November 25, 2017

The future of tree diseases

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What has caused the epidemics that are currently decimating our trees? The factors involved are intimately linked to economic development – and the only hope for our trees lies in de-industrialisation.

Published in The Ecologist Vol. 9, Nos. 4–5, August 1979.


Trees are threatened with extinction. Those that survive the woodman’s axe and the developer’s bulldozer are struck down or menaced by an increasing number of epidemics. At the beginning of this century the American chestnut Castanea dentata which once made up 25 percent of the forests in the eastern half of the US was stricken by the fungus Endothia parasitica. It has now virtually disappeared from the landscape. The loss is incalculable.

Among other things its timber was highly prized for its beautiful grain and its resistance to dry rot. Thirty thousand million board feet of it are estimated to have been lost. The wood contained tannin that is used for making leather. The industry that extracted it is now bankrupt. The chestnut also provided a habitat for vast populations of squirrels and deer that fed on the nuts and great flocks of wild turkey. These have been decimated. What is more the fungus crossed the Atlantic where it is now wiping out European chestnut groves in Southern Italy. The loss was estimated 12 years ago at $1,000 million. [Carefoot and Sprott, 1967] But is money the right currency for expressing such a loss?

In 1930, forests containing 1,000 million American elms were struck with Dutch elm disease, which spread from the Atlantic along the St. Lawrence watershed around the Great Lakes, from Maine to Minnesota. Forty years later few survivors remained. Another 500 million or so elms scattered on farmland, along country roads and city streets were also annihilated.

The cost was estimated, twelve years ago, at $50,000 million, which included $12,000 million spent on cutting down the dead trees and burning them to kill the fungus; $3,000 million for replanting resistant trees; and $250 million a year to inject amenity trees with chemicals. [Carefoot and Sprott, 1967]

In the 1940s it was the turn of the oaks of which there were in America an estimated 12,000 million belonging to 35 different species. The disease was triggered off by a sap fungus which entered the tree through punctures in the bark mainly caused by an oak bark beetle. The disease is still spreading at the rate of about 50 miles a year. It is not as lethal as Dutch elm disease or chestnut blight, but is nevertheless decimating oak stands over a wide area.

The cost of this damage is enormous. The oak forests of America represented, among other things, over a billion feet of timber. At the low price of $30 per thousand this would mean $30 billion, and at today’s prices possibly as much as $100 billion. But this is nothing compared to what would be the biological and ecological, not to mention the aesthetic costs of the destruction of America’s oaks.

Only the Ash to remain?

I have mentioned the three tree epidemics that have attracted the most attention, but these are by no means the only ones that have broken out in recent years. For instance, a disease of the maple tree has been spreading southwards from Nova Scotia. A fungus associated with a tree scale insect is afflicting the hemlock, the red pine and the beech in New England and threatens to spread across much of America and possibly to Britain.

Another fungus, Ceratocystis ulmi, is killing off the plane trees in the southern part of France, while yet another, Coiyneum cardinale, is killing cypresses in south and central Italy. In California the Monterey cypress is in trouble. In South West Australia a particularly virulent disease is destroying vast tracts of eucalyptus forests. In New Zealand, willows are dying and in Jamaica and Florida the precious coconut palm, on which tens of millions of people depend for their livelihood, is being annihilated by Lethal Yellowing disease which is now spreading to the neighbouring islands.

In Britain the countryside is studded with dead and dying elms, our beech trees appear to be affected by some yet unidentified ailment which may or may not be related to the disease affecting the beech trees in New England, [Lonsdale, 1979] while sycamores in certain areas are afflicted with Sooty bark disease.

Elm, beech, sycamore, oak and ash make up the vast majority of the larger deciduous trees in this country. If the first of these is already being annihilated, and the second and third are afflicted – no-one knows with what consequences – the prospect is indeed grim. If oak wilt were to cross the Atlantic, this would leave us with only the ash – a truly terrifying prospect.

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Playing down the Problem

The temptation for foresters and plant pathologists is to play down the current epidemics. They like to think that things are much as they have always been. This leads to the comforting thought that everything they learned at university is as valid today as it was then, and that there is no need to bring about any radical change to current attitudes or current forest practices.

