Annosus Root Disease
Heterobasadion annosum
Key Wildlife Value:
Heterobasadion annosum creates short-term snags and down wood in the form of whole green trees and fallen snags, by killing and decaying the root system and butts of host trees, especially true firs, hemlocks, and in some areas, pines and western juniper. Concentrations of down wood that vary in their density and arrangement typically occur in root disease infection areas, usually with a corresponding decrease in canopy cover. Canopy gaps caused by annosus root disease expand slowly, resulting in a more diverse stand structure and at times a more diverse plant species composition, as resistant or non-host trees, shrubs, and forbs are released or become established behind the slowly expanding fringe of dying host trees. Bark beetles frequently are attracted to trees infected with H. annosum, providing good foraging habitat for woodpeckers. Infected trees with extensive butt rot, especially old western hemlocks growing in coastal areas, sometimes become hollow at their bases. Although the degree of wildlife use of trees hollowed at the base by annosus root disease is not well known, these trees potentially could provide denning habitat while standing, or while lying on the ground if they fall with their hollow bases intact. However, the likelihood of a standing tree breaking at a hollow base is fairly high.Distribution in Oregon and Washington:
Found throughout both states.Hosts:
Most conifers and a few hardwoods may be infected by H.annosum, but susceptibility and damage vary greatly by tree species (Table fid-10) and location. Grand fir and white fir east of the Cascades crest (Eastside) and in southwest Oregon are frequently killed, as are Eastside ponderosa pine and associated western juniper growing on poor, dry sites. Other species are less likely to be killed, but some may develop extensive decay and experience breakage and windthrow. Western hemlocks growing west of the Cascades crest that are older than 120 years are frequently butt rotted and may develop hollow butts. H.annosum commonly causes extensive decay in old-growth mountain hemlock stands.Diagnosis:
Annosus root disease is often difficult to identify because it causes variable symptoms and diagnostic fruiting bodies may be hard to find. It sometimes is effective to arrive at a diagnosis after methodically eliminating the possibility of other major root diseases. However, the most reliable way to diagnose annosus root disease is to find fruiting bodies.In seedling/sapling/pole size stands regenerated following harvest activities, look for a pattern of dead or missing trees associated with large stumps. Note the species of the affected trees and stumps to see whether they fit the host profiles of H. annosum. Look for spore-producing fruiting bodies, called conks, on dead trees by pulling away the duff and examining below the root collar and on the roots for small, leathery, buff to white pustules or popcorn-like “button conks”. Larger, mature fruiting bodies sometimes may be found inside nearby hollow or extensively decayed stumps. A mature H. annosum conk is woody and often has a shelving form. It typically has a dark brown upper surface with concentric furrows, and a smooth, cream-colored lower surface with tiny round pores. A narrow strip of cream-colored, non-pored area (sterile margin) divides the upper and lower surfaces of the conk along the underside of the leading edge. Stumps also provide good locations to examine decay. The best place to look usually is on a root below the soil line, but diagnostic decay sometimes is also apparent in the aboveground portions of the stump. Advanced decay is either 1) laminated (laminae may be thin and dry or thick and spongy), with oval to oblong pitting on only one side of the laminae and no cinnamon-brown setal hyphae present, or 2) stringy and white with small black flecks (especially true of hemlocks).
In stands of small-size and larger trees, look for patches of forest that have a more open canopy than the surrounding stand and that contain standing dead trees and windthrown trees with decayed roots or “rootballs” having characteristic annosus decay. Resistant or immune species such as hardwoods and non-host conifers, may sometimes populate an infection area, mitigating the effects of the root disease upon canopy cover. Disease infection centers may contain an array of trees that are in various stages of dying and decay, indicating a progressive rate of mortality from the center to the fringe. Live trees with root disease may display one or more the following symptoms (although true firs frequently die without showing crown symptoms because they are often killed by bark beetles before foliage symptoms become evident): shortened terminal growth, sparse foliage, small needles, chlorosis (yellowing), and stress cone crops. These symptoms and patterns are fairly typical in true fir and pine stands east of the Cascade crest and in southwestern Oregon. However, in western hemlock stands, especially those west of the Cascades crest in northwestern Oregon and Washington, H. annosum tends to produce less obvious symptoms and patterns. Infected hemlock trees rarely die standing, but windthrow or broken stems are sometimes present. Often the presence of H. annosum in western hemlock stands is not evident until harvest activities result in exposure of decay or stain in the lower boles of cut trees.
Bark beetles frequently colonize trees infected with root diseases. With the exception of droughty years and periods following large-scale outbreaks of defoliators, a very high percentage of true fir trees killed by fir engraver are infected with H. annosum. Armillaria root disease also is frequently found on tress infected with H. annosum.
Life History:
Spores germinate on freshly exposed wood surfaces and develop microscopic fungal threads (hyphae) that grow through woody tissues, secreting enzymes that convert the cellulose and lignin of wood cells to simple carbohydrates that the fungus uses for food. When there is contact between an infected root and a healthy root, the hyphae (masses of hyphae are called a mycelium) can grow across the contact and invade the healthy root. Mycelial growth results in wood decay and death of living cells and often occurs in the heartwood, but also occurs in some tree species, such as pines and true firs, in the living sapwood. Conks form on infected host tissues at sheltered locations on the bases and roots of host trees and in decaying stumps.In Oregon and Washington, two strains of H. annosusm occur, each with distinct host preferences. The “S-group” infects true firs, hemlocks, spruces, and very rarely Douglas-fir. The “P-group” infects pines, western juniper, and rarely incense-cedar. Although both strains will readily colonize stumps of host species preferred by the other group (e.g. the S-group strain will colonize and produce fruiting bodies in ponderosa pine stumps), these stumps usually do not become disease infection centers, meaning that annosus root disease rarely develops in adjacent trees.
