PPA 660: Pathogenesis--concepts and definitions

Assigned Reading

Baker, B., P. Zambryski, et al. (1997). "Signaling in plant-microbe interactions." Science 276(5313): 726-733.

Lecture Notes

There is much argument about terminology, and for good reason. A crisp and consistent use of terms is essential for scientific communication. Also essential is a common language; that is, the definition of terms should be similar among the conversants.Unfortunately, that is not always the case in plant pathology (or any other discipline, really). Therefore, I will introduce terms and their various uses, and also indicate what definitions we will adopt for this course. In that way, I hope that you will be provided both the means to read and understand the literature, and a common language to be used in this course.


Causing disease. rel. Disease


Intimate or physiological interaction of organisms of different species. The symbiotic continuum encompasses antagonism (often called parasitism), commensalism, mutualism etc., though some restrict the definition to mutualism. rel. Symbiotum; Aegricorpus; Basic compatibility


Physiological or developmental change in a host due to the persistent action of a biotic or abiotic factor (we will use this def.). An abnormal physiological or developmental condition. A detriment to host fitness. Any condition of an organism that is economically detrimental. rel. Pathogenesis

Disease triangle

Plant, parasite and environment contributing to disease development. (Also can be represented by Venn diagrams or Boolean algebra)


Organism that can cause disease. rel.: Parasite


Organism that obtains nutrients or other benefits from a living host, exacting a cost from the host. Many definitions specify that host fitness is reduced (i.e. antagonism), but we will specify only that a component of the host fitness be reduced (e.g. nutrient cost), allowing for the possibility of net fitness increase for both host and parasite (i.e. including mutualism). Note, also, that a parasite may or may not cause disease.

Obligate parasite

Organism that can only live as a parasite in nature. rel.: Biotroph; Facultative saprophyte (Note that this allows for organisms that can be cultured on rich or defined medium, provided that they do not exist as saprophytes in nature). rel. Biotroph


Organism whose exclusive, natural growth environment is in or on living host cells. ant. Necrotroph, rel. Hemibiotroph, Obligate parasite


Pathogen that causes necroses usually after a biotrophic phase. rel. Necrotroph


Pathogen that kills host cells in advance of its own growth, and obtains nutrients from the dead host cells. (Luttrell, E. S. 1974. Parasitism of fungi on vascular plants. Mycologia 66: 1-15). syn. Perthotroph


syn. Necrotroph


Parasite that requires living host cells during part of its life cycle . rel. Biotroph

Facultative saprobe (facultative saprophyte)

Organism that normally lives as a parasite, but can also live as a saprobe. rel. Obligate parasite

Facultative parasite

Organism that normally lives as a saprobe, but can also live as a parasite. rel. Saprobe

Saprobe (saprophyte)

Organism that does not have any apparent interactions (beneficial or harmful) with any host.


Association of an organism with the surface of a plant.


All or most of the life cycle of a symbiont completed within host plant tissues (Note: symbiont may grow intercellularly, intracellularly, or both). (Note: some restrict this definition to mutualistic symbioses, others to mutualisms and latent infections by pathogens).


Ability of an organism to infect a susceptible host genotype and cause disease (we will use this def.). The degree of disease (we will call this Virulence). rel. Basic compatibility. Note that the use of "virulence" with this definition is very common in some subdisciplines.


Inability of a species to infect and cause disease of another species. (Generally, such terminology would refer to an organism that is a pathogen of another host species). rel. Nonhost resistance.


Aggressiveness. The degree of disease caused by a pathogen genotype under appropriate conditions. rel.: Compatibility; Susceptibility.


No virulence. Characteristic of a pathogen that causes little or no disease after infecting its host. rel. Incompatibility; Resistance


Pathogen or parasite colonization of the host.


Ability or degree of ability to colonize susceptible host tissue. rel. Infection


Propensity of a host to be infected by a pathogen, determined genetically (and developmentally if we talk of susceptible tissue). The propensity to exhibit disease due to infection by a specified pathogen. rel. Virulence; Compatibility


Genetic susceptibility of a host to a parasite and virulence of the parasite on the host. rel. Virulence; Susceptibility


Genetically determined reduction or elimination of infection of a host by a pathogen, manifested as a reduced potential for the disease caused by that pathogen. rel.: Incompatibility; Avirulence


Genetic resistance of a host to a parasite and avirulence (or reduced infectivity) of the parasite on the host. rel. Avirulence; Resistance


Absolute (or nearly absolute) plant resistance to infection by a pathogen. (Note that this is a very different definition than used for animal systems.) rel. Resistance; Incompatibility

Systemic acquired resistance

Host plant resistance, manifested following initial infection or other stimulation, to subsequent infections by the same pathogen or a diverse array of pathogens. syn. Induced resistance

Induced resistance

syn. Systemic acquired resistance


Genetically determined reduction or elimination of potential disease, without a corresponding reduction in colonization by the pathogen.


The absence of disease in a susceptible individual when disease could reasonably have been expected to develop.

Basic compatibility

Inherent (genetically determined) property of some or all genotypes of a host species and some or all races of a parasite species that enables them to interact and cause disease under appropriate conditions. rel. Disease triangle

Nonhost resistance

The genetically determined resistance of a species to infection by a pathogen of another host species. rel. Nonpathogenicity. ant. Basic compatibility

Hypersensitive response

Expression of plant resistance involving rapid death of host cells at or near the site and time of infection. rel. Resistance


Visible or detectable characteristics of a disease.


Visible structures of a pathogen or parasite.


"Disease body": entity composed of both host and pathogen. rel. Symbiotum


Entity composed of two or more organisms in symbiosis. rel. Aegricorpus

Disease Cycle

The life cycle of a pathogen in association with its host.
  1. Inoculation
  2. Penetration
  3. Infection/establishment
  4. Colonization (invasion)
  5. Growth and reproduction of the pathogen
  6. Survival of the pathogen (e.g. overwintering or oversummering)


Agrios, G. N. 1988. Plant Pathology. Academic Press, San Diego.

Andrivon D (1993) Nomenclature for pathogenicity and virulence:the need for precision. Phytopathology 83: 889-890

Hunt RS (1994) Comment on the letter by Andrivon--re: pathogenicity and virulence. Phytopathology 84: 874-875