Annosus Root Disease
Annosus root disease (ARD), caused by the fungus Heterobasidion annosum, produces significant losses of conifers across the South. On sandy, well-drained sites, this disease causes growth loss or kills trees. It is most often associated with thinning of loblolly, longleaf, shortleaf, slash, and white pine plantations. Slash and loblolly pines are the most commonly planted species in the South and are both very susceptible to ARD (Robbins 1984, Stambaugh 1989).
A survey of ARD in the South documented 2-3 percent mortality and a 44-60 percent rate of disease occurrence in planted pine. Documented rates of radial and height growth are significantly less for diseased than for healthy pines (Applegate 1971, Froelich and others 1977, Morris 1970). The fungus enters a stand by infecting freshly cut pine stumps. It progresses into roots, and, thereafter, it grows from tree-to-tree via root contacts and grafts. First entry into a stand can be prevented by treating susceptible new stumps with borax.
The primary risk factors associated with ARD are the amount of host type available, the timing and degree of management activity, and the soil and site conditions. Risk of damage caused by ARD decreases as clay content in the surface layer of soil increases, giving us an effective risk mapping tool (see figure at left). In the Southeast, risk of ARD is high or moderately high on an estimated 163.5 million acres, not all currently forested (Hoffard and others 1995). Silvicultural and chemical controls can be used to minimize the impact of ARD on high-risk sites. A biological control that appears to be effective does not have EPA registration and is currently unavailable to managers.
The most crucial stage in the disease cycle of this pathogen is the entry of the fungus into the stand through freshly cut stump surfaces. These cut surfaces provide a suitable niche in which airborne basidiospores can germinate and subsequently bring about mycelial colonization of the stump and root wood. Direct infection of roots through root wounds or possibly unwounded roots can occur in southern pines such as slash pine (Hendrix and Kuhlman 1964) and in Abies species (Garbelotto and others 1999). Once present in a stand, infection spreads from stumps to healthy trees via root contacts or grafts, creating ever-widening mortality centers. The fungus derives its nutrition from the enzymatic decomposition of woody tissues, particularly lignin and to a lesser extent cellulose, resulting in a physiological white rot. Thus, wood rotted by H. annosum has a characteristic delaminated appearance, and later becomes lighter to almost white as the lignin is removed from cellulosic wood fibers. Infected trees are subject to windthrow as a result of these structural changes in decayed wood.
Roots infected by the fungus in living trees become highly resinous in advance of the invasion front containing active mycelia. Resin production is a physiological, host-defense response of the tree to invasion and may slow and sometimes contain the advance of the infection. The production of resinous compounds in response to infection is metabolically very costly in terms of expended energy and may result in the weakening of the tree over time. The expense of energy for host defense in response to extensive root infection by H. annosum predisposes conifers to attack by bark beetles and other root diseases (Alexander and others 1981, Schowalter and Filip 1993). The fungus can persist saprotrophically in the highly resinous stumps and stump roots in longleaf pine for at least 7 years after thinning, providing inoculum potential to infect healthy residual trees via root grafts and contacts (Otrosina and others 2002). Mortality is a dramatic effect of H. annosum root disease, but growth reduction usually results from sublethal infections. Because root disease infection in trees is invisible until very advanced stages, considerable growth increment loss can occur in affected stands without significant mortality (Alexander 1989, Alexander and others 1981). On the other hand, slash and loblolly pines may be able to sustain considerable root infection before growth reduction occurs (Bradford and others 1978, Froelich and others 1977).
Considerable research has been done regarding risk assessment with respect to H. annosum root disease in the Southeastern United States. Edaphic factors are important elements associated with occurrence and hazard associated with this disease. Sites classified as high risk have well-drained soils containing sand, low organic matter, and low water table (Alexander 1989). These edaphic risk factors have been used to develop hazard-rating maps (Anon. 1999). While these maps provide correlations between certain soil types and H. annosum root disease, there is little information available to explain why or how soil factors affect disease development. Soil type affects factors such as water stress, microbial activity, aeration, and root habit, and root configuration can affect the root-infection processes.
Control of H. annosum root disease is achieved primarily through prevention. The most effective means to date is the prophylactic application of powdered borax formulations to freshly cut stump surfaces. Borax is toxic to basidiospores and conidia of H. annosum (Hodges 1970). Prevention and control is achieved only if borax applications are timely, ideally within a few hours after tree cutting. Technology that automates application of powdered borax by devices that attach to feller buncher equipment is now under development (Karsky 1999). Another avenue for H. annosum root disease control is through silvicultural management. Research by Ross (1973) revealed that thermal inactivation of basidiospores is achieved when stump surfaces reach >35 °C, resulting in no stump colonization. These temperatures are common during the summer months south of 34° N. latitude and form the basis for the recommendation that southern pine stands south of this latitude be thinned in the summer. On the other hand, high temperature may not be the sole factor responsible for lowering rates of stump infection. Some research suggests that microbial synergy at the stump surface may be affected by high temperatures on stump surfaces, since the fungus could be reisolated from surface sterilized and inoculated wood bolts at temperatures up to 40 °C (Gooding 1964).
Private industry generally favors intensive plantation management of loblolly and slash pine on short rotations of 30 to 35 years. Severity of ARD in this type of management is directly related to the number of thinnings in the stand and the proportion of sand in the soil. Industrial owners are more likely to use a full range of management options. Short rotations and intensive management generally result in low ARD caused mortality on industry lands.
On managed public land, the current trend is to restrict the amount of intensive plantation management in favor of longer rotations for watershed protection and recreation. Restoration of longleaf pine is being promoted. Of the southern pines, longleaf is considered the least susceptible to root disease and its restoration on sites currently occupied with other pines will lessen the impact of ARD.
When pine stands managed on longer rotations have few intermediate cuts, the risk of ARD development is generally reduced. However, strategies that promote uneven-age management with frequent cuts will likely increase incidence and severity of ARD. Management for red-cockaded woodpecker habitat, which requires frequent mid-rotation thinnings, may also increase ARD on high-risk sites (Cram 1994).
On public reserved land, where management activities are minimal, ARD will have little impact.
Private nonindustrial land, which includes 69 percent of the South’s forest lands, is managed in a variety of ways, creating a range of risk for ARD. The Conservation Reserve Program (CRP), which has assisted private landowners to reforest thousands of acres of erodable cropland, has resulted in increased risk for ARD in the plantations it supports by favoring early thinning. Approximately 400,000 of the 2 million acres enrolled are on high-risk soils for ARD development (Anderson and Mistretta 1982).
Less emphasis has been given to H. annosum root disease control in recent years. Preventive measures such as application of borax after thinning are becoming less common. While some data suggest that mortality of trees planted in severely infested sites is minimal up to 22 years after planting (Kuhlman 1986), multiple stand entries and thinning without the proper preventive measures, combined with longer rotation lengths, will increase the importance of this disease in coniferous forests in the Southern United States.
Encyclopedia ID: p990
