocarpa var. oliviformis, an upland variety of bur oak characterized by its relatively small, olive-shaped acorns. Susceptibility of bur oaks to BOB is highly variable, and trees of all sizes are at risk. Occasionally, BOB can infect swamp white oak (Q. bicolor); it is unknown whether hybrids of bur oaks are susceptible. Repetitive defoliation of trees by BOB increases susceptibility to secondary problems such as the two-lined chestnut borer (Agrilus bilineatus) and Armillaria root rot, which ultimately cause tree decline and death.
Symptoms of BOB become visible during mid-July with the presence of small circular leaf spots displaying colors of reds, purples, and browns. Next, the major leaf veins start dying and display similar colors (Fig. 1). Small, black fruiting bodies, which contain fungal spores become visible along the undersides of these veins.
Fig. 1 Vein discoloration due to bur oak blight. Photo: Dr. Tom Harrington and Doug McNew, Iowa State UniversityFig. 2 Chlorotic-dead areas on leaf due to bur oak blight. Photo: Dr. Tom Harrington and Doug McNew, Iowa State University
As the disease progresses, wedge-shaped chlorotic-dead areas (Fig. 2) of the leaf become prominent and the bases of the petioles die and turn brown. In late August to early September, the dying petioles turn entirely brown, become swollen at the base, and develop black pimple-like fruiting bodies on the surface that are visible with the naked eye (Fig. 3 and 4). The presence of these fruiting bodies on the petiole is essential to the confirmation of BOB. Dead leaves and/or dead petioles will remain attached to the stems throughout the winter.
Fig. 3 Black pustules are seen on the petiole (center of photo). Photo: Dr. Tom Harrington and Doug McNew, Iowa State UniversityFig. 4 Close up of black pustules on petiole. Photo: Dr. Tom Harrington and Doug McNew, Iowa State University
The disease starts in the lower, inner sections of the canopy and spreads upward and outward as it progresses.
Fruiting bodies on dead petioles from the previous season release spores during the spring and this is considered the primary infection. The spores are spread by splashing rain and infect the newly-emerged foliage. The intensity of the disease within a season is dependent on spring moisture. Although infection occurs in spring, leaves will appear healthy until mid-to-late July. The foliar symptoms progress throughout the month of August and early September. Secondary infections will occur throughout the season from spores developed under leaf veins during wet periods. Fruiting bodies are produced on dying petioles in late summer. Entire leaves, or just petioles, will remain attached to the stems until the following spring and repeat the cycle.
Cultural – There are no promising cultural methods to manage BOB. Removing leaf litter under the tree will not reduce the infection rate the following year. Plucking off petioles is likely not an effective method because the fungus can survive within the stems and it is not practical on large trees.
Chemical control – A fungicide treatment in spring after full leaf expansion is the most effective method for managing BOB. Results from studies conducted at Iowa State University showed injections of propiconazole (Alamo formulation) reduced symptoms in the fall and in the following year. Repeat applications are not recommended until the infection becomes severe in later years. Fall applications of fungicides are not recommended for BOB.
Amid this year’s titanic political and social upheavals, the town of Basking Ridge, New Jersey, found time for quiet mourning. One of the East Coast’s most famous trees, a centuries-old great white oak that spread its serpentine limbs over a cemetery beside the Basking Ridge Presbyterian Church, was dying. In October, the tree was pronounced dead; a month later, distraught residents held a memorial service.
To mourn a tree as if it were human is understandable, even natural. There are ways, after all, that trees really are like us. We appear to share a basic body plan, with thick structures stabilising us on the ground and thinner limbs reaching into space. Inspired by recent research into plant communication and nutrient sharing through underground root networks, the ecologist Peter Wohlleben in The Hidden Life of Trees (2016) even writes about trees as familial and socially networked. Tree-like characters also appear in mythology, from Daphne (whose arboreal transformation helped her escape the amorous Apollo) to the forest-dwelling Ents in The Lord of the Rings.
