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INT 256: Tree Pests and Disease
Introduction to the
Concept of Disease Complexes
What are Disease Complexes?
Disease complexes refer to all the factors involved in causing disease. I use this concept because I have found it difficult
to teach traditional concepts of disease. For example, most students think of tree disease
as being a fungus. I have generally found that traditional approaches to plant pathology
will only reinforce this perception. By using the term "Disease Complex", the
student should immediately realize that disease is not merely the presence or absence of a
single organism. Also, I have tried to develop the concept so that all forms of stress on
a tree can be dealt with using the same approach. Therefore, my goal is to develop one set
of concepts for dealing with tree problems caused by different kinds of stress agents
including fungi, insects, or abiotic stresses.
These concepts are not perfect and will be revised as needed. Please send me any
comments you have concerning the contents of this or other pages at this web site.
William H. Livingston
Associate Professor of Forest Resources
University of Maine
School of Forest Resources
5755 Nutting Hall
Orono, ME 04469
ph: 207-581-2990
fx: 207-581-4257
e-mail: WilliamL@maine.edu
March 3, 1997
Key Concepts - Listing
Key Concepts - Details
 | Persistent, detrimental functioning in a system.
 | Detrimental functioning is caused by a disruption of the equilibrium between energy
generating and energy utilizing processes ( see Bateman, D.F. 1978. The dynamic nature of
disease. p. 53-83 in Horsfall, J.G., and E.B. Cowling, eds. Plant Disease: An Advanced
Treatise. Vol. III. How Plants Suffer from Disease. Academic Press, New York. 487 pp.). |
| Disruption persists; the system's recovery to normal functioning is delayed such that it
no longer responds normally to the environment. |
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| Implications of this definition.
| Other definitions of disease refer to a cause (e.g. "caused by a persistent
agent"). I believe this helps to reinforce the perception that a disease is a fungus.
By not including reference to a cause in a disease definition, the emphasis is on the
detrimental functioning and whether it persists or not. |
| Causes of tree disease can be any factor causing a persistent, detrimental functioning
in a tree. This does include fungal infection, insect feeding, and injury due to an
abiotic stress. Insects do cause disease in trees. |
| Using the concepts of disease as a basis for defining health
becomes more feasible. |
| Referring to a "system" means that disease can be defined for any level of
biological organization: Cells, tissues, organism, and forests can all be diseased. |
| Disease at one level doesn't necessarily mean disease at the next level.
 | Natural thinning agents (the affected trees are diseased) is necessary for a healthy
forest. |
| Tissue disease and tree disease.
| Tissue disease is a persistent,
detrimental functioning in a tissue. This does not necessarily mean that the other
tissues in a tree are functioning detrimentally. If other tissues in a tree are responding
normally to the environment, then the tree system is not diseased. |
| Tree disease is when there is a persistent,
detrimental functioning in the tree organism, i.e., most or all tissues are affected. When
a tissue disease results in the persistent, detrimental functioning of
other tissues, then there is a tree disease. |
| For the purpose of fiber production, tissue disease such as a foliar
disease is not important until it results in a tree disease. |
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| Persistent, normal functioning in a system.
| System functioning is set by an equilibrium between energy generating and energy
utilizing processes. |
| Can recover from equilibrium disruption such that responses to the environment are
normal (see normal functioning). |
| Can maintain vitality. |
| Has vigor. |
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| Adaptation: Functioning is set by how a system (tree) has evolved
(adapted) to environmental stimuli.
| In a healthy tree, the adapted responses to stimuli should persist over time (e.g., if
it is wet and warm in June, there should be good growth). |
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| Age: The level of functioning as it has evolved to the environment is
influenced by the age of a system (tree). |
Disease Complexes: Factors Involved in Causing Disease
Summary.
Details.
| Cause and effect.
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| Disease complex name:
| This is the common name for the system of factors causing disease, e.g. "white pine
blister rust." |
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| Components.
| Tree.
| Name of susceptible species. |
| Symptoms: the visible expression of disease. |
| What is not functioning properly?
| Tissue disease: Which tissue functions are being
directly affected by the disease? For example, photosynthesis in leaf diseases. |
| Tree disease: Are other tree functions being
adversely affected by the disease? For example, is the leaf disease causing growth
reduction or dieback? |
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| Primary Stress(es): A factor(s) adversely
affecting tree functions.
| Types of stress agents.
| Abiotic. |
| Biotic: biotic stress agents are commonly called pathogens. |
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| Signs: Visible evidence of the stress agent. |
| Mechanism: How does the stress agent cause the
detrimental functioning in the tree? |
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| Environment.
| Where a disease complex is likely to occur. |
| Examples: natural forest, plantation, urban forest, nursery. |
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| Disease development - interaction of
components in a disease complex - if and how much disease occurs.
| Predisposing factors: Factors making
disease possible.
| Factors Affecting Tree Responses:
| Factors indicating how well adapted a tree is to the primary stress(es)(pathogen).
| Resistance: Ability to prevent disease. |
| Resistant: Has ability to keep stress from
causing disease under given conditions. |
| Susceptible: If stress is present, disease is
likely under given conditions. |
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| Adaptation to site, climate, management.
| Trees less adapted to their environment are more susceptible
to adverse effects of predisposing stresses ; less stress
is needed to have an effect. |
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| Tree Age:
| The very young and the very old are usually the most susceptible to stresses . |
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| Degree of Stress: Stress is always or
frequently present.
| For biotic stress:
| Environment and tree interactions influence population numbers; life cycles must be
known. |
| Spatial (landscape) interactions influence population numbers. |
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| For abiotic stress:
| The environment (site and climate) influence the severity of stress (e.g., sand and
drought). |
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| Healthy trees can recover from natural variations in
predisposing stresses. |
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| Inciting stress initiating
disease. Tree recovery from stress is delayed or does not occur; the tree can’t
respond normally to the environment.
| Usually a Pathogen if:
| Little adaptation by tree to Pathogen (e.g. exotic
pathogen). |
| Stand or landscape changes (forest management) favor pathogen. |
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| Usually abiotic (e.g. climatic) if:
| Little adaptation by tree to site &/or climate (e.g. growing species
off-site). |
| If tree adapted to pathogen and site. |
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| Other? |
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| Contributing factors: Factors
detrimentally affecting the tree only after the inciting
conditions have initiated the disease . |
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Complexity in causes of disease.
| Energy available for survival. |
| Vigorous trees can be less susceptible to disease because they have more energy to resist pathogens and to
repair equilibrium disruptions. |
| Even a vigorous tree is susceptible to disease if it is not well adapted to a stress
(e.g., infection by Cryphonectria parasitica) |
| Ability to survive. |
| This depends on a system's (tree) adaptation as well as available
energy. |
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