Back to Table of Contents

Potential for Biologically Based Control
of Beech Bark Disease
in the Southern Appalachians

David R. Houston
USDA Forest Service
Hamden, CT 06514


Beech bark disease results when bark of Fagus spp. is altered by the beech scale Cryptococcus fagisuga and then invaded and killed by fungi of the genus Nectria. There are biologically based factors or agents that are potentially useful for controlling both members of the causal complex. On several sites in Nova Scotia, bark epiphytes (lichens) provide significant levels of protection against the insect. Also, several invertebrate predators are effective in reducing scale populations on individual trees, though they offer little promise for control at the stand level. The effects of the entomogenous fungus, Verticillium lecanii, found associated with collapsed scale populations in England, have not been studied in North America. Although Nectria spp. are parasitized by the mycoparasite, Nematogonum ferrugineum, and other fungi are suspected competitors or antagonists, control of the Nectria pathogens offers less potential for controlling the disease than factors and agents that affect C. fagisuga. Most important is that some American beech trees are resistant to the scale. Resistance offers the greatest opportunity for exploitation at this time.


In North America, beech bark disease (BBD) is a complex affecting American beech (F. grandifolia Ehrh.). Its etiology includes the predisposing attack of bark by the beech scale insect, Cryptococcus fagisuga Lind. (Fig. 1), and subsequent invasions and killing of infested bark by several fungi of the genus Nectria (Ehrlich 1934). The principal fungus is N. coccinea var. faginata Lohm. and Watson (Lohman and Watson 1943) (Fig. 2), though N. galligena Bres. also attacks and kills bark predisposed by C. fagisuga (Cotter 1974; Houston 1994a; Mielke et al. 1982). A general framework for its etiology can be expressed as:

Beech trees + C. fagisuga + Nectria spp. => BBD

This correctly implies that a specific chronology of events is required for disease development, and that while the effects of the insect are necessary, the disease is expressed only when Nectria spp. attack scalealtered tissues and, conversely, Nectria attack does not occur unless trees are infested by beech scale.

Brought to Nova Scotia accidentally around 1890 (Ehrlich 1934), C. fagisuga has steadily spread westward and southward through the natural forests of Canada and the United States (Fig. 3). It now is found throughout New England, New York, much of Pennsylvania, northeastern Ohio, northeastern West Virginia, northwestern Virginia, and in a small area of the Great Smokey Mountain National Park on the TennesseeNorth Carolina border (Houston 1994 a, b). Generally, Nectria infections and tree mortality occur 1 to 4 years following a heavy buildup of the insect (Fig. 4) on large trees. The area of current heavy mortality is termed the "killing front" (Fig. 5); regions in which severe mortality occurred earlier comprise the "aftermath zone" (Shigo 1972). In aftermath forests, causal agents are established on small trees of root sprout and seedling origin that often develop in dense stands. Most of the new emerging trees and old survivors become cankered and are rendered highly defective by the scaleNectria complex (Fig. 6).

Biological Factors and Agents Affecting C. Fagisuga

Host Resistance

In affected stands, some trees remain free of beech scale and disease (Fig. 7). Challenge trials have shown them to be resistant to C. fagisuga (Houston 1982, 1983a). Resistant trees are found in relatively low numbers (< 1 percent of the beech stems) and commonly occur in groups (Houston 1983a). The occurrence of resistant trees in groups is encouraging because they are easier to recognize than isolated individuals, and are potentially easier to protect in forest management operations designed to discriminate against diseased trees. Isozyme genetic studies have shown that groups of resistant trees originate both from root sprouts and seed (Houston and Houston 1986, 1990).

Increasing the relative number of resistant trees is important in reducing the impact of BBD. The results of trials to determine the effects of various harvesting regimes on the initiation, development, and survival of root sprouts are being analyzed. In addition, studies to determine how to clone selected resistant genotypes have been conducted. Tissueculture techniques in which sprouts from root segments and forced buds of mature resistant trees are used have brought several genotypes through to rooting (Barker et al. 1995). Still needed are trials to develop ways to grow the tissueculture plantlets in soil and introduce them into the forest.

Bark epiphytes

Some epiphytes growing on beech bark offer favorable spatial habitats for C. fagisuga (Ehrlich 1934; Houston et al. 1979). Infestations often develop initially beneath patches of moss and lichen. However, not all epiphytes enhance infestations. In Europe, the common bark fungus Ascodichaena rugosa sometimes produces a dense, relatively continuous stromatic layer on European beech, F. sylvatica L. (Butin 1977); as a consequence, C. fagisuga often is absent on densely infected bark (Houston et al. 1979). However, trials initiated in 1975 revealed that although infestation by C. fagisuga of bark infected by A. rugosa remained low, stromatic patches sometimes were not sufficiently dense or complete to preclude significant infestation and subsequent development of BBD (D. Lonsdale, pers. commun.). In North America, stromatic patches of A. rugosa often are thin and fractured; they can offer refuges for C. fagisugasometimes on trees too small to be infested otherwise (pers. observation).

