2. PARASITE ?
An organism that lives on or in some other organism and
obtains its food from the latter is called a parasite.
A plant parasite is an organism that becomes intimately
associated with a plant and multiplies or grows at the
expense of the plant.
The relationship between the parasite and the host is
known as parasitism
The removal of nutrients and water from the host plant
usually reduces efficiency in the normal growth of the
plant and becomes detrimental to the further
development and reproduction of the plant.
3. Whether viruses are true parasite ?
Viruses and viroids are pathogens but not parasites in
the same sense as the fungi and bacteria etc.
They lack the Lipman enzymatic system for conversion
of high energy into potential energy required for
biological activities. Their parasitism is at genetic level.
While inducing malfunction of the host nucleic acid they
do not directly draw any food from the host.
4. PATHOGENICITY ?
When parasitism results in visible disease symptoms, the
capacity of the parasite or the pathogen to cause disease
is known as pathogenicity
It is the ability of a pathogen to cause disease. The
ability of the parasite to invade and become established
in the host generally results in the development of a
diseased condition in the host.
In some cases, both the plant and the microorganisms are
benefited from the association. eg. Root nodule bacteria of
legume plants and the mycorrhizal infection of feeder
roots of most flowering plants (Symbiosis)
Damage caused by a parasite is not always proportional to
the nutrients removed by the parasite from its host.
In many cases of parasitism is intimately associated with
pathogenicity
5. Additional damage results from substances secreted by
the parasite or produced by the host in response to
stimuli originating in the parasite.
Tissues affected by such substances may show increased
respiration, disintegration or collapse of cells, wilting,
abscission, abnormal cell division and enlargement, and
degeneration of specific components such as
chlorophyll.
These conditions are not good for the welfare of the
parasite. Pathogenicity, then, is the ability of the
parasite to interfere with one or more of the essential
functions of the plant, thereby causing disease.
Parasitism frequently plays an important, but not always
the most important, role in pathogenicity
6. Pathogens are successful Parasites because they can
invade a host plant, feed and proliferate in it, and
withstand the conditions in which the host lives.
A parasite is not necessarily a pathogen and a pathogen
is not necessarily a parasite
Parasitism mostly results in disease development, there
is not much difference between parasitism and
pathogenicity
7. Obligate parasites/biotrophs: Parasites that can grow
and reproduce in nature only in living hosts. Eg. viruses,
viroids, mollicutes, some fastidious bacteria, nematodes,
protozoa, and fungi causing downy mildews, powdery
mildews, and rusts.
Nonobligate parasites: Parasites that can live on either
living or dead hosts and on various nutrient media.
Facultative saprophytes/semibiotrophs: Nonobligate
parasites that live most of the time or most of their life
cycles as parasites, but, under certain conditions, may
grow saprophytically on dead organic matter.
Facultative parasites: Those live most of the time and
thrive well on dead organic matter but, under certain
circumstances, may attack living plants and become
parasitic.
8. Saprophytes:Organisms which obtain nutrition on from
dead organic matter either completely or for a part of
their life. Eg. Rhizopus, Mucor
Necrotroph: An organism that kills the host in advance
of penetration and derives nutrients from dead plant
tissues eg. Alternaria spp. Botrytis cinerea, Sclerotinia
sclerotiorum
Hemibiotroph: The parasite s which attack living tissues
in the same way as biotroph but will continue to grow
and reproduce after the death of tissue. Phytophthora
infestans, Magnaporthe oryzae, Colletotrichum spp.
