Agri Books

Terminology of Plant Pathology

Disease: According to Horsfall and Diamond (1959), disease may be defined as a malfunctioning process that is caused by continuous irritation by a pathogen and/or environmental factor resulting in some suffering producing symptoms.

Disorder: The diseases caused by the deficiency of nutrients or unfavourable environmental are sometimes termed as disorders or physiological disorders.

Pathogen: It is the agent responsible for inciting ̳pathos‘ i.e. ailment or damage.

Parasite: These are the organisms which derive the food materials needed for their growth from other living organism (the host). All the pathogens are parasites but all the parasites are not pathogens. As some of the parasites live on their hosts without causing any damage to them as symbiotic relationships, e.g., Rhizobium bacterium in legume roots, mycorrhizae and lichens.

Biotrophs are the organisms which regardless of the ease with which they can be cultivated on artificial media obtain their food from living tissues only in nature in which they complete their life cycle). They were earlier also called obligate parasites, e.g., rusts, smuts, powdery mildews etc.

Saprophytes/saprobes are the organisms which derive their nutrition from the dead organic matter. Some parasites and saprophytes may have the faculty or (ability) to change their mode of nutrition.

Facultative saprophytes are ordinarily parasites which can grow and reproduce on dead organic matter under certain circumstances. They are also called hemibiotrophs which attack the living tissues in such a way as biotrophs but continue to grow and reproduce after the tissues is dead.

Perthotrophs or perthophytes: A parasite is called necrotroph when it kills the host tissue in advance of penetration and then lives saprophytically, e.g. Sclerotium rolfsii and Pythium species. Similar to necrotrophs are facultative parasites which live as saprophytes but under favourable conditions they can attack living plants and become parasites. The necrotrophs are also known as perthotrophs or perthophytes.

Pathogenicity is the ability of a pathogen to cause disease under a given set of environmental conditions. Whereas, pathogenesis is the chain of events that leads to development of a disease in the host.

Parasitism is a phenomenon by which a plant parasite becomes intimately associated with the plant; it draws nutrition and multiplies and grows at the expense of the plant host.

Virulence is a measure or degree of pathogenicity of an isolate or race of the pathogen. The term aggressiveness is often used to describe the capacity of a pathogen to invade and grow in the host plant and to reproduce on or in it. This term like virulence is used as measure of pathogenicity.

Immunity of a plant against a disease is absolute quality. It denotes the freedom of plant from disease, when the pathogen cannot establish parasitic relationship with the host. High resistance and low susceptibility approach immunity.

Disease resistance is the ability of an organism to overcome completely or in some degree the effect of a pathogen or other damaging factor; whereas susceptibility in the inability of the plant to resist the effect of the pathogen or other damaging factor.

Hypersensitivity is the extreme degree of susceptibility in which there is rapid death of the cells in the vicinity of the invading pathogen. It halts the further progress of the pathogen. Thus, hypersensitivity is a sign of very high resistance approaching immunity.

Infection is the establishment of the parasitic relationship between the pathogen and host following entry or penetration.

Incubation period is the time elapsing between penetration and completion of infection i.e. development of the disease symptoms.

Invasion and colonization is the growth and multiplication of the pathogen through the tissue of the host varying extent.

Disease Incidence the number of plants affected by a disease within a population. E.g. soil borne diseases, nematodes.

Disease Severity the measure of damage done by a disease. or Amount of disease present in a population. E.g. Leaf, stem, seed diseases.

Pathogenesis is the chain of events that lead to development of disease in the host (or) sequence of progress in disease development from the initial contact between the pathogen and its host to the completion of the syndrome.

Symptom The external or internal reactions or alterations of a plant as a result of a disease.

Syndrome The set of varying symptoms characterizing a disease are collectively called a syndrome. or The series of symptoms of a disease collectively known as syndrome.

Primary infection The first infection of a plant by the overwintering or over summering pathogen.

Primary inoculum The overwintering or over summering pathogen, or its spores that primary infection.

Secondary infection Any infection caused by inoculum produced as a result of a primary or a subsequent infection. OR An infection caused by secondary inoculum.

Secondary inoculum Inoculum produced by infections that took place during the same growing season.

Immune Cannot be infected by a given pathogen.

Resistant: Possessing qualities that hinder the development of a given pathogen.
OR Infected little or not at all.

Susceptible Lacking the inherent ability to resist disease or attack by a given pathogen; non-immune.

Tolerance The ability of a plant to sustain the effects of a disease without dying or suffering serious injury or crop loss.

Antagonism Injury, killing or inhibition of the growth of one species of microorganism to another microorganism.

Antagonists An organism having the capacity or ability to inhibit the growth or interfere with the activity of another microorganism.

Hyperplasia It is the abnormal increase in the size of a plant organ due to increase in number of cells of which the organ is composed.

Antibiosis Inhibition or destruction of one microorganism by a metabolic products of another microorganism.

Competition The ability of one organism to utilize a substrate more efficiently than another microorganism.

Hyper parasitism The parasitism of a parasite by another parasite.(Rhizoctonia solani on Pythium spp., Fusarium maxima on Rust fungi).

Hypersensitivity Excessive sensitivity of plant tissue to certain pathogens. Affected cells are killed quickly blocking the advancement of obligate parasite.