J. D. Low, Forestry Commission, United Kingdom
Fomes annosus is a serious cause of forest disease in many countries. The severe loss which it can occasion has been known for a long time and its spread now causes great concern. This paper, adapted from a Fuller account due to be published shortly, describes the findings of research carried out to date in the United Kingdom in the endeavor to control the further increase of the infection. It does not deal with the treatment of established infections which is also an important current research project.
The main importance of Fomes annosus is as a cause of but-trot, though it is also responsible for serious losses through windblow, death and reduced volume increment. Most of Britain's forests consist chiefly of first rotation crops, initially free of Fomes infection. These contrast strongly with the limited areas where conifers have previously been grown, where the fungus is now generally becoming well-established, and where there is a clear picture of progressive deterioration. For this reason active steps are now being taken to control its further spread and increase.
The mechanism of infection
Newly-cut stumps serve as the principal mode of entry for the fungus. Airborne spores alight on them and grow down into the root systems and thence by root contact or fusion into neighboring standing trees. The most common means of initial infection of a plantation is through stumps created during thinning and, to a lesser extent, during operations such as rack-cutting. 1 Stumps are likewise provided at the time of clear-felling. The regular provision of stumps is probably the main reason why this disease is so much more serious in plantation than in natural forest. Since stumps remain infected for a very long time, frequently for over 40 years, the disease is readily transmitted by carry-over in them from one crop to the next.
1 Rack-cutting is the cutting of extraction paths.
The crucial infection factor is thus the upsetting of the normal balance which exists between tree and fungus by thinning and clear-felling operations, and the provision of stumps which are such an ideal mode of entry into the crop. The substrate value of the stump of a tree felled in full vigor, the apparent reduction in the natural resistance of its tissues to fungal growth, and the very numerous root contacts and fusions between trees, provide a combination of factors such that the silvicultural operations might almost have been designed as the best possible way of introducing infection. In addition, stumps usually provide the main sources of sporophores. Infected stumps in the ground lead to the infection of adjoining trees and also of succeeding crops on the same ground, but the presence of such stumps with their associated sporophores has also by the production of spores, a very considerable effect on the rate of development of the disease in all surrounding and neighboring conifer plantations.
The fungus is not free-living in the soil, but almost entirely confined to the woody tissues of its host tree or stump. Infection can enter a tree either from infected thinning stumps, from another infected tree or from old infected stumps of a previous crop. Other modes of infection have been noted, through brushing and pruning wounds, through extraction damage to the bole of the tree and to surface roots, through the removal of double leaders and through animal damage to the base of trees, but so far these do not appear to be very important. The most dangerous of them is probably extraction damage, particularly to the roots. There is no evidence whatsoever to confirm the view that infection can enter directly from the soil or that dead roots are a normal mode of entry. Infection has never been found in first-rotation crops before thinning except in a very few cases where some special mode of entry could be traced; for instance the cutting of young stock for Christmas trees, has, on occasion, been the cause of early infection.
Factors in infection and development
Although many factors, such as variations in species reaction, in site factors, in tree health and vigor and in fungal competition, play a part in controlling the level of infection, the overriding limiting factor is the size and availability of the infection sources. By reason of the relatively limited history of past conifer plantings in the United Kingdom, the present level of infection is often low. Increase and build-up infection is measured in decades even under conditions most favorable to the fungus, so that for many years most of the large new areas of coniferous forest would, in the absence of crop protection, be going through the early stages of initial infection and build-up, with relatively little damage to show. Some of the older forest areas on the other hand are now showing the effects of a prolonged period of build-up, and damage is already quite severe in young second rotation crops.
One may expect any conifer plantation, however isolated, to become infected to some degree during the course of a rotation. Isolation never appears to be sufficient to ensure immunity from attack; though the likelihood of early infection occurring and the degree of initial infection is certainly very considerably affected by this factor. On the other hand, where spore sources are nearer to hand, more rapid establishment of infection occurs and it is in such forests that the position is most immediately dangerous.