But when we give trees our own attributes, we should realise that we are misleading ourselves. We are seeing the tree not as a tree but, essentially, as a person. The great naturalist John Muir chastised his contemporaries for assuming that trees would ‘bemoan their imprisonment rooted in the ground’. In the nearly 13 decades since those words were written, there has been no shortage of tree anthropomorphising, but it has shed little light on actual trees. Rather, we have blinded ourselves to more profound realities that could help us better appreciate our own place in the natural world.
Trees are indeed like us in some ways: they grow, they respire, they metabolise, they pass on genetic information via DNA. But in just as many ways they are not like us. They cannot move from the place where they take root, and must get everything they need to live under this constraint. They make their own food from sunlight, carbon dioxide and water. They respond to their environment on vastly slower time scales. Most significantly, I would argue, they do not age like us or die like us.
Every old tree sheds parts of itself over the years. The Basking Ridge oak, for example, lost limbs and part of its interior, and eventually had to be held together with cement and cables. Still, year after year, it made sugar in its leaves and drank water from the ground. In his book Wild Trees (2007), the writer Richard Preston describes California redwoods whose tops broke off, perhaps centuries ago. Yet these redwoods live on, too, incubating entire ecosystems in their crowns 300 feet above the ground.
For humans, ageing invariably leads to loss of function and eventual death. When we lose a limb, it’s a major loss. In the terrible event that we lose a head, it’s game over. For trees, it is almost the opposite: the older they get, the better they get at being trees. The rate at which they sequester carbon increases each year, and the amount of life they can sustain increases proportionally. Scientists know of no fundamental reason why trees must inevitably die, and many times one or more genetically identical ‘scions’ grow where a mature tree once stood. There is no equivalent reincarnation in the human world.
Even when a tree dies, it continues to give life. First it nourishes insects that bore into its wood and birds that eat them. Eventually it falls, rots, and becomes a seed bed for new trees and plants, turning into a long, linear mound, whose original identity can be divined only by an astute observer. ‘A tree has half its life as a live tree, and then a second life as a rotting log,’ is how University of Wisconsin-Madison ecologist Don Waller put it to me. In other words, as long as you’re in a forest, there’s no need to mourn a fallen tree.
When a loved one passes, we grieve because we recognise that even among the 7.4 billion living today, or all who will be born in the future, the person we loved is unique. No one else will have the exact same genes and epigenome and, more to the point, none will have the exact same mix of personality quirks, intelligences and talents. Projecting that same individuality onto a tree risks veering into imaginary nonsense, however. Trees become notable precisely when their ecological context has been removed – when they are not fully their natural selves.
What made the Basking Ridge oak a celebrity was its age, its shape, and its intersection with human history. The surrounding land was cleared by colonial settlers, allowing the tree to grow out instead of up, and develop a Medusa-esque arrangement of curving limbs that made it identifiable in a photo, even stripped of locational cues. Sitting beside a church established in 1717, it served as a preaching site during the Great Awakening, sheltered George Washington and Marquis de Lafayette along with their soldiers, and eventually came to define a town.
Yet from an ecological point of view, the Basking Ridge oak led a rather poor life. It surely did some of the common things trees do, such as produce seed and shelter birds and mammals, but it was not able to fully share nutrients with other trees below ground. Since it is surrounded by human structures that need to be protected from falling limbs, it will be deprived of its second life as well. Instead, it will be carved up and removed, never to nurture the next generation.
The human concept of individuality, so strongly associated with the Basking Ridge oak, has far less meaning in a tree’s proper home. Trees in a temperate forest typically live among hundreds or thousands of members of the same species; they might vary slightly genetically but they are identical functionally. A grove of almost 50,000 quaking aspens in Utah appears to share a single root system and is regarded as a single colony, perhaps tens of thousands of years old, possibly much older. The loss of any one of those aspens is trivial, like clipping a fingernail.
As members of a species that hopes to thrive well into the future, we might take a lesson from the ecological success that trees have achieved by choosing the communal, rather than the individual, life. Despite the extensive deforestation of the US by the mid-20th century, very few of its hundreds of native tree species have been lost in the wild. Globally, forests still claim around 30 per cent of Earth’s land area. When a farm or lot is abandoned, except in extremely cold or dry areas, trees are what come back – and fast. If through global warming, nuclear war or other means we ever make Earth uninhabitable for us, trees will still be here.