In Nova Scotia, some stands on steep, southfacing slopes contain many beech trees that are remarkably free of disease compared to others in the general area. These trees are heavily colonized by mosaics of crustose lichens. Several of the predominant lichen species are rarely colonized by C. fagisuga (Houston 1983b). Such lichens have thalli that are dense, smooth, and epigenous in contrast to the loosely compact, granularsurfaced hypogenous thalli of readily colonized species.

Predators and Parasites

To date, no invertebrate parasites of C. fagisuga have been found, but several predators are known. In North America, Chilocorus stigma Say. is the most common predator. C. stigma is most abundant when scale populations are dense and, although it responds numerically to high scale densities, its predatory effectiveness is limited by its propensity to disperse, by its failure to feed on all life stages of scale, and especially by the high rate of scale reproduction (Mayer and Allen 1983). Although scale populations on individual trees have been markedly reduced when populations of coccinellids were high, their overall effectiveness is limited.

Microbial Pathogens

In North America today, scale populations are low in some stands and regions where they were once high. In some forests, we have observed precipitous and unexplained population declines. Similar crashes in other forest insect populations have been associated with attacks by microbial pathogens.

In England, the entomogenous fungus Verticillium lecanii Viegas was common where infestations of beech scale were or had been heavy (Lonsdale 1983). The presence of V. lecanii depended on high scale density or on coalescense of scale colonies. It was absent from small, isolated, or new scale colonies because it spreads from one colony to another by hyphal growth rather than by aerially dispersed spores (Lonsdale 1983). We do not know whether V. lecanii or another pathogen is responsible in North America for observed sharp declines in scale populations or the maintenance of collapsed populations at low levels.

Agents Affecting Nectria spp.


Nematogonum ferrugineum (Pers.) Hughes (Gonatorrhodiella highlei) is a biotrophic contact mycoparasite (Barnett and Binder 1973) that obtains its nutrients from living cells of its host. The first association of the fungus with BBD was in North America (Ayers 1941). N. ferrugineum also was commonly associated with N. galligena both on cankers of several hardwood species and on beech with BBD (Houston 1983c; Mielke and Houston 1983). The effects of parasitism by N. ferrugineum in nature are not known even though high populations of the fungus sometimes occur after severe outbreaks of BBD.

In culture, growth of parasitized Nectria spp. is little affected (Blyth 1949; Gain and Barnett 1970); though production of conidial and perithecial initials is reduced (Shigo 1964). In inoculation trials, parasitized isolates of N. coccinea var. faginata and N. galligena spread more slowly in bark and cambial tissues, and persisted for shorter periods in bark tissues than unparasitized isolates. Although cankers resulting from parasitized N. coccinea var. faginata isolates produced fewer perithecia (Houston 1983c), this fungus appears ineffective as a biocontrol agent because in nature it becomes abundant only following severe outbreaks of BBD.

Biological Control: Discussion and Conclusions

The beech scale is now well distributed throughout the Great Smokey National Park (K. Johnson, pers. commun.), and BBD is causing significant mortality in upper elevation beech gap stands. While this situation creates a sense of urgency, it also offers the opportunity to exploit what we have learned about the disease in other areas.

BBD is complex because of its dual organism etiology. While this duality might seem to offer added opportunities for biological control, earlier studies indicate that approaches focused on reducing the effects of the scale initiator hold the most promise. The following actions seem feasible at this time:

  1. Areas in the Southern Appalachians now severely affected by BBD should be searched for trees exhibiting resistance to the beech scale. Identified trees will serve as candidates for future cloning and reintroduction into severely impacted forests.

  2. Tissueculture techniques designed to preserve and increase the relative numbers of resistant individuals and clones need to be improved, and steps toward transferring plantlets from tissueculture media to soil and establish them in forest settings need to be developed.

  3. Potential sites on which beech can be grown and protected from beech scale by lichens should be identified.

  4. Western Europe may not be the site of origin of C. fagisuga. Should its true home range be identified elsewhere, a search should be made there for pathogens, parasites, or predators.

Literature Cited


Figure 1.Beech scale nymph (about 0.3 mm long).

Figure 2.Sexual fruiting bodies (perithecia) of
N. coccinea var. faginata (about 0.3 mm in diameter).

Figure 3.Known distribution of beech scale (black areas) as of
1996 in relation to the range of American beech (gray areas).

Figure 4.Heavy infestations of beech scale
can cover tree boles with white wax.

Figure 5.Tree mortality (trees with bare and discolored crowns)
can be high when forests are affected
by the causal complex for the first time.

Figure 6.Trees in aftermath forests can become
severely defective as cankers accumulate over time.

Figure 7.Some trees are resistant to beech scale
and remain free of disease (left)
in contrast to their susceptible neighbors (right).

Back to
Back to
the Proceedings
Email us!