Once infection is present in a plantation, a slow spread through root systems gradually takes place and in addition, and this is almost certainly of more importance, the local production of sporophores increases the Fomes air spore in the vicinity and thus greatly increases the likelihood of further stump infections following later thinnings. Since spores travel freely for quite long distances the total risk of stump infections in any one plantation is dependent also upon its position in relation to the remainder of the forest. Mutual build-up of infection between plantations is quite as important as the developments in any single one. This factor is one of the main differences between present-day conditions and those of the past. Most of the new conifer areas are in relatively large blocks, where the arrival of infection in any one part will in time considerably affect the position as regards Fomes in the remainder. In the past, when small scattered areas of woodland were more usual, isolation effects were considerably greater. Secondly, most of the new forests start off with at least a small area in or near the forest on which conifers have already been planted and where infection sources are thus already established. The conifer plantings of the last century were not usually exposed to infection in this way.
Perhaps the most significant finding in the research work to-date is that rot infection is worse, and usually much worse, in second rotation crops than in first. There is no evidence of rot infection dying out or decreasing in a crop or on a site once it has become established.
Rate of development
It is essential to have the time factor in mind in considering the future behavior of Fomes. This fungus is not one of those pathogens the whole development of which occurs in a single season and the incidence of which varies with yearly changes in conditions. Fomes is a fungus which, once it enters the tree, stays with it until long after death and continues as part of the birthright of future forest generations. From its initial entry point it only slowly invades the tree and spreads to other surrounding trees. Rates of Fomes development vary considerably, and rot attack and build-up normally proceeds much more slowly than in the case of death. On average, some six to ten years elapse between a stump infection and the appearance of fungal stain in the butt of the nearest tree, and the appearance of sporophores, even at a low level of frequency, in most species, is even longer delayed. Thus both spread by growth through the root system and the increase in the local air spore necessary for fresh infections will be very limited for quite a long time after initial infections from an outside source. It will be readily understood that, during the first rotation, Fomes often proceeds only as far as the stages of entry and establishment with a limited degree of damage. It is usually only in the second rotation that the attack tends to assume really serious proportions. Where, by lack of thinning, infection does not enter until quite late in the first crop, development is even longer delayed. Thus, although it is possible clearly to follow the trend of development and to get evidence all pointing to the eventual occurrence of widespread and serious levels of attack, the change over short periods of time is often so slight as to go unnoticed.
Killing attack
Although killing attack by Fomes is such a striking phenomenon and although in some localities it can do quite severe damage, its importance in the country as a whole is limited. Compared with the problem of butt-rot, that of death gives relatively little concern. The other aspects of Fomes attack, namely windblow and loss of increment, are likely in the long-term to prove of greater importance than death. Nevertheless, where death of trees does occur, its effect can be quite severe and the presence of the fungus always has other implications than the killing it causes. Pine is the main species attacked in this way, although most species can occasionally succumb, especially in the first few years after the replanting of an infected site. Even in the case of pine, however, only a small proportion of plantations are at all seriously affected by killing attack and most of those, which are affected, have an alkaline soil reaction or a history of agricultural usage.
Lost increment and windblow caused by root-rot
Fomes is a root-rot fungus and by causing death of roots it is able to affect to a considerable degree their two primary functions, uptake and anchorage, as well as causing the more extreme condition of death of the whole tree. A great deal is not as yet known about the effect of root-rot on increment, but work carried out elsewhere has strongly suggested that such losses can under some circumstances be greater than those caused by butt-rot. Losses due to such a cause can very easily go unnoticed, or be ascribed to site factors rather than the direct effect of long-term loss of roots. It is very easy to remain unaware of a factor so completely hidden as this, and foresters are accustomed to a pathology involving noticeable symptoms. In fact, among all four effects of Fomes attack, it is only death, possibly the least important, which is not to some extent hidden.