Even Dr. Burdekin, the tree pathologist of the Forestry Commission, tries to convey this impression to the public. In an article in The Times, he quotes a letter that appeared on 11 June 1977 in the same paper, lamenting the disappearance of elms from the British countryside and showing that identical sentiments were expressed 40 years earlier – on November 11th 1930, also in a letter to The Times.

This is supposed to justify his conclusion that ‘times do not really change’. The opposite is in fact the truth. Times have changed, and very dramatically at that, and the question we must ask ourselves today is whether our trees will survive these changes?

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The technological approach to tree disease

It is the object of this article to try and answer this question. To do this we must undoubtedly review the major diseases affecting trees today. But I propose to inquire more deeply into the subject. Tree diseases are merely instances of diseases in general and must be bound by the same set of principles. So it is disease itself that we must consider.

Now there are two very conflicting approaches to the study of disease. The first is the technological approach. A disease is empirically associated with a parasite. The parasite is taken to be the ’cause’ of the disease, which it is assumed, can only be cured by eliminating it.

The second approach is the ecological one which I shall consider later. The technological approach is very convenient since it provides the rationale for indulging in precisely what our society is organised and motivated to indulge in – economic enterprises, in this case, in the form of large-scale spraying programmes which contribute to GNP and provide research grants for scientists, development grants for technologists, profits for industrialists, dividends for shareholders and jobs for all. It also brings the rapid results which are required in a society that is so little concerned with medium to long term consequences.

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Spray, spray and spray again

Systematic spraying is, of course, very irresponsible in that its effect on populations of insects, fungi and the various micro-organisms that inhabit the forest soil, cannot be predicted with accuracy. Even if it could, this would not necessarily help, as the exact ecological functions of the different populations in maintaining the fertility of the soil and contributing to the health of the trees and to the ecosystems as a whole is generally not known. What is known is that these populations will he significantly affected. [Kuhnelt 1976] Populations of some species will increase, others will decrease.

Since pesticides accumulate up the food chain, those at the top, i.e. predators, will be most adversely affected and it is these, it must be remembered, that in normal conditions, are responsible for controlling the population of target species. All sorts of micro-organisms live in the tree’s roots, in symbiosis with the tree, contributing in all sorts of subtle ways to its long term health – these also can be affected.

Resistance, too, builds up very quickly among insects and micro-organisms to chemical poisons. Hundreds of insect species are already resistant to DDT and other pesticides. In any case insecticides can only really eliminate an epidemic if they can exterminate the pest population involved. But all they in fact do is thin it out, killing at most 80-90 percent of it.

As a result, the survivors, now in possession of an ample food supply, will tend to proliferate. What is more, the pesticides will give rise to a new population which, being descended from the survivors, must display some resistance to the chemicals used and is likely to be tougher and more difficult to eliminate.

More often than not, the plant pathologist faced with such a situation simply orders further spraying, which can only lead to a further increase in resistance to the chemical used, and of course to further biological and ecological deterioration. For these reasons attempts to control major plant pests by large scale spraying programmes have almost always failed.

What is more, except in the worst cases, the outbreak, if allowed to take its course, would have died a natural death. Natural controls would have eventually restored ecological stability, as shown in the following examples.

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The Douglas Fir Tussock moth

Under ‘normal’ conditions this moth causes little or no noticeable defoliation. However on certain occasions, for reasons that are probably related to climate, the tussock moth population of a particular area can explode and cause a great deal of defoliation. Some 25 – 30 percent of the trees can die over a period of three years.

Most of them, however, die as an indirect consequence of defoliation, actually succumbing to bark beetles and other invaders. What is particularly important is that tussock moth outbreaks usually only last three years – after which the entire population tends to collapse as a result of an attack by their natural enemies, in particular a polyhedrosis virus.

For the last 25 years, the tendency has been to spray the affected forest. There is no evidence however that, this has had any effect. A study by the US forest services, for instance, concluded that “limited comparisons in California of two chemically treated areas with two untreated areas showed no significant differences in total tree mortality”.

