New England is home to a variety of oak species that are often some of the largest and most culturally significant trees in cities and towns, providing a variety of services (e.g. carbon sequestration, particulate and storm water capture, shade, and aesthetics that increase property values). As these trees age, the damage inflicted by root and butt rot fungi can reduce their structural stability. Subsequent failure, usually during strong winds, can result in serious damage to property or people. Root and butt rot of oaks can be caused by one of many wood-decaying fungal pathogens that are native to our region. The most common and destructive of these fungi include: Armillaria spp. (honey fungus), Inonotus dryadeus (warted oak polypore), Laetiporus cincinnatus and L. sulphureus (chicken of the woods), Bondarzewia berkeleyi (Berkeley's polypore), Ganoderma sessile (reishi mushroom), G. applanatum (artist’s conk), Grifola frondosa (hen of the woods) and Meripilus sumstinei (black-staining polypore).
All oak species growing in northeastern North America are susceptible to infection. In New England, the most abundant native oak species include: red oak (Q. rubra), pin oak (Q. palustris), white oak (Quercus alba), black oak (Q. velutina), scarlet oak (Q. coccinea) and chestnut oak (Q. prinus).
Symptoms & Disease Cycle
The damage caused by root and butt rot fungi may be exposed only when trees suffer uprooting or stem failure during strong winds. In some cases, symptoms of root and butt rot are present but they provide very little information on the volume of decay in the roots and lower trunk. Symptoms of root and butt rot can be very general in nature (e.g. progressive canopy dieback, stunted shoots, undersized leaves, pale-colored to chlorotic foliage and premature fall color change) to more overt (e.g. open cavities at the base of the tree, seams or cracks with sap/resin flow and extensive flaring at the base of the trunk). Many wood-decaying fungi invade a susceptible tree through a wound on the roots or lower trunk. Additionally, they may directly parasitize small feeder roots and gradually progress into larger roots until finally reaching the lower trunk. Once established, many of these fungi decay the heartwood and do not invade the sapwood until the late stages of infection. It is hypothesized that these fungi have adapted this strategy to circumvent the active defenses of the tree, since the sapwood, cambium and phloem are aggressively defended, whereas the heartwood is non-functional and has only passive chemical defenses against pathogens.
Management of oaks infected with root and butt rot fungi can be extremely difficult. First, the occurrence of a wood-decaying fungal pathogen must be confirmed. In many cases, the only confirmation is the presence of an annual or perennial fruiting body (mushroom or conk) growing directly from the trunk or from a lateral root close to the base. Regular lawn mowing often destroys fruiting bodies growing from nearby roots before they can mature and be properly identified. In addition, there are numerous beneficial mycorrhizal fungi that may produce annual fruiting bodies at the base of oaks, creating confusion. Regular scouting from mid-July through mid-October is required to find and identify a potential pathogen. Minimizing mechanical wounds that may serve as a potential infection point is essential to limit the introduction of these fungi. These wounds may be created by lawnmowers and weed trimmers, automobiles, or careless placement of tools and equipment. Maintaining a large mulch ring around the base of landscape trees can help protect them from basal wounding.
An important point to consider is that wood-decaying fungi grow very slowly. It can take these fungi many years to decades to cause a defect large enough to cause uprooting or stem failure. The rate of decay varies with tree species, fungal pathogen and occurrence (roots or lower trunk). The presence of fruiting bodies from certain pathogens (e.g. Grifola frondosa and Ganoderma sessile) does not always indicate extensive decay is present, whereas others (e.g. Ganoderma applanatum and Laetiporus spp.) often signify extensive decay has taken place. Accurate identification is critical. Fungicides are of little use against wood-decaying fungi because the pathogen lives within the roots and/or lower trunk, often in the heartwood, making it very difficult to contact.
