Treesearch

Trees heavily infested with beech scale are commonly observed either singly or in small groups within infested forests. This appears to be due partly to the presence of resistant trees and also, in the U.K. at least, to the existence of sub-populations of scale insects, some of which appear to be adapted to individual host trees. It is suggested that the greater susceptibility of American beech to attack by beech scale may be due in part to the absence of co-evolved defence mechanisms.

Surveys of beech scale infestation among progeny of single beech trees demonstrated significant variation in susceptibility between the progenies. Relative differences in susceptibility of some progeny were maintained on three different sites in southern England.

American beech trees that were free of beech bark disease in forests long-affected by beech bark disease were challenged with C. fagisuga using the 'foam' technique. Trees were resistant: no insects reached maturity. In Nova Scotia, 12-15 disease-free trees per hectare occurred in the stands examined. Many of these trees occurred in groups.

The status of beech bark disease in Great Britain is summarised with respect both to historical perspectives and to the contemporary situation. Features of the disease which relate particularly to its occurrence in Great Britain are listed. Some tentative findings from recent observations and experimental work are presented.

The two organisms involved in beech bark disease are endemic everywhere in France. Nevertheless the disease is restricted to the northern part of the beech's range, where it grows on the plains. During the last five years beech bark disease has shown a general decrease in severity but there have been local increases in the north and north-eastern parts of forests where the disease has been present for a long time. Some French foresters feel that beech bark disease is a sanction against the growth of beech in inappropriate ecological conditions.

Beech bark disease can be found at present time in young and old stands (20-150 years old) of Fagus sylvatica. The present state of the disease may be described as "normal" and apart from some cases, it is no threat to the existence of the stands.

Beech bark dieease first reached North America about a 100 years ago and within a few decades rendered beech an almost useless weed species in the Maritimes. A survey in 1980 to determine the current status of the disease found the scale insect active, though infestations generally light, on over 90% of trees in Nova Scotia and Prince Edward Island but on only about 50% i n New Brunswick; Nectria coccineavar. faginata distributed throughout; some degree of stem cankering on trees; mortality in New Brunswick (3.7%) less than half of that found in Nova Scotia (9.1%) or Prince Edward Island...

The distribution of beech bark disease has not changed significantly in recent years in Quebec. It has remained as scattered infected stands located within relatively old beech-scale-infested areas. This scale covers the southeastern half of the beech natural range in the province and it is spreading slowly westward.

The advancing front of beech bark disease in the northeast is now located in western New York and Pennsylvania. The disease is killing trees as far west as central New York. Cryptococcus fagisuga scale was found on nearly every tree examined during a northern New England disease survey. From that survey and Resource Evaluation plot data, beech mortality is estimated as high as 50 percent in Maine and about 30 percent in New Hampshire and Vermont.

Results of periodical surveys for beech bark disease in Pennsylvania from 1958 to 1982 reveal that the disease is slowly spreading in a south- and westward direction. Although the disease complex is still confined to the northern and eastern portions of the state, beech mortality is occurring in the areas infested longest.