So feel free to mourn the Basking Ridge oak, or any other beloved tree, as a loss – a loss of shade, landmark, symbol of strength and endurance, or simple beauty. But don’t worry about the loss of life. To the Basking Ridge oak, that would be meaningless.
Oak Wilt Destruction near Dripping Springs, TX
Oak Wilt is Widespread
Oak Wilt is one of the most destructive tree diseases in the United States and is found in 25 states. In Central Texas, and especially the Hill Country, our vast forest of Live Oaks is being devastated, impacting 65 counties in Texas. Oak Wilt is caused by a fungus with the scientific name of Bretziella fagacearum (previously known as Ceratocystis fagacearum). This fungus infects our Oaks and colonizes the water conducting tissues. The reaction of the tree to this infection results in the blockage of these water conducting tissues; ultimately resulting in severe die-back or tree death.
Oak Wilt Spreads Above and Below Ground
The disease spreads in two ways; via sap feeding insects (Nitidulid beetles) carrying spores on their bodies (overland transmission) and through interconnected roots among trees (root transmission). Other methods of transmission may occur but have not been scientifically proven.
Overland transmission of Oak Wilt begins with an infected red Oak (Shumard Oak, Spanish Oak, Blackjack Oak, Texas Red Oak, etc.). When a red Oak dies, one or more fungal spore mats may form under the bark. The mat grows and expands causing the bark to crack open. The spore mat emits an odor that attracts Nitidulid beetles. The beetles enter the spore mat to feed and spores stick to the insect. These beetles then travel to other trees to feed on the sap from a fresh wound. The Oak Wilt spores may then infect that tree, starting a new disease center. Experiments have shown that under their own power these insects can travel a mile or more and may appear on fresh wounds in 15 minutes or less.
Root transmission is largely a problem for Live Oaks. The root system of one Live Oak is highly interconnected to neighboring Live Oaks. The fungus travels through the roots from one Live Oak to the next. The disease can spread in this manner from an infected tree at a rate of 75 to 100 feet per year on average. Due to the high concentration of Live Oaks in Central Texas, root system transmission destroys large areas of Live Oak forest. Spore mats do not form on Live Oaks as they do on red Oaks. Contrary to popular belief, simply removing dead trees either by cutting them down or bulldozing them, does not eliminate the root transmission of the disease. Treatment of trees does not prevent root transmission either. The only way to prevent root transmission is to completely sever root connections between healthy and sick trees.
Identification of Oak Wilt
Confirmation that Oak Wilt is present in a particular location might be challenging. However, a Certified Arborist that specializes in Oak Wilt can usually make a diagnosis in a fairly short period of time. A professional will make sure that other factors such as chemical poisoning, drought, construction damage and lightning strikes are not misdiagnosed. There are four primary ways to identify Oak Wilt; foliar (leaf) symptoms, patterns of tree mortality, fungal mats and laboratory tests.
The prominent symptom most commonly associated with Oak Wilt is the distinctive browning out of the veins in Live Oak leaves. This pattern that often reminds people of “fish bones” is called veinal necrosis. The veins in the leaf become yellow or brown and the leaf soon falls off the tree. Even after drying out and turning completely brown, the symptomatic leaf retains the distinctive darker veins. Unfortunately, veinal necrosis does not occur in red Oaks. Trees in this family defoliate in a manner similar to autumn with the leaves turning brown from the edges in and then falling off. Defoliation of a red Oak at an odd time of the year should always be investigated. In addition to veinal necrosis in Live Oaks, there are other types of foliar symptoms including interveinal necrosis (the reverse of veinal necrosis) or veinal banding, tip burn (where the end of the leaf turns brown) and margin burn (where the entire edge of the leaf turns brown). Major defoliation (leaf loss) of a Live Oak other than during spring (when they normally replace their leaves) should be investigated.
Common Causes of Oak Mortality
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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In this part of the world, we have oak trees. Technically they are called live oaks – but I don’t get it. Of course they are alive. I was at a soccer game and this is the tree I always look at.
