Diseases and Forest Ecology


Management of most diseases is covered on other pages with the diseases, so we won’t get into details here. Following is an overview and a few relevant odds and ends.


Epidemiology is the study of disease in populations and factors that affect rate of disease increase. Here are the types of the things that fall under the rubric of epidemiology:

  • Mathematical analysis of disease over time in populations.
  • Study of factors affecting rate of disease progress: time of leaf wetness, weather, plant development, etc. Mostly environmental factors.
  • Modelling of disease epidemics. Allows one to vary starting conditions and see the effects. In some cases these are set up like games where you are a farmer and try to maximize income. No games for forest diseases!
  • Prediction of effects of disease management methods.
  • Monitoring for infection conditions to determine when fungicides or other measures are necessary. Reduces pesticide use. This is an important part of IPM for diseases of orchard crops and other agricultural crops. Instead of spraying according to the calendar on a regular basis, the grower may have a little weather station with a computer attached that tells him when sprays are really necessary. Or may subscribe to a disease forecasting service for the same purpose.


Control vs. management of disease. “Control” implies to some reduction of disease to zero and absolute control over nature. Management is the word that is in vogue now, because it has an implication of tolerating a certain level of disease. It acknowledges that we may influence nature, but do not have total control over it.

The following terms tend to overlap, and are defined variously. Here are some general definitions though:

  • Symptoms: these, of course, are observable changes in the plant that indicate the presence of disease.
  • Damage, effects: decrease in quantity or quality of trees or their products (can include non-timber values here). Growth reduction may be a symptom if you are thinking of it as a way to detect disease, or an impact if you look at it from that point of view.
  • Loss, impact: decrease in economic returns and cost of measures taken to reduce it.

Quantifying diseases

Although it may not seem very interesting, quantification of disease is an important concern.

  • It is necessary for justifying control measures.
  • It is one way that institutions and scientists decide what is important to study. It can be used to justify funding for research or management.
  • Ecologically, it can be a way of determining what factors are important in determining forest structure and dynamics.

Loss to disease is difficult to quantify in forests:

  • We often don’t have enough research to know how much growth loss, mortality and cull are attributable to the disease. This can be a major study just to quantify those losses.
  • Is it a loss if the tree was not going to be harvested? We might say 4 million board feet were lost, but was anyone really going to get all those trees anyway?
  • What about compensatory growth? When one tree dies, trees beside or under it may be released and increase growth as a result. How can our loss figures account for that? If mortality is scattered randomly, compensatory growth from neighbors may cover the loss. If it is clumped into a disease center, release is less of a factor and the net loss may be more. How can we account for that?

Approaches to Management


The three most important approaches to management of tree diseases are silviculture, silviculture, and silviculture. By this we mean activities that are incorporated into stand management, using the usual techniques of silviculture. Included are such things as:

  • thinnings that remove diseased individuals or those with infection courts, removing inoculum and releasing healthy individuals.
  • practices that select resistant species when likelihood of a disease is high, by planting or favoring.
  • choosing even-aged vs. uneven-aged management based in part on disease considerations.


Breeding or genetic engineering for resistance is another approach that is increasingly important in intensive forestry. Breeding programs are underway for white pine blister rust, fusiform rust, dutch elm disease, and chestnut blight, and have already had significant success in all but the latter. Resistant cultivars or hybrids are known for some other diseases such as sycamore anthracnose. Selecting the best, probably resistant trees to serve as seed trees is a low-tech approach to “breeding” (artificial selection).


Other means of control are not commonly used. Most direct methods of control, such as fungicides, are used only in nurseries and on especially valuable landscape trees. An exception is stump treatment or removal to reduce inoculum of root-rot pathogens. It seems to be approaching the level of being operational in high value stands in the Pacific Northwest. Another is fire, which could be considered a silvicultural technique. It is sometimes used for mistletoe to kill residual small trees after a clearcut, or for brown spot needle blight on longleaf pine to reduce inoculum when seedlings are in the grass stage.



Forest pathology has always been strongly ecological. The only significant control measures that have been available have necessarily been based on understanding the ecology of diseases rather than on any chemicals that could kill the pathogen.

If ecology is study of the interaction of organisms with their environment, pathology can even be viewed as a branch of ecology. Certainly the interaction of a pathogen, a host, and the biotic and abiotic environment to produce disease can be viewed through an ecological lens. So any aspect of pathology can be considered ecology.

One can think about ecology of diseases on a small, or organismal scale. For instance, one might study conditions that affect the interaction between a pathogen and a host, or a third organism that inhibits a pathogen by competition or hyperparasitism.

But what I mean by “ecology” of forest diseases is on a larger scale. How do diseases fit into an understanding of forest ecology? How do diseases influence forest composition and dynamics? What role do they play in succession? Looked at from the other side, what characteristics of forest communities influence the distribution and abundance of diseases?