Symptoms and Signs of the Common Wood-Decaying Pathogens of Oak
1. Armillaria species (honey fungus)
Armillaria is widespread on the landscape and trees may be infected singly or in small groups. This fungus has a very broad host range on hardwoods and conifers, but is most frequently found on oaks and sugar maples in the landscape. Armillaria primarily causes a butt rot of landscape oaks that may advance many feet upwards in the main trunk. Symptoms of infection are often cryptic and go unnoticed. When cavities are present at the base of the tree, the presence of Armillaria is easier to detect. Infection by Armillaria results in a white rot that targets cellulose and hemicellulose at first and then lignin at later stages. Decayed wood may range in color from brown to a bleached white and is often stringy when wet but disintegrates easily. Often, irregular black zone lines are present in the decaying wood. Armillaria produces mushrooms in late summer and fall at the base of infected trees. They may appear from late August through October but in southern New England are most abundantly found in late September and early October. These gilled mushrooms grow in dense clusters, are golden tan to pale brown in color and often have a conspicuous white ring on the stem. White-colored spores are locally wind-dispersed and are capable of initiating new infection centers when they establish. The primary modes of disease spread are through rhizomorphs and root grafts (root to root contacts) between diseased and healthy trees. Rhizomorphs are root-like filaments of fungal tissue (mycelia) that are encased in a black, melanized rind. Rhizomorphs are produced from infected trees and grow through the soil until they come into contact with nearby roots or the lower trunk of a healthy tree.
2. Inonotus dryadeus (warted oak polypore, weeping conk)
Inonotus is most common on old oaks growing on unfavorable or stressful sites. The pathogen primarily targets the roots and extensive decay may be present with few aboveground symptoms. General dieback and decline of the canopy, with undersized and chlorotic foliage may be present. When decay has advanced to the primary lateral roots close to the base of the trunk, the bark and cambium will die before sapwood begins to decay and irregular patches of white mycelia may be present on the exterior of the roots. The fungus causes a mottled white rot that often does not advance very far above the soil line in the trunk. Large and highly irregular conks are produced annually on the trunk at the soil line or exposed roots. Fresh conks have a light yellow to yellow-brown upper surface while the interior is porous, reddish-brown to chestnut-brown and corky. The conks darken as they age, becoming black and develop a rough, cracked surface. The presence of multiple conks on the lower trunk or lateral roots often indicates extensive root decay. The fungus spreads through airborne dispersal of spores so trees are usually affected singly rather than in groups.
3. Laetiporus cincinnatus & L. sulphureus (chicken of the woods)
Both Laetiporus cincinnatus and L. sulphureus are primarily found on oaks in southern New England, but are able to attack other hardwoods. While the two species are very similar, distinguishing them by their fruiting bodies is possible. Laetiporus species cause a brown rot in infected trees, decaying the cellulose and hemicellulose while the lignin remains in an altered form. Brown rot leads to a dramatic reduction in bending strength and infected oaks can suffer uprooting or stem failure under strong winds. Laetiporus attacks the heartwood, meaning that extensive decay may be present with no external symptoms as the sapwood remains largely unaffected until the late stages of decay. Annual mushrooms are yellowish-orange to bright orange and appear anytime from July onward. Mushrooms produced by L. cincinnatus have a white pore layer and are typically produced in a rosette pattern. They grow directly adjacent to the trunk at the soil line or from infected roots and may be several feet away from the base of the tree. Mushrooms produced by L. sulphureus have a yellow pore layer and occur in overlapping shelves growing directly on the main trunk of infected oaks. The presence of the mushrooms frequently indicates that extensive decay is present and removal should be carefully considered.
4. Bondarzewia berkeleyi (Berkeley’s polypore)
Bondarzewia is a common and destructive pathogen of oak in the region, occurring on forest and landscape trees. It has been reported sparingly from other hardwood hosts, such as chestnut and maple. The fungus causes a white stringy rot of the heartwood in the roots and lower trunk, but typically does not extend upwards in the lower trunk beyond 3–5 feet. Because the decay is mostly restricted to the heartwood, excessive tapering at the base may be the only external symptom present. As such, serious reductions in structural stability can result from advanced infections with few observable symptoms. The fungus produces annual mushrooms consisting of one to five overlapping, cream-colored caps produced on a short central stem. The pore layer is white and typically does not bruise when pressed. At times, the fruiting bodies can be several feet in width, making them highly conspicuous when they appear. The mushrooms typically grow from infected roots a few feet away from the main trunk, but can also grow directly from the main trunk at the soil line. The mushrooms can appear anytime from July onward and may be highly distorted in appearance if the site is exposed.
