Beech Bark Disease

This is another introduced disease. But this is a bit more complicated than the others as far as the causal agent goes.

Host

The host in North America is Fagus grandifolia, American beech.  It is a component of the northern hardwood forests of northeastern North America.  In Europe the host is Fagus sylvatica, European beech.

Pathogen

Two organisms must be present: one is a scale insect, the other a fungus.  The scale Cryptococcus fagisuga was introduced from Europe to Nova Scotia about 1890.  The disease was first observed there about 1920. The scale was first found in the US in Massachussetts in 1929.

The fungi are now named Neonectria ditissima and N. faginata ​[2]​.  Neonectria ditissima was found in North America as early as 1897.  It is genetically diverse and is thought to be native to North America ​[2]​. It occurs in Europe as well, but additional sampling in Europe is needed to determine if it was introduced there.  Neonectria faginata was long thought to have been introduced to North America, but currently the species is known only from Fagus in North America.

Neonectria ditissima is generally the pathogen involved in recently affected stands (the disease front), while N. faginata is more common in stands affected for long periods (aftermath stands) ​[5]​.  See Disease Cycle below for details.

In Europe, Neonectria coccinea sensu stricto is the important pathogen ​[2]​.  It is closely related to N. faginata.

Environment

It appears that stand diversity does little to reduce the amount of disease in beech ​​[6]​​.  In fact, since N. ditissima is thought to initially reside in other species (see Disease Cycle below), and attacks beech in the advancing front of scale movement, this could be an example where stand diversity actually makes the disease more severe, at least initially.

A mycopathogen, Nematogonum ferrugineum, can parasitize Neonectria spp. ​[4]​.  It can reduce reproduction and slow the progress of Neonectria spp. in bark.  However, it seems to have little effect on the course of disease.  Similarly, there are a few predators of the scale insect in North America, but they are not effective in reducing populations at the stand level, and the most important one can even vector Neonectria species when it disperses ​[4]​.

In general, bark becomes more vulnerable to attack by the scale as it matures, because more cracks, crevices and other irregularities are an advantage for the scale.  Mosses and crustose lichens growing on the bark can render it vulnerable at an earlier age.  However, certain crustose lichens, when their cover is unusually high, can inhibit scale establishment ​[4]​.

Disease Cycle

Neonectria ditissima infects other species associated with beech, including Acer saccharum (sugar maple), A. rubra (red maple), and Betula alleghaniensis (yellow birch) ​[5]​.  The fungus is naturally widespread in northern hardwood forests.  On those hosts it typically causes a perennial target canker.

The beech scale insect first begins feeding on the bark of beech.  Another, native scale insect, Xylococculus betulae, may sometimes facilitate the other agents ​[1]​.  Scale feeding creates minute wounds and alters bark physiology such that it becomes susceptible to the Neonectria sp.  Neonectria ditissima from the other hardwoods can readily infect the altered beech bark.  The fungus kills phloem and cambium as it invades, and the scale is no longer necessary.  Cankers on beech are typically diffuse and spreading.

Usually, the more competitive N. faginata replaces N. ditissima where the disease has been present for several years or more, although this does not occur consistently ​​[1]​​.  Neonectria faginata is a more aggressive pathogen on beech than N. ditissima, causing larger cankers and consequently producing more perithecia ​​[5]​​.

Disease Spread and Distribution

Because the pioneering pathogen (N. ditissima) was already well distributed in northern hardwood forests, the spread of the disease has been determined by dispersal of the scale insect after its establishment in North America ​[4]​.  Although it can be wafted in winds in small numbers, the scale cannot fly and is not highly mobile.  The disease has spread much more slowly than did chestnut blight.  Its progress has been measured in decades, but it is relentless.  The main disease front is now in Pennsylvania, Virginia, West Virginia, and Ohio.  Occasionally it made leaps of hundreds of miles, west to the north and east of Lake Michigan and south to North Carolina and Tennessee.  It still has much of the beech range to invade.

Symptoms, Signs, and Damage

The canker may be diffuse, or there may be host response that stops the fungus. Irregular, blocky bark is an indication of some kind of resistance response.

All of these species make red, superficial perithecia in clusters.  What a nice sign!

Management

As it began killing beech in large numbers, there was a flurry of interest in it. Pathologists sorted out the causal agents, determined how the disease was spreading and the response of the trees, etc. But interest soon dropped because foresters initially saw this as a good thing. Beech is an OK lumber tree, but very abundant, and maple and birch are usually more valuable. In fact, the reason beech was so abundant is that it was left behind when the more valuable birch and maples were harvested preferentially. (That practice of taking the most valuable trees and to hell with the rest is called high-grading, and is responsible for some modern problems in forest management.) Here was a pathogen helping the foresters for a change!

However, it soon became apparent that the disease was leaving behind, not stands of maple and birch, but thickets of beech sprouts. Beech sprouts very well when the top is killed or when a stand is salvaged. Some old trees survive, but they are deformed and defective.  The important associated species become less abundant as beech sprouts dominate the understory.

Silvicultural approaches to reducing these impacts have been tested in stands dominated by sugar maple and with similar basal area of beech and yellow birch ​[3]​.  Selective harvesting of larger beech, with about 45% basal area reduction, improved overstory growth of all species, especially sugar maple and birch.  However, beech regeneration, primarily suckers, grew quickly and remained dominant among the larger regeneration. The treatment did promote adequate regeneration of maple and birch, but did not control profuse regeneration of beech. The disease remains a substantial challenge to managing northern hardwood forests.