If you have an interest in forest ecology and have looked over these pages, you already have the information at your disposal to compose tentative answers to these questions. However, the discussion that follows will put some of that information in perspective and provide additional information and views on the role of diseases in forest ecology.


Where reasonably well-defined successional sequences exist in forest development, diseases may play a role in the removal of one stage to make room for the next. The following discussion is perhaps a simplistic one but will give you the general idea without a lot of confusing qualifications and exceptions.

In general, trees and other plants that colonize and dominate disturbed areas are adapted for rapid growth. They invest in rapid height growth and root growth to maximally exploit the site before other plants can. Then they invest in reproduction to maximize the chance that seeds will find and dominate another such site when it appears. They invest relatively little in resistance mechanisms that are carbohydrate- and nitrogen-intensive. These plants are said to be r-selected.

Those at the other extreme must cope with heavy competition during establishment and for most of their lives. There would seem to be little point in rapid growth when the short-term winners are already way ahead of you, nor would rapid growth be possible with resources coming so slowly. These plants are adapted for competition and for survival over the long haul. They invest more than r-selected plants in defense against herbivores and pathogens. These trees are said to be K-selected.

These categories are the extremes; there are intermediates. Also, there are some exceptions. (An alternative or extension to the r-K system, the CSR system (competitive, stress tolerant, ruderal) is no great improvement in my opinion.)

Examples of trees that might be considered more or less r-selected include striped maple, paper birch, aspen, many willows. Such tree species generally do not have durable heartwood (preformed, antifungal chemicals formed as sapwood converts to heartwood). Stem decay often begins at a fairly young age and progresses rapidly. They also tend to be relatively susceptible to common root and butt rots such as Armillaria root rot. Damaging cankers are often common in such trees.

At the other extreme, trees like coast redwood, red spruce and white oak may serve as representatives. Many have durable heartwood. Stem decay occurs in all species of trees, but in these it usually starts later and progresses slowly. Root diseases seem to be compartmentalized more effectively in these trees, so that infections stop or progress very slowly as trees get above the sapling stage. Cankers don’t seem to be both common and damaging.

Thus, diseases (and often insects) may play a major role in the process of succession. How long would those r-selected trees survive without diseases? It is impossible to say. Probably much longer. And in a broad sense of the word disease, they would last forever!

Natural Thinning

We tend to think of tree mortality as something that happens mostly to old trees. But if you look at the facts, or even think about it for a moment, you will realize that just the opposite is true. Most trees die within a few years of germination, if not during germination. Studies of diseases of seedlings in nature are rare but suggest that they can play a major role at that stage. Problems in establishment of certain species, such as red oak, may involve seed or seedling pathogens in some cases, but this has not been studied.

Although it is probably a continuum, we might identify the next stage as the sapling stage of thinning. One can often find many trees dying at this stage and observe a disease such as Armillaria root rot or cankers or rusts that have girdled and killed the trees.

The next time you are fighting your way through a thicket of young trees, try to think about what is happening to this population and put it in evolutionary terms:

  • Sometimes, most of the competing trees survive for quite a long time. This happens, for instance, with some stands of white fir in the Sierras of California. They are called dog-hair stands because the saplings are as thick as the hair on a dog’s back! The stand stagnates for long periods and no trees can grow well, but they don’t die either. Such a stand would be completely killed by fire and would be unlikely to make a mature stand of large trees.
  • In most cases, pathogens or insects thin the trees as the stand matures so that part of the population reaches reproductive maturity without other problems, such as fire or competing species, arising.
  • Now think about this in terms of selection and evolution. Could there be selection within the population as a whole for susceptibility to certain insects or pathogens that would take out some individuals without building up and decimating the population? Could this explain why some species, are susceptible to certain diseases when they are young, but become resistant when they are older? That is my hypothesis. (An example of this phenomenon is pines and spruces with Armillaria root rot. Another possibility may be black walnut and Nectria canker, but I don’t think the phenomenon is described in that case.)

Disturbance – Canopy Gaps

One of the major themes in forest ecology has been the role of small-scale disturbance (read “mortality”) on forest composition and dynamics. The gist of the idea is that disturbance creates openings (gaps) in the canopy. Nutrients, moisture and light may be more available in the gaps, allowing vegetation to become established which differs from the canopy and the normal understory in composition and age structure.

Such disturbance introduces several kinds of heterogeneity into the forest. The heterogeneity persists to some extent as trees in the gaps mature, so that the resulting forest is a patchwork of species, densities and sizes that depends on the disturbance history. To some extent, this concept is replacing the concept of a “climax” forest type that maintains itself indefinitely.

There have been few studies on the agents of disturbance but, as you might expect, diseases are frequently involved. Moreover, there is evidence that the agent of disturbance may dictate conditions in the gap and thus the vegetation that gets established.