5. Ganoderma sessile (previously G. lucidum; reishi mushroom)
Ganoderma sessile attacks a variety of hardwoods and can be readily found on landscape and urban oaks. A light-colored, stringy white rot develops as a result of infection of the sapwood and heartwood tissues. The roots and/or the lower trunk can be attacked by G. sessile, leading to structural instability over time. Unlike some other wood-rotting pathogens of oak, the appearance of the conk does not necessarily indicate that extensive decay is present. The pathogen may produce conks when only localized decay in the roots or lower trunk has occurred. As a result, conks may develop before any significant decline develops in the canopy. When infections are advanced, general canopy dieback symptoms can often be observed. The annual conks are soft and amorphous when young, becoming shelf-like and tough, but still pliable, as they mature. Conks occur singly or in overlapping groups, growing from lateral roots several feet away from the trunk or directly from the trunk at the base of the tree. The upper surface of the conk is reddish-brown with a bright white margin and white pore layer on the underside. The upper conk surface is also waxy giving it a varnished appearance. The conks may persist for more than one year but after freezing they appear brown to black.
6. Ganoderma applanatum (artist’s conk)
Ganoderma applanatum has a broad host range among hardwoods and conifers, but on oak, it’s usually encountered on large and mature trees. The fungus decays lignin, cellulose and hemicellulose resulting in severe loss of wood strength. Trees may suffer stem failure without showing any prior external symptoms. The decay occurs in the sapwood and heartwood of the lower trunk and sometimes large roots that are close to the base. The decaying wood is light-colored and mottled in appearance. Perennial conks are usually produced singly (sometimes overlapping) on the lower trunk, often within 10 feet of the ground. At the same time, infections can also develop in the upper trunk, a condition often referred to as top rot. The conks are shelf-like and fan-shaped, with a tannish brown to gray-colored upper surface and brilliant white pore layer on the underside. The upper surface is not waxy, but dull in appearance, and develops concentric “rings” as it grows annually. Conks typically grow perennially for five to ten years (in some cases longer), reaching 8–12 inches in width and becoming rigid and woody with age. The presence of even a single conk frequently indicates extensive internal decay and a high likelihood of stem breakage during strong winds. Spread is primarily through airborne dispersal of spores from conks.
7. Grifola frondosa (hen of the woods)
Grifola is capable of attacking several hardwood hosts but is most common on oak. Grifola causes a white pocket rot of the roots and lower trunk that advances very slowly. As a result, the localized pattern of decay reduces structural stability slowly over time. Oaks with fruiting bodies growing at the base are not necessarily a high risk for uprooting or stem failure under strong winds. The late stages of decay appear as a white pocket rot in the heartwood surrounded by less decayed wood that is darker in color, usually orange to reddish-brown. Annual mushrooms are similar to Laetiporus and Meripilus, growing in a large rosette pattern of overlapping caps. The caps are fan-like, attached to a central stalk at the base with the surface appearing white when young, becoming greyish-brown to greyish-blue with age. Older specimens exposed to freeze-thaw cycles late in the season can be very dark in color. The pore layer is usually bright white becoming tan to brown with age. The mushrooms are usually found growing from the soil at the base of the tree but at times can be growing from roots a few feet away. Decay can extend for many feet up the main trunk but is usually most severe in the roots.
8. Meripilus sumstinei (black-staining polypore)
Meripilus is found on a variety of deciduous hardwoods and some conifers, but primarily targets oaks in southern New England. Meripilus is principally a root rot pathogen and while it’s classified as causing a white rot, studies have shown the patterns of decay are very diverse. Because of the damage caused to the roots, upper canopy dieback and thinning can sometimes be observed on trees with advanced infections. The appearance of Meripilus mushrooms does not automatically warrant removal of the tree. The annual mushrooms are similar to Grifola and are composed of overlapping caps growing from a central stalk, typically in a rosette pattern. The surface of the caps is often a yellowish-brown to brown with concentric rings and a white margin when young. The underside of the caps are bright white when fresh, staining black when touched. This black-staining feature is useful for distinguishing Meripilus from closely-related fungi like Grifola. The fungus can be found fruiting several feet away from the main trunk on lateral roots or adjacent to the trunk at the soil line. Meripilus is known to persist in infected roots for long periods after a dead tree is removed and can be regularly found fruiting from stumps. Therefore, stump grinding and root removal is recommended prior to replanting at the site.
References for further reading:
1. Sinclair, W.A. and H.H. Lyon. 2005. Diseases of Trees and Shrubs. Cornell University Press, Ithaca, New York.
2. Schwarze, F.W.M.R., Engels, J. and C. Mattheck. 2000. Fungal Strategies of Wood Decay in Trees. Springer, Berlin, Germany.
3. Luley, C.J. 2005. Wood Decay Fungi Common to Urban Living Trees in the Northeast and Central United States. Urban Forestry, LLC.