Introductory Plant Pathology

Class 3

Disease, Koch's Postulates, and Disease Triangle

Objectives for Today's Class
Reference: Agrios Chapter 1		Mastery Topics

What is Disease?

"a malfunctioning process that is caused by continuous irritation. Of course, this process must result in some suffering, and produce symptoms"

Horsfall and Cowling

"The term plant disease is properly applied to any deviation from normal growth or structure of plants that is sufficiently pronounced and permanent to produce visible symptoms or to impair quality and economic value."
- Stakman and Harrar
Any disturbance of a plant that interferes with its normal growth and development, economic value, or aesthetic quality; a continuously, often progressively affected condition in contrast to injury, which results from momentary damage."

Schumann

Any disturbance brought about by a pathogen or a consistent environmental factor which interferes with manufacture, translocation, or utilization of nutrients

Failure to reach full genetic potential due to the activities of another

Disease is not a Condition

In agreement with Horsfall and Dimond, a condition is a symptom complex. A disease is "deeper" than the symptom. A disease is the totality of the biological activity of all interactants both overt and covert.

Disease is not the Pathogen

Pathogens are the causal agents of disease. Imprecise usage of terms has lead to careless application. One hears " Phytophthora infestans is Late Blight of Potato". This misstatement fails to recognize that the organism is not the disease and that disease cannot occur in the absence of a host.

Disease is not infectious

Following the above logic, since disease is the result of host and parasite interaction; only the parasite/pathogenic partner is infectuous.

Disease is not mobile, is not disseminated

Propagules and inoculum are disseminated and the disease host may be transported; but it is incorrect to equate disease and inoculum when speaking of epidemiology or dissemination. In strictu sensu only inoculum is disseminated.

Disease and injury are not the same

Mowing a lawn may remove as much a 60% of the biomass of the grass and may cause wounding by the mower; but it is a single non-recurring event that does not cause constant irritation. As such disease is not the result of tissue removal. However, one should not ignore the tremendous wound sites produced by tissue removal and their potential for entry sites for opportunistic parasites that may lead to disease.

Principle and Concept: Koch's Postulates

In order to substantiate that an organism, group of organisms, or combination of organisms and environmental factors are causal agents for a disease one must fulfill Koch's postulates. While this principle and concept are universally accepted by all pathobiologists (plant and animal), it is all too often over looked and there are diseases with putative causal agents that have never been demonstrated by Koch's Postulates. Years of research may have been spent on organisms never proven to cause particular diseases. Whenever one speaks about, or hears about, a disease, one should always seek to find out if Koch's Postulates were performed.

In effect, Koch's Postulates are the scientific method applied to pathobiology. As such, without them Pathobiology becomes an art and not a science.

Pathogen must ALWAYS be associated with disease in ALL diseased plants. There are no exceptions allowed.
Pathogen must be isolated and established in PURE culture. This may be difficult with obligate parasites, but methodologies have been developed to fulfill this requirement even with obligate parasites.
Inoculation of a healthy plant of the same variety must reproduce EXACTLY the same symptom(s). Inoculation must be of a healthy plant of the same species and cultivar. This may be difficult if one isolates from a plant of unknown cultivar. The symptoms must be reproduced essentially identical to the initial diseased plant, taking into account differences between the initial plants environment and the health inoculate plant.
Pathogen must be reisolated from inoculated plant and its identity confirmed as the same as the original isolate. The organism recovered must be the identical to the original isolate. There are no exceptions.

The "Disease Triangle" is a central concept of plant pathology. It is based on the principle that disease is the result of an interaction between a host, a potential pathogen, and the environment. If any one of these factors is missing then disease will not occur. It is important to remember that all three components are necessary pre- and post- infection. Too often the importance of environment is forgotten once the host-parasite interaction is established. To do so is to limit potential disease management approaches.

In order to fully understand this concept one needs develop definitions and "working definitions" of; what is a host, a pathogen, and environment? Definitions are not just carefully worded phrases. They are concepts that imply meaning by usage therefore; one needs to develop a depth of cognition so that one is thoroughly familiar with the term, concept(s) and usages implied by the term.

Disease Cycle:

Both the host and the parasite have life cycles. While the life cycles of higher plants can be easily stylized and summarized, the life cycles of the organisms that cause plant disease are widely varied. The simplest is, perhaps, the viruses; which require only a vector and can reproduce nearly autonomously after infection. On the other hand, the macro-cyclic rusts (we'll get to them don't panic just yet) with alternate hosts have not only numerous spores types, that must be developed in sequential order, but also require multiple hosts in order to complete their life cycle.

Just as it is both inaccurate and misleading to refer to a causal agent and the disease it causes as one in the same, so to it is equally poor to refer to a disease life cycle. Because a Disease Cycle, is the result of the intimate association of both the host and the pathogen.

Terms and Terminology

You will notice that the terms used to describe disease are phytocentric in nature, reflecting plant pathology's mission and perspective towards plant health

Causal Agent: The biotic or abiotic agent which, through prolonged irritation, causes the disease.
Disease: A host condition is the result of the activities of a causal agent.
NOTE: Causal agents are mobile and "infect". Diseases are neither mobile nor infectious
Sign: The physical manifestation of the causal agent. May be in the form of spores, mycelia, thallus, nema, bacterial ooze (when the bacterium is present), etc.
Symptom: The physical manifestation of the host's response.
Symptoms may be localized (confined to a small area of the plant) or systemic (spread through out the plant).
- primary symptom: symptom proximal to the infection site produced as a direct response to the causal agent.
  A common primary symptom is a lesion; a well defined localized diseased area.
- secondary symptom: symptoms which result from indirect or "chain of events" interactions and occur some distance from the infection point and in many cases distal to the causal agent.
- Whether primary or secondary symptoms; tissue may be verdant (greener than normal), chlorotic (yellower than normal), or necrotic (dead, often tan or brown in color).
- Hypertrophy and Hyperplasia are symptoms that result from increase in tissue volume (growth) due to increase in cell volume and cell numbers, respectively.
- Hypotrophy is a stunting due to retarded growth of lack or development. Ironically, it may involve increased cell division but without the corresponding increase in cell volume.

Not all signs and symptoms are macroscopic (visible by the unaided eye). Describing a disease may require histologic (tissue level) and microscopic (even electron microscopic) examination as well as morphologic and macroscopic description.

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:

Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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