David R. Houston1 and James T. O'Brien2
1Principal Plant Pathologist, U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station, Hamden, Conn.
2Plant Pathologist U.S. Department of Agriculture, Forest Service, Northeastern Area, State and Private Forestry, Durham, N. H.
Beech bark disease causes significant mortality and defect in American beech, Fagus grandifolia (Ehrh.). The disease results when bark, attacked and altered by the beech scale, Cryptococcus fagisuga Lind., is invaded and killed by fungi, primarily Nectria coccinea var. faginata Lohman, Watson, and Ayers, and sometimes N. galligena Bres.
History and Distribution
Accounts from Europe indicate that the disease was killing beech (Fagus sylvatica) before 1849. The scale insect, readily visible on the trees, was considered the cause of death until 1914, when it was learned, that a fungus, then identified as Nectria ditissima Tul., infected trees infested by the scale.
Around 1890, the scale was accidentally brought to Nova Scotia. By 1932, the scale and an associated nectria fungus were killing trees throughout the mature beech areas of the Maritime Provinces and in localized areas of eastern and southcentral Maine. In addition, isolated infestations of scale were occurring in southwestern Maine and eastern Massachusetts. The scale insect has continued to spread to the north into Quebec and to the west and south throughout New England, New York, New Jersey, and northern and eastern Pennsylvania. In 1981, a 70,000-acre area was found infested in northeastern West Virginia.
The pattern of insect spread and the subsequent occurrence of nectria infection and tree death have led to an arbitrary classification of disease development over time and space:
The Causal Complex
The scale - C. Fagisuga is a soft-bodied scale insect. At maturity, it is yellow, elliptical, and 0.5 to 1.0 millimeter3 long (fig. 1). It has reddish-brown eyes, a 2-millimeter stylet, rudimentary antennae and legs, and numerous minute glands that secrete a white "woollike" wax.
3 0ne millimeter = 0.04 inch.
The wingless larvae (also called crawlers or nymphs) emerge from the eggs with well-developed legs and antennae (fig. 2). Some larvae remain under the females, which die after the eggs are deposited. Some migrate to cracks and other protected areas; others are washed down or fall to the ground where most of them die; and still others are carried, usually by wind, to other beech trees. If a suitable location is found, the insect forces its tubular stylet into the bark and begins to feed. It then transforms into a second-stage nymph, without legs and covered with woollike wax. The insect overwinters in this stage and, in the spring, molts to become an adult female.
The fungus - In North America, two species of the nectria fungi are associated with beech bark disease. The principal one, N. coccinea var. faginata, is considered a weak parasite; the second species, N. galligena, is a common pathogen inciting perennial cankers of many hardwood species. In some areas, for example in West Virginia, N. galligena appears to be the major species involved. Both organisms produce several types of spores.
One type of spore is produced in fruiting bodies called perithecia that occur in clusters on the bark. The perithecia, are tiny, bright red, and lemon shaped (fig. 3). Each perithecium is filled with elongated sacs, each containing eight spores. The production of these spores constitutes the sexual or perfect stage of the fungus.
The perithecia mature in the fall. Spores are forced out when the perithecia have been sufficiently moistened; when dry, they appear as white dots on the tips of the perithecia. Perithecia on the dead bark continue to produce viable spores the next year.
Other spores are formed by an asexual or vegetative process. Frequently, small white cushions of spores burst through the bark before the perithecia appear (fig. 4). These asexual spores range from single-celled, oval spores to eight-celled, sickle-shaped spores and are produced in a dry head, well suited for dissemination by wind. The asexual spores can be found from mid-summer until fall, and can easily be mistaken for small isolated colonies of the scale insect.
Symptoms and Course of the Disease
On some trees, a red-brown exudate called a slime flux or "tarry spot" oozes from dead spots (fig. 6).
Bark infected by Nectria becomes inhospitable for the beech scale. If the outer bark is cut away, a distinct orange color may be seen where Nectria is actively invading the bark. The fungi may infect large areas on some trees, completely girdling them. On such trees, the perithecia that often form can redden large areas of the bark (fig. 7). On dying trees, leaves that emerge in the spring do not mature, giving the crowns a thin, open appearance. Later, the leaves turn yellow and usually remain on the tree during the summer. (See cover.)
Frequently the fungus infects only narrow strips on the bole, and the subsequent symptoms differ from those of trees that have been girdled. Callus tissue forms around these strips, and the bark becomes roughened (fig. 8). Small nectria cankers may be walled off from the sapwood by callus tissue (fig. 9).
Other insects and wood-rooting fungi quickly invade the wood beneath bark killed by beech bark disease. Species of Hypoxylon that decay sapwood are among the first to invade. Ambrosia beetles make holes that allow other fungi to enter. The shoestring root rot fungus, Armillariella mellea, sometimes invades weakened trees and hastens their death. Attacks by these organisms make it difficult to judge when trees will succumb to beech bark disease. Many trees that are partially girdled remain alive, in a weakened state, for years. Many are broken by the wind - a condition termed "beech snap" (fig. 10).
In the aftermath zone, attacks of a second scale insect, Xylococculus betulae, create severe defects on young beech stems. Roughened areas resulting from X. betulae attack are, in turn, infested by beech scale and then by Nectria.
A ladybird beetle, Chilocorus stigma, feeds on the scale; and a fungus, Nematogonum ferrugineum (Gonatorrhodiella highlei), parasitizes the nectria fungi. The effects of these organisms on the disease agents and on the course of the disease have not been critically evaluated.
Scales on high-value ornamental trees can be controlled with insecticides. Consult your local forest pest management specialist or county agricultural agent to obtain current information on chemicals registered for beech scale control.
The disease in forest stands cannot be controlled at a reasonable cost, and a program of timely salvage cuttings is the only way presently know to reduce disease losses.
Vigorous trees free of the disease are often found in heavily affected areas (fig. 11). Recent trials with some of these trees have shown them to be resistant to the scale. This offers hope that methods can be developed to increase the levels of resistance in affected forests.
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Revised February 1983
Formatted for the Internet April 1998