In both cases, the virus infection seemed to be the main cause of the decline of the moth population. Another report showed that 99.9 percent of the population of tussock moths in the Blue Mountain epidemic in 1972 had died by the end of the following year of natural causes. The polyhedrosis virus being mainly responsible. [Hermann, 1973]

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The gypsy moth

The gypsy moth feeds on oak leaves, and when its population explodes it can cause considerable defoliation. However these population explosions cannot be sustained for long.

“Two years of defoliation are usually followed by a population crash with dispersal, disease and parasitization all contributing to a drastic reduction in gypsy moth numbers.” [Hinckley 1972]

The affected trees tend to be leafless by the end of June but can refoliate during July and August. Some branches of older trees may die, but it is largely the pines which have little capacity for refoliation or deciduous trees in the forest under-storey that tend to succumb. As Hinckley points out,

“if this process continues long enough, a different forest emerges, one no less interesting and more inbalance with the gypsy moth. Lumber interests would have to put up with a temporary loss in production, but this would be made good if they had the patience to wait for the forest to recover. Unfortunately they do not have and insist on spraying.”

Until 1961 DDT was used in massive spray programmes in New England and New York. Caterpillars were killed in May and a lot of defoliation was prevented. The eminent ecologist Kenneth Watt showed that these spray programmes had but little effect on the population dynamics of the gypsy moth. He considers that weather conditions are more important than anything else in determining the growth and the fall of gypsy moth populations. [Kenneth Watt, 1968]

More recently Sevin (Carbaryl) has been introduced to replace DDT which is now banned in the USA. It is non-persistent and breaks down on contact with water. But to be effective it must be applied at a very specific time: just after the majority of caterpillars have begun feeding, and before the leaf canopy has built up.

Because of micro-climatic difficulties, it is impossible to apply Sevin with this sort of precision over large areas, and as a result the pesticide merely tends to thin out the caterpillar populations, increase survival to pupation – because of the reduced competition for food – and hence prolong the outbreak. [Doane, 1968]

In addition this pesticide will also kill many insects, including parasitic flies and wasps, that are the natural predators on the moth. [Kamram, 1971] It will also kill bees which must thereby reduce pollination and food production.

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The spruce budworm

The spruce budworm infests spruce forests in Canada, and the response has been to spray them, as in May 1978. In 1952 DDT was sprayed on 200,000 acres of forests in New Brunswick. The main effect was to increase the acreage affected and the need for further spraying operations. In 1963, about 25 percent of the land area of the province was affected – but by 1973, after 20 years spraying, the figure was closer to 90 percent. In 1976 9.5 million acres of forest had to be sprayed.

Eventually DDT was abandoned and an organophosphate called fenitrothion was used instead, but this has not been any more successful. As Elizabeth May writes,

“the killing of a large proportion of the budworm population (about 85 percent) was termed successful but left the survivors with an abundant food supply; starvation was no longer a limit to population growth. The lethal effect of both DDT and fenitrothion on the birds, small insects, spiders and wasps which prey on the budworms removed another check. The only remaining check on unlimited population growth was the annual dousing with chemicals from the air – the very thing which allowed the infestation to continue and spread.”

The perpetual epidemic created in New Brunswick created a breeding ground for infestations in other areas, and forced both Quebec and Maine to start spraying too (Robert Paelhke 1978).

The biological and ecological damage done by all this spraying was of course immense. Among other things it caused an outbreak of a disease called Reye ‘s Syndrome – unknown before the 1950s – which causes liver and brain damage and is frequently fatal, primarily affecting rural children.

Recently, Prince Edward Island decided against spraying and instead set about replanting more varied, more resistant species, to replace the dying spruces. However, as Paellike points out,

“in all of the provinces where forestry was big business spraying was either carried out or fought hard for by the big forest companies. Their lack of concern with anything but maximising short term yields is truly disgraceful.” [Paelhke, 1971]

In Nova Scotia where spraying had not occurred, the infestation came to an end by itself. When Nova Scotia’s Forest Industries (NSFI) requested a permit to spray 100,000 acres of Cape Breton forests with fenitrothion, Nova Scotia’s Deputy Minister of Lands and Forests turned down the request stating that

“bud worms have hit Cape Breton before but they died out on their own . . . We feel that it is far better from a forestry point of view to suffer our losses now, rather than spray and prolong the inevitable, as New Brunswick has done. The forests of New Brunswick after 25 years of spraying certainly are not the envy of any one involved in proper forest management.”

