Forest Health > Insects and Diseases publication
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 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.
Figure 7. Some common types of disease problems affecting trees
Figure 8. Generalized life cycle for tree disease pathogens
TABLE V.
Some Common Causes of Tree Disease |
|
Non-Living
(Abiotic) Factors |
Living
(Biotic) Agents
|
Pathogens
|
| Temperature
Extremes |
Fungi
(currently constitute the largest and best known group of tree pathogens) |
Lower
plants with threadlike (filamentous) vegetative stages
(single threads being referred to as hyphae: sing.,
hypha). Typically reproduce via spores produced in or
on various structures called fruiting bodies. Fruiting
bodies range from microscopic to large (mushrooms, etc.)
depending upon the particular fungi involved. |
| Moisture
Extremes |
Bacteria |
Microscopic,
single-celled plants with cell walls. Reproduce by fission
(splitting) or budding. |
| Soil
Compaction |
Viruses |
Sub-microscopic
particles consisting specific nucleic acid molecules
covered with protein coats. |
| Nutrient
(Fertilizer) Deficiencies |
Seed
Plants |
Higher
plants that reproduce by seed. |
| Chemical
Injuries (herbicides, excess fertilizers, etc.) |
Mycoplasmas |
Submicroscopic
organisms more complex (advanced) than viruses, but
less complex (lower) than bacteria. Mycoplasmas have
no cell walls. Reproduce by budding or fission, and
are typically found in the phloem of host plants or
trees. |
| Air
Pollutants |
Nematodes |
Minute
to microscopic worms. |
Contact your local Division of Forestry Field Office if you have questions or need assistance with a pest problem or tree care and maintenance.
Bulletin No. 196-A | Printed October, 1983 | Contact the Forest Health Section |
