GENERAL CONCEPTS AND CLASSIFICATION OF PLANT DISEASES DEFINITIONS AND CONCEPTS Flashcards
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
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
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
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.
Necrotroph
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
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
it 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
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
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
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
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
Infection is the establishment of the parasitic relationship between the pathogen and host following entry or penetration.