Important Habitats and Spread Dynamics:
In the Cascade Mountains and west of the Cascades, H. annosum causes extensive root and butt rot in western hemlock and mountain hemlock stands, most commonly in trees older than 120 years, but at some northwest coastal Washington locations western hemlock trees as young as 50 years have extensive decay caused by H. annosum. Annosus root disease is very common and causes considerable mortality east of the Cascades crest and in southwestern Oregon in stands having a high proportion of grand or white fir. It also is common and causes mortality of ponderosa pine growing on very poor, dry Eastside sites. Due to dissemination and spread characteristics of H. annosum, annosus root disease is more prevalent and widely distributed in mature stands that have experienced multiple entries associated with selective harvest activities than in unharvested stands or in those that have had a single entry. Incidence within a stand is often higher along skid trails and yarding corridors where residual tree wounding occurs more frequently than in other parts of the stand.Annosus root disease spreads long distances aboveground via airborne spores, and locally underground through mycelial growth across host tree root contacts. Freshly cut host tree stumps generally greater than 45 cm (18 in) in diameter (except for subalpine fir, in which 20 cm (8 in) diameter stumps can act as infection centers) and host trees with large (> 900 cm2, or 1 ft2) wounds provide favorable and important habitat for H. annosum colonization and spread. Stump and wound surfaces remain susceptible to infection for 2 to 4 weeks following cutting or injury. Colonized host stumps (those exceeding the previously mentioned diameter thresholds), and colonized wounded host trees serve as “centers” of gradually expanding root disease infection areas as the fungus slowly moves down colonized tree roots and up the roots of previously uninfected individuals. Rates of expansion are estimated at 0.5 to 1.5 m (1 1/2 to 3 ft) per year. H. annosum dies out within a few years in small stumps less than 15 cm (6 in), but may persist in large stumps for 50 to 60 years.
Opportunities for Manipulation to Increase Wildlife Habitat:
Buffered areas that include entire annosus root disease infection centers with some surrounding non-infected area may be retained in managed stands to provide a steady supply of short-term snags and down wood. Non-host buffers may require periodic maintenance to prevent unintended spread of the root disease into other areas of the stand. This approach would be appropriate for stands where root disease infection centers were discreet and not diffuse, and for stands being converted to non-host species.Potential Adverse Effects:
H. annosum can have profound effects upon forest composition, density, structure, and succession when widely distributed throughout a stand or landscape composed of highly susceptible host trees. H. annosum may cause undesired reductions in stand densities, limit the species of trees which can be grown to a large size on a site, and cause high rates of mortality. The abundant down wood resulting from high levels of annosus root disease infection may increase fire hazard. Losses are greatest east of the Cascades crest and in southwest Oregon in partially cut stands of pure grand fir and white fir, or where fir basal area exceeds 20 m2 per hectare, total stand basal area exceeds 75 m2 per hectare, and stand age exceeds 120 years. Pine mortality may be high on eastside poor, dry sites where few other tree species can grow. On recreational sites, trees infected with H. annosum have a high potential for failure and present a significant hazard to public safety.How to Minimize the Risk of Adverse Effects:
When managing highly susceptible species such as grand fir or white fir, adverse effects are best minimized by limiting rotations to 120 years or less, minimizing the number of harvest entries, and limiting tree wounding during harvest entries. New infections by H. annosum also can be minimized by treating recently cut stump surfaces with a registered borate fungicide (e.g. Sporax® or Tim-Bor®) to prevent spore germination. Borate stump treatment is not effective on stumps that are already infected with H. annosum.Other strategies include discrimination against highly susceptible host species by regenerating with non-host or resistant species, by preferentially removing susceptible hosts during thinning and partial harvest operations, and by using prescribed fire.
References
Goheen, E.M., and E.A. Willhite. In prep. Field guide to the common diseases and insect pests of Oregon and Washington conifers. USDA Forest Service, Pacific Northwest Region, Forest Health Protection.Hadfield, J.S., D.J. Goheen, G.M. Filip, C.L. Schmitt, and R.D. Harvey. 1986. Root diseases in Oregon and Washington conifers. USDA Forest Service, Pacific Northwest Region, R6-FPM-250-86. 27 pp.
Harvey, R.D., and P.F. Hessburg. 1992. Long-range planning for developed sites in the Pacific Northwest: the context of hazard tree management. USDA Forest Service, Pacific Northwest Region, Forest Pest Management, FPM-TP039-92. 120 pp.
Otrosina, W.J., and R.F. Scharpf, tech. cords. 1989. Research and management of annosus root diseases (Heterobasidion annosum) in western North America. USDA Forest Service, Pacific Southwest Research Station, Gen. Tech. Rep. PSW-116. 177 pp.
Schmitt, C.L., J.R. Parmeter, and J.T. Kliejunas. 2000. Annosus root disease of western conifers. USDA Forest Service, Forest Insect and Disease Leaflet 172. 10 pp.
Website links
Annosus root disease links, An Online Catalog of Western Forest Insects and Diseasesnot yet available: Field Guide to the Common Diseases and Insect Pests of Oregon and Washington Conifers