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.

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.

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.

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.

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.

FIGURES

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).

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