For instance, compare a tree struck and killed by lightning with a tree uprooted by wind. The windthrow gap not only has more light and moisture reaching the forest floor, but the soil disturbance also provides opportunities for establishment not available in the lightning gap.

Now compare the windthrow gap to one caused by root and butt rot. Depending on the pathogen, host and conditions, the tree may die standing or snap near the ground level, creating different conditions. Furthermore, the gap will continue to expand in some cases as the pathogen infects neighboring tree via rhizomorphs or root contacts and grafts. The expanding gap may reach a critical size necessary for establishment of a particularly intolerant species, whereas the single-tree gaps would not.

More complex interactions are also likely, in some cases obvious. Gaps initiated by one agent may permit another agent to go to work. For example, in areas where windthrow is likely, occasional mortality from root diseases may establish foci for windthrow or chronic wind stress to work on the trees at the margin of the gap.

It is also possible that a pathogen may select among competing gap colonists. In a gap created by a root pathogen, would the species that was killed in creating the gap be taken out of the regeneration by the same pathogen?

It is important to understand these processes if we are to understand how natural ecosystems function and practice “ecosystem management.” Much research remains to be done to characterize these and other aspects of diseases and gap-phase disturbance.

Disturbance – Introduced Diseases

Introduced diseases have catastrophic effects on forest ecosystems. The permanent removal, debilitation, or reduction of a dominant canopy species dramatically changes the composition and dynamics of forests. Of interest is how the remaining species adjust to the new situation. At a coarse level, the initial changes are relatively obvious and need not be considered further here.

A Case Study of Forest Changes and Diseases

In at least two forest regions of the United States, rather rich and interesting stories have emerged that portray the ecological interactions between forest changes and forest diseases. One is the development of fusiform rust in southern pines. Here I will relate another one. This history was put together by Jim Byler, a forest pathologist formerly with the Forest Service in Montana.

The Inland Empire (eastern Oregon and Washington, western Montana and northern Idaho) were old-growth stands of mixed conifers before European influence. Frequent, low-intensity groundfires kept stands open and park-like. Douglas-fir and grand fir were components of mixed stands on some sites, but ponderosa pine, western white pine and western larch were dominant on many sites, maintained as a fire climax. Western red cedar was also present.

Timber harvesting began in the late 1800’s. The large, valuable cedar, white pine and ponderosa pine were removed first. Shade-tolerant understory species, Douglas fir and grand fir, grew up to become dominant. These species are less valuable.

Then came fire exclusion (i.e., active fighting of fires as well as forest changes, such as grazing and roads, that limit their spread). Douglas- and grand fir are relatively susceptible to fire, so fire exclusion favored them also. Sometimes, because the natural, low-intensity ground fires were prevented, larger wildfires occurred. Douglas and grand fir seedlings also became established on many of those burned sites.

Then, white pine blister rust came along. This disease, along with pole blight and mountain pine beetle, killed or stimulated the logging of remaining old-growth white pine. Furthermore, it was seen as generally inadvisable to manage for white pine. Grand fir came up in those stands.

These changes have dramatically influenced the status of diseases in those forests. Here is a capsule summary:

Changes in major forest diseases in the Inland Empire
associated with human influence

Stem decays (caused primarily by Porodaedalea piceina and Echinodontium tinctorium)Stem decays were of course abundant in the old-growth stands and have declined relative to other pests. Still, they continue to cause significant damage, particularly because true firs are relatively susceptible.
Brown cubical root and butt rot (caused by Phaeolus schweinitzii)Like the stem decays, this disease was very damaging in the old-growth forests. It is not nearly so today. Besides tree age, this is because fire scars, the major infection court for most species, are less common. Douglas fir, which can be infected directly through roots, is still damaged in some stands.
Laminated root rot (caused by Coniferiporia sulphurascens and C. weirii)In the early days, this disease was a problem only on cedar (caused by Coniferiporia weirii in the strict sense). Today it is the most serious problem in Douglas and grand fir stands (caused by C. sulphurascens). Douglas and grand fir are the most susceptible.
Armillaria root rot (caused by you know who)In early days, it commonly killed young growth of many species, but otherwise seemed to act as a weak pathogen. Today it is much more serious, probably because Douglas and grand fir are the most susceptible species.
Dwarf mistletoes (Arceuthobium spp.)These are serious problems today. There is evidence and clear mechanisms for their increase from presettlement times. Suppression of fire (which plays a natural role in sanitizing stands) and other activities have increased the damage from mistletoes.
White pine blister rust (caused by Cronartium ribicola)The introduced pathogen was not present before, now limits management for white pine.

Have we just exchanged one set of problems for another? That is an optimistic way of looking at it. The diseases that have increased are in some ways more serious because they affect trees at younger ages.