Forest and Shade Tree Diseases (Pathology)
Concepts of Tree Disease
Disease,
as it pertains to trees, may be defined as a sustained and progressive
impairment of the structure or function of any part of a living
tree. Diseases are caused by a variety of factors or agents which
are divided into two general groups: non-living (abiotic) and
living (biotic). Biotic agents are called Pathogens. Generally,
it is essential to know the specific cause or causes of a tree
disease to determine
(a) what course of remedial action (treatment), if
any, might be effective, and
(b) the prognosis for survival and
recovery of the diseased tree.
Accurate identification of specific
causes of tree diseases is important (if not essential) for
preventing or avoiding repeat problems in the future. Determining
the cause of tree disease can be compared to the work of a detective.
This process is commonly known as Diagnosis. One begins
by examining the available evidence and considering all related
circumstances. Visible evidence used in the diagnosis of disease
is typically classified as either a symptom or a sign. Symptoms
are the abnormal characteristics expressed by a diseased tree
(e.g., dieback, foliage discoloration, decay, galls, wilting,
etc.).
A Sign is the physical presence of a causal agent (e.g.,
fungus, parasitic plant, an empty herbicide container, etc.).
Related Circumstances are often extremely important in properly
diagnosing a tree disease problem. For example, has the tree
been exposed to severe or unusual weather conditions? Has there
been a history of site disturbance such as building or road
construction near the tree? What is the history of tree removal
or thinning in the area (particularly important in forest stands)?
Has the tree been fertilized lately? What are the predominant
soil and/or drainage features in the area? Have any chemical
spills occurred in the area? Has a herbicide been used? Is there
a source of an air pollutant nearby? Good detectives often make
good diagnosticians.
The role of the environment in the development of tree disease
cannot be overemphasized. It is important to understand that
in every situation, environmental influences such as temperature,
moisture, and soil conditions influence the biology of the tree,
the activity of the pathogen or causal agent, and thus the development
of disease. Disease development may be enhanced or retarded
by changes in the surrounding environment, but the environment will influence disease. In fact, many tree diseases are
more a function of environmental influences (e.g., abiotic factors)
than they are of specific pathogens per se. In other
cases, certain pathogens are able to cause disease only because
of prevailing environmental influences. Always consider the
environment!
Don't be fooled by the appearance of a suspicious looking fungus
on or near an ailing tree. It may be a result, not a cause,
or it may be totally unrelated to the tree's disease. Many fungi
develop only on dead or dying trees and are not pathogens. These
fungi are Saprophytes and are living on dead tree tissues or
organic debris as opposed to pathogens which usually gain their
sustenance from living trees as Parasites.
Other fungi, especially certain mushroom or toadstool types,
are actually beneficial to trees. Many of these types of fungi
form highly specialized, mutually beneficial associations called
Mycorrhizae with the roots of living trees. In these associations
the fungi receive sugar and other dietary essentials from the
trees, and in return enhance the tree's ability to extract phosphorus
and other nutrients from the soil.
Careful application of these simple principles and concepts,
together with a basic understanding of the growth or site requirements
of trees being examined, provides even the novice with a sound
approach for identifying many of the causes of tree disease.
However, not all disease problems can be identified in the field,
not even by trained experts. Some diseases require detailed
laboratory analyses for positive identification of specific
causal agents. The following pages provide brief and simple
descriptions of some of the most common diseases affecting Florida's
more important tree species. The use of these descriptions,
together with appropriate consultation of trained specialists
should provide the interested forester, landowner, arboriculturalist,
etc., with a reasonable base of information for dealing with
tree disease problems.
There are many ways to catalog or categorize tree diseases.
The following groupings are provided to facilitate retrieval
of specific information available on this website.
Foliage Diseases:
Includes leaf spots, anthracnoses, blights,
foliage rusts, needlecasts (of conifers), powdery mildews, sooty
molds, leaf blisters and others.
Note: discolored, wilted, stunted, or dropping
leaves are often indicative of problems other than foliage diseases.
Stem Diseases:
This group is diverse and includes galls (hard,tumor-like
growths of infected plant/tree tissue), cankers (infections
of and necrosis of cambial tissues resulting in stem or branch
girdling and depressed or flattened areas on infected stems),
decays (the enzymatic destruction of heartwood or sapwood tissues,
typically by basidiomycetous, mushroom- or conk-forming fungi)
and vascular wilts (infections whereby pathogens invade and
occlude or render dysfunctional the vascular elements of their
hosts' xylem tissues).
Root Diseases:
A vast array of infections ranging from
feeder root necrosis to vascular occlusions and root rots (decays)
that, depending upon disease type and severity, interfere with
nutrient and water uptake, tree growth and survival, and tree
stability.
Forest Tree Nursery and Regeneration Diseases:
A dissimilar
grouping of a variety of diseases (and related issues) that
are characteristically limited or pertinent or problematic in
forest tree nurseries and/or forest regeneration settings.
Miscellaneous:
Forest Health Publications
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