Lecture 3, PARASITISM AND DISEASE DEVELOPMENT
Parasite: an organism or virus that lives on or in another living organism and obtains food from that organism.
Pathogenesis: the sequence of processes in disease development from the time of infection to the final reaction in the host; production and development of disease.
Pathogenicity: the capability of a pathogen to cause disease.
Virulence: the degree of pathogenicity of a given pathogen.
Virulent: capable of causing a severe disease; strong pathogenic.
Avirulent: lacking virulence.
Aggressiveness (this word has been used in several senses): race of pathogen that differ in the severity of their pathological effects but do not interact differewntially with host varieties or cultivars are said to differ in aggressiveness (Van der Plank, 1968).
Symbiosis: a mutually beneficial association of two or more different kinds of organisms.
Biotroph: an organism that can live and multiply only on another living organism.
Hemibiotrophic: a microorganism that parasites in living tissues for a period and continues its life cycle on dead tissues.
Necrotroph: a microorganism feeding only on dead organic tissue.
Obligate parasite: a parasite that in nature can grow and multiply only on or in living organisms.
Facultative saprophyte: an organism that is usually parasitic but may live as a saprophyte (having the ability to be a parasite).
Facultative parasite: an organism that is usually saprophytic but may become parasite.
Saprophyte: an organism that can obtain nutrients from dead organic material.
Host: a living organism from which parasites or pathogens obtain nutrients.
Host range: the various kinds of host plant that may be attacked by a pathogen.
Development of diseases in plants
Disease triangle: a concept describing the simultaneous occurrence of a pathogen, a susceptible host, and a favorable environment such that a disease may develop.
Disease Stages/disease cycle: the chain of events involved in disease development, including the stages of development of the pathogen and the effect of the disease on the host.
Life cycle: the stage of successive stages in the growth and development of an organism that that occur between the appearance and reappearance of the same stage of organism.
Disease cycle and life cycle are different in polycyclic diseases.
The primary events in a disease cycle include inoculation, penetration, establishment of infection, colonization (invasion), growth and reproduction of the pathogen, dissemination of the pathogen, and survival of the pathogen.
Inoculation
The arrival or transfer of a pathogen onto a host.
Inoculum: the pathogen or its parts that come in contact with a host and can cause disease.
Primary inoculum: the overwintering or oversummering pathogen, or its spores that cause primary infection.
Secondary inoculum: inoculum produced by infections that took place during the same growing season.
Infection the establishment of parasite within a host plant.
Primary infection - infection caused by primary inoculum.
Secondary infection - infection caused by secondary inoculum.
Incubation period - the time interval between inoculation and appearance of disease symptoms.
Latent infection - the state in which a host is infected with a pathogen but does not show any symptoms.
Penetration
In relation to diseases, the initial invasion of a host by a pathogen.
Prepenetration phenomena: attachment, germination and differentiation.
Penetration:
a). Direct penetration through intact plant surface
b). Penetration through wounds
c). Penetration through natural openings
Host recognition
Recognition determines the specificity of host-parasite interaction, following by a compatible or incompatible relationship.
Invasion(Colonization)
The spread of a pathogen into the host.
Local infection - infection involves a single cell, afew cells, or a small area of the plant.
Systemic infection - A plant pathogen occurs throughout the plant.
Dissemination
Air
Rain
Germplasm
Human
Vectors
Overwintering and/or oversummering of pathogens
Fungal overwinter as spores, mycelium and sclerotia in infected tissues of the plant and the plant debris or in the soil.
Bacteria overwinter and oversummer as bacteria in essentially the same ways as for fungi.
Viruses, viroids and mollicutes survive only in living plant tissues or in the vectors.
Nematodes usually overwinter or over summer as eggs in the soil and as eggs or nematodes in plant roots or in the debris.
Soil inhabitants
Plant debris
Contaminated germplasms
Alternate and alternative hosts
Vectors
REFERENCES:
The Biochemistry and Physiology of Plant Disease. R.N.Goodman, Z.Kiraly & K.R.Wood, 1986
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