Introductory Plant Pathology

Class 2

Health, Disease, and Plant Pathology

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

"Plant Pathology is the study of;

the living entities and the environmental conditions that cause disease in plants;
the mechanisms by which these factors produce disease in plants;
the interactions between the disease-causing agents and the diseased plant; and
the methods of preventing or controlling disease and alleviating the damage it causes."

G.N. Agrios.

This definition says nothing about production issues nor does it consider the esthetic value of a plant or plants. Plant pathology is about plants and the organisms that cause diseases on them. While the economic value of plants is important, it is plant health rather than plant production that is the focus of plant pathology.

Principle and Concept: Health, Disease

What is health?

The ability to carry out normal physiological functions at a acceptable level consistent to genetic potential.

Normal physiological functions include:

Normal call division, differentiation, and development,
Absorption of water and minerals from the soil and translocation;
Photosynthesis and translocation of photosynthates;
Utilization and storage of photosynthates;
Metabolism of metabolites and synthates;
Reproduction;
Storage of reserves for overwintering or reproduction.

The primary causes of disease are either biotic or abiotic. Biotic causes are living organisms(pathogens) while abiotic causes usually involve the environment. Organisms may kill plants directly or they may so severely debilitate them that they die of starvation or the effects of secondary infections.

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. The term "pathodeme" is used to express the altered metabolic result of the contributing interactants, but the pathodeme is not disease.

Disease is not the Pathogen

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

Disease is not infectuous

Following the above logic, because 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-recuring 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.

How does one study Disease??

Lucas presents the concept that in order to understand the nature of disease one must first understand the processes that occur during the life cycle of a normal healthy plant. He suggests that the analysis should be conducted at three levels:

the sequence of events comprising the normal plant life cycle;
the physiological processes involved in plant growth and development;
the molecular reaction underlying these processes.

What is Plant Pathology?

Greek - Pathos (suffering) + Logos (study) = The study of the suffering plant

Plant Pathology has two parts;

Science - the theoretical consideration of the suffering plant.
- i.e. How do plants defend themselves?
- How do pathogens invade?
- What causes symptoms?
- Why don't all plants die?
Art - The application of the Science
- i.e. Diagnosis
- Control

Disease theory / spontaneous generation - concrete mind sets

Classification of Plant Disease

Infectious - Biotic
Non-infectious - Abiotic

Non-Disease Plant death

Apoptosis - Programmed Cell Death

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:

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

E-mail Home Page General References