It is a matter of common observation that blown trees often exhibit rotted roots. Some of the bad blows in North America have been directly ascribed to the effect of root rots. In the United Kingdom, Douglas fir appears to be the species most affected in this way. Even quite young trees have had all their main roots completely rotted. There is often a strong production of fine secondary feeding roots in this species and therefore growth does not appear to suffer. There can be no doubt, however, about the lack of stability of such damaged root systems. It is particularly interesting that Douglas fir, which is relatively resistant to the development of butt-rot, is one of the conifers most subject to root-rot by Fomes, with a consequent tendency to windblow.
Windbreak associated with rot has only been found in older trees. Some trees have broken off in the ground at the rotted base, some have snapped at ground level and some have crumpled at a point in the rotted #fem. This type of damage may appear more frequently as crops grow older.
The principle of crop protection
The point at which it is easiest to control the development of Fomes is at the critical stage of initial entry into the crop. This applies not only to ground carrying its first conifer crop, but also where infection is already established. The newly-cut stump, which provides such an ideal medium for the germination and growth of Fomes spores, is in a sense selective, since Fomes can colonize some conifer stumps, and especially those of pine, more readily than most other fungi. There are several ways in which this situation may be changed; by providing a complete barrier against all fungi and relying upon the invasion in time by saprophytes from the soil; by using a protectant which is selective, allowing harmless fungi to become established but keeping out the pathogen; or by using a material which alters the substrate value of the stump and alters its selectivity. The whole subject of the fungal ecology of stumps is in its infancy, but it is already known that a number of substances give good protection and that among these creosote, if properly selected and applied, remains the surest. It is likely that in time a change will be made to a chemical or to a mixture of chemicals which, while still providing a surface barrier, also causes more rapid killing of the stump, thus altering its condition rather than tending to preserve its initial state, as creosote does.
The practice of protection
Creosote is not a single substance of constant composition but a general term for a product of considerable variability. For stump treatment, as indeed for wood preservation, some creosotes are not nearly so effective as others and care must be taken to avoid those which are unsuitable. A creosote of standard specification or a gasworks creosote of known quality is recommended. Care must always be taken to avoid adulteration. Where poor quality creosote has been used, serious breakdown in protection has been found to occur.
Two other important causes of breakdown have emerged: the use of insufficient quantities and delay in application. It is false economy to make the creosote cover the maximum area. What is wanted is copious application in order to get good penetration. While too much run-off over the bark would be wasteful, some waste is preferable to an over-light application. Care must be taken to avoid missing any bare wood such as axe cuts, and particular care must be taken to cover jagged breaks. Since the creosote only accounts for approximately one fifth of the total cost, the rest being labor, the obvious aim is to do the job as quickly as possible and not to be over-careful with the material. A short-handled tar brush with soft blistles which hold a large volume of material is most suitable. Under easy, level conditions, the open pot and brush method has proved satisfactory but the hilly nature of many forests presents difficulties. The best answer so far found is a one-gallon (4.5 liter) spouted and handled paraffin can as shown in the Figure. The creosote is poured onto the stump and then spread with the brush. The wide base and the small spout considerably reduce the danger of spill. Cans can now be purchased with a polythene spout which is less subject to damage.
Regarding time of application it has been found that, although quite good results may follow treatment the same day as the felling, breakdown sometimes occurs, particularly where resin production from the cut surface interferes with creosote penetration. The possibility of spores being carried some distance into the wood by rainfall is another reason for avoiding delay and in practice the aim must be immediate application, delay not being allowed to exceed half an hour at most. The men felling the trees should carry the creosote with them and treat the stumps an they go. This will avoid going over the ground twice and eliminate the difficulty, when a series of trees are felled, of stumps becoming covered with brushwood before they have been treated. A few untreated stumps left covered over with debris can still let infection into a crop, so also may stumps subsequently damaged in extraction operations. Creosote should never be used on wounds of living trees.