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Other spraying failures: when will the experts learn?

There have also been attempts to use insecticides in the fight against Dutch elm disease in the US. Its use, to quote Frank Graham Jr., provided scientists with . . .

“one of the classic environmental horror stories. Enormous amounts of long-lasting insecticides were sprayed on American cities and towns. Robins, feeding on earthworms which earlier had fed on the sprayed leaves, died in untold numbers. Sewers carried DDT residues from city streets into rivers and lakes, where destruction became magnified. Yet, alter great financial and environmental cost, the cities lost their elms anyway, and the disease kept spreading into new areas.”

Attempts to save trees affected by serious tree diseases by injecting them with chemicals have been a total failure. Efforts to inject trees afflicted with chestnut blight or Dutch elm disease have failed. First of all the cost is very much too high which means that the method can only be used for saving a few amenity trees around public buildings, for instance, or in private gardens.

In any case resistance soon builds up against the chemical. According to William B. Ennis of the Agricultural Research Centre, efforts to inject palm trees threatened with Lethal Yellowing disease can slow the progress of the disease but that is all, the tree cannot be cured and will eventually die.

Unfortunately, the experts never learn – largely of course, because they do not want to. In spite of the almost universal failure of spraying campaigns they remain the main weapon in the armoury used by plant pathologists. In this country, the Forestry Commission in 1978 mounted what was in its own words “its biggest ever aerial spraying operation”. [Press release 1, 1978]. Twelve thousand acres of lodgepole pine plantation infested with the larvae of the pine beauty moth (Flammea panolis) were sprayed with fenitrothion – precisely the same pesticide use so unsuccessfully in New Brunwick and elsewhere.

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Health and Stability: the ecological approach

We have seen that the technological approach to tree disease is an abortive one. If the parasite is the only cause of the disease then there is no solution to the problem because once the parasite is established we shall never succeed in eliminating it. Fortunately, the parasite is only the ’cause’ of the disease in the very narrowest sense of the term. The problem is a very much more complex one, and cannot be understood in terms of the simple cause and effect relationships so beloved by our misguided empiricist philosophers.

To understand the real cause of the disease we must look much more closely into the question of disease in general, and this we cannot do until we can first explain what is ‘health’ – from which ‘disease’ is nothing more than some sort of deviation.

The term is normally applied to biological organisms only. A biological organism, however, is simply an instance of a natural system. If ‘health’ is a basic term, like such terms as ‘control’, ‘stability’, ‘order’ etc, then we should also be able to talk of the health of other natural systems such as ecosystems, social systems etc.

If a system is healthy, this can only mean that it is stable, i.e. that it functions properly. That state can only be established if one knows what is the system’s goal. Since we know this to be the achievement of stability or continuity or homeorhesos, to use C. H. Waddington’s term, [Waddington, 1957] then one can regard a system to be functioning properly to the extent that it achieves this goal. In other words health equals stability.

A natural system is hierarchically organised, i.e. made up of sub-systems and sub sub-systems. To maintain its structure, these must all fulfil their appointed functions and thereby cooperate towards the achievement of a common goal. Those that do not and have thereby ceased to be viable tend to be eliminated by natural selection. In this way ‘noise’ or ‘randomness’ or ‘entropy’ is reduced to a minimum, organisation or negative-entropy is maximised and the viability of a system, and hence its adaptiveness, stability or health, is maximised.

Disease in a stable society is, in fact, but a means of natural selection which explains why it eliminates mainly the weak and the sickly. If it does not eradicate the healthy, it is because it is not adaptive to do so, still less to wipe out whole populations of healthy organisms. It is the strategy of nature precisely to avoid such things. To achieve stability means precisely that: reducing discontinuities of this sort to a minimum.

Hence epidemics do not occur in stable ecosystems any more than do other major discontinuities such as large scale droughts, floods or massacres. Their occurrence is a sign that something has gone wrong, that the system has ceased to be stable, that a serious maladjustment has occurred.

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