L.16-G.Biology
Mycology
D.Ebtihal Muiz
Parasitism and Pathogenicity
An organism that lives on or in some other organism and obtains its
food from the latter is called a parasite . The relationship between a parasite
and its host is called parasitism . A plant parasite is an organism that
becomes intimately associated with a plant and multiplies or grows at the
expense of the plant . The removal by the parasite of nutrients and water
from the host plant usually reduces efficiency in the normal growth of the
plant and becomes detrimental to its further development and reproduction
. Thus , in many cases , parasitism is intimately associated with
pathogenicity , since the ability of the parasite to invade and become
established in the host generally results in the development of a
development of a diseased condition in the host .
In some cases of parasitism , as with the root nodule bacteria of legume
plants and the mycorrhizal infection of feeder roots of most flowering
plants , both the plant and the microorganism are beneficial to the others
development . This phenomenon is known as symbiosis .
In most plant diseases , however , the amount of damage caused to plants
is often much greater than would be expected from the mere removal of
nutrients by the parasite . This 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 in themselves
do not seem directly to improve the welfare of the parasite . It would
appear therefore that the degree of pathogenicity exhibited by a parasite is
not always proportional to the nutritional affiliation of the parasite and its
host . Pathogenicity , then , may be defined as the ability of the parasite to
interfere with one more of the essential functions of the plant , with
parasitism frequently playing an important , but not always the most
important , role .
Of the large number of groups of living organisms , only a few members
of a few groups can parasitize plants : fungi , bacteria , mycoplasmas , and
parasitic higher plants , nematodes and protozoa , and viruses and viroids .
These parasites are successful becausethey can invade a host plant , feed
and proliferate in it , and withstand the conditions in which the host lives .
Some parasites , including viruses , viroids , ycoplasmas , some fastidious
bacteria , nemataodes , protozoa , and of the fungi those causing downy
mildews , powdery mildews , and rusts are biotrophs , that is , they cn grow
and reproduce in nature only in living hosts , and they are called obligate
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parasites . Other parasites ( most fungi and bacteria ) can live on either
living or dead hosts and on various nutrient media and are , therefore ,
called nonobligate parasites . Some nonobligate parasites live most of the
time or most of their life cycles as parasites but , under certain conditions ,
may grow saprophytically on dead organic matter and are called facultative
saprophytes ; others live most of the time and thrive well on dead organic
matter but , under certain circumstances, may attack living plants and
become parasitic – these parasites are called facultative parasites . Usually
no correlation exists between the degree of parasitism of a pathogen and
the severity of disease it can cause . since many diseases caused by weakly
parasitic pathogens are much more damaging to the plant than others
caused even by obligate parasites . More - over , certain fungi , for example
, the slime molds and those causing sooty molds, can cause disease by just
covering the surface of the plant without feeding at all or by feeding on by
feeding on insect excretions rather than by parasitizing he plant .
Inoculation
Inoculation is the coming in contact of a pathogen with a plant . The
pathogen or pathogens , that land on , or are otherwise brought inot contact
with the plant are called the inoculum . Inoculum is any part of the
pathogen that can initiate infection . Thus , in fungi inoculum may be
spores , sclerotia ( a compact mass of mycelium ) or fragments . In bacteria
, mycoplasmas , viruses , and vieoids , inoculum is always whole
individuals of bacteria , mycoplasmas , viruses , and viroids , respectively .
In nematodes , inoculum may be adult nematodes , nematode larvas , or
eggs . In parasitic higher plants inoculum may be plant fragments or seeds .
Inoculum may consist of a single individual of a pathogen , for example ,
one spore or one multicellular sclerotium , or of millions of individuals of a
pathogen , for example , bacteria carried in a drop of water . One unit of
inoculum of any pathogen is called a propagule .
Types of