Image: Rhett Allain
Look how far those limbs extend horizontally. That branch is about 12 meters long. Why is this amazing? Have you ever tried to hold an 8 foot 2 x 4 board horizontally by holding one end? Pretty tough. How about I calculate the forces needed to hold that branch in place? I will do a simple model and then maybe later I can make it more complicated. Suppose I replace that limb with one straight uniform limb that looks like this:
In this replacement limb, I am going to say it is a cylinder that is 9 meters long and 30 cm in diameter. Let me assume that this limb is connected at two points to the tree (the white dots). So, for this limb to stay there, the following must be true:
The first two equations say that the total force must be zero. The last one says that the torque about any point must be zero (since it is in rotational equilibrium about any point). First for the forces. There is the gravitational force. This pulls on all parts of the limb, but I can represent this as one force pulling on the limb at the center of mass (long ago, I said I would explicitly show this – but I haven’t yet). Then there are two other forces. Let me pretend like there are two pins that hold the limb to the tree. Each of these pins can exert a force in the vertical and horizontal direction I will call these F1-y F2-y etc…where the top pin will be 1. That is 5 forces.
For these forces and the first two equations, I get:
So, already I have some constraints. The horizontal components of the forces from the two pins must be equal and opposite. The vertical components of the forces from the pins have to add up to the weight of the log. Now for the torque, I am going to add up the torques about the lower pin. Let me draw a distorted view of the log so that the important distances can be seen.
What is torque? Torque is like a rotational force. Here is an example, what if you try to open a door by pushing near the hinges? It is much harder than pushing near the handle, right? When rotating about some axis, the torque is:
Here F is the applied force, r is the distance from the point where the force is applied to the axis. Theta is the angle between F and r. I will call torques that would make a rotation counterclockwise positive (really, torque is a vector). So, what is the some of torques about axis O (that passes through point O)? First, there are some forces that have zero torque. Both of the vertical pin forces have either theta = 0 or r = 0 so that the torque is zero. The same is true for the horizontal force on the bottom pin. This just leave two forces that have non-zero torques:
Now that I have the horizontal force on the top pin, the bottom pin has the same value (but in the opposite direction). I don’t have an expression for the two vertical pin forces. Let me just say that each has a force equal to have the weight of the limb.
How about some values? First, I need the mass of the limb. If this is a cylinder of wood (with a density rho) then the mass is:
So the magnitude of the two horizontal forces on the pins would be:
If I use my values from above and an estimation of the density of wood, I get:
Wow. Oh, I know I made some estimations but even 50,000 Newtons would be huge. Impressive, most impressive. I salute you mighty tree.
Chronic oak dieback
A complex disorder or syndrome in which several damaging agents interact and bring about a serious decline in tree condition.
This resource will help you to identify trees suffering from chronic oak dieback and decline.
British oaks have been affected by a condition now known as chronic oak dieback or decline for much of the past century. Pedunculate or English oak (Quercus robur) tends to be the most commonly affected.
This disorder is widespread, prolonged and complex. The causes of the condition often involve abiotic factors for example poor soils, recurrent drought, high winds, disturbed environments and air pollutants. Biotic agents that are involved in the causes of this disease include insects and fungi that are destructive to weakened trees.
Predisposition to the disease is considered a very important aspect. Current specific evidence of the factors involved and how they affect the tree to make it susceptible to biotic agents is insufficient. Current research is addressing this so that a better understanding can be obtained and lead to management of the disease.
This resource will help you to identify trees suffering from chronic oak dieback and decline.
- Most commonly affects English or pedunculate oak (Quercus robur)
- Caused by the complex and combined interaction of damaging abiotic and biotic agents including soil compaction, low soil moisture holding capacity, poor soil drainage, poor fertility high winds, recurrent drought and opportunistic attack from insects and diseases on weakened trees
- Also known as oak decline and dieback-decline
Chronic oak dieback is different to acute oak decline (AOD), which causes dark, weeping patches on the tree stems. Trees with AOD deteriorate rapidly and may die within four or five years.
Incidence and distribution
British oaks have been affected by dieback for much of the past century, caused by a complex combination of abiotic and biotic agents.
The most serious recent episode of chronic dieback occurred in 1989–1994, when drought damage weakened trees which were then attacked by the two-spotted oak buprestid beetle, Agrilus biguttatus, frequently leading to tree death.
Reported cases of oak decline are widely distributed. Since 2002 there has been a significant increase in reported sites with rapid and severe decline: these are cases of acute oak decline.
- Early foliage deterioration
- Progressive death of branches over several years
- Extensive dieback and secondary diseases in weakened trees leads to death in some cases
- Affected trees often have dead branches or dieback in the crown
You can report oak trees with symptoms of dieback or decline to our Tree Health Diagnostic and Advisory Service.
Dieback occurs in oak trees when a number of damaging agents or events combine to weaken branches or trees. They are then susceptible to opportunistic attack from insects and diseases, or dieback caused by physiological stress or physical damage. See the Manion disease spiral model.
Forest Research has studied the role of different agents in oak dieback.
We are currently using a multidisciplinary holistic approach to investigating the environmental factors that weaken trees or predispose trees making them more susceptible to Chronic Oak Decline.
In some cases of Chronic Oak Decline certain biotic agents can play a dominant role, for example Armillaria. We are studying the role of Armillaria in oak declines, and the interactions between the various biotic and abiotic agents involved in this disease.
The definition and concept of Chronic Oak Decline are still evolving and the relationship between Acute Oak Decline and Chronic Oak Decline needs resolution.
More about different forms of oak decline and dieback
- Oak declines – New definitions and new episodes in Britain
- Dieback of pedunculate oak
If you suspect a tree is affected by chronic oak dieback you can report it using TreeAlert.
We can also help you identify diseased trees and provide advice on prevention and management. Please contact our Tree Health Diagnostic and Advisory Service .
Common Oak Trees: Oak Tree Identification Guide For Gardeners
Image by ollirg
Oaks (Quercus) come in many sizes and shapes, and you’ll even find a few evergreens in the mix. Whether you are looking for the perfect tree for your landscape or want to learn to identify the different types of oak trees, this article can help.
Oak Tree Varieties
There are dozens of oak tree varieties in North America. The varieties are divided into two main categories: red oaks and white oaks.
Red oak trees
Reds have leaves with pointed lobes tipped with tiny bristles. Their acorns take two years to mature, and sprout the spring after they drop to the ground. Common red oaks include:
White oak trees
The leaves on white oak trees are rounded and smooth. Their acorns mature in one year, and they sprout soon after they fall to the ground. This group includes:
Most Common Oak Trees
Below is a list of oak tree types that are the most commonly planted. You’ll find that most oaks are massive in size and not suitable for urban or suburban landscapes.
- White Oak Tree (Q. alba) – Not to be confused with the group of oaks called white oaks, the white oak tree grows very slowly. After 10 to 12 years, the tree will stand only 10 to 15 feet tall, but it will eventually reach a height of 50 to 100 feet. You shouldn’t plant it near sidewalks or patios because the trunk flairs at the base. It doesn’t like to be disturbed, so plant it in a permanent location as a very young sapling and prune it in the winter while it is dormant.
- Bur Oak (Q. macrocarpa) – Another massive shade tree, the bur oak grows 70 to 80 feet tall. It has an unusual branch structure and deeply furrowed bark that combine to keep the tree interesting in winter. It grows further north and west than other white oak types.
- Willow Oak (Q. phellos) – The willow oak has thin, straight leaves similar to those of a willow tree. It grows 60 to 75 feet tall. The acorns aren’t as messy as those of most other oaks. It adapts well to urban conditions, so you can use it a street tree or in a buffer area along highways. It transplants well while it is dormant.
- Japanese Evergreen Oak (Q. acuta) – The smallest of the oak trees, the Japanese evergreen grows 20 to 30 feet tall and up to 20 feet wide. It prefers the warm coastal areas of the Southeast, but it will grow inland in protected areas. It has a shrubby growth habit and works well as a lawn tree or screen. The tree provides good quality shade despite its small size.
- Pin Oak (Q. palustris) – The pin oak grows 60 to 75 feet tall with a spread of 25 to 40 feet. It has a straight trunk and a well-shaped canopy, with the upper branches growing upward and lower branches drooping down. The branches in the center of the tree are nearly horizontal. It makes a wonderful shade tree, but you may have to remove some of the lower branches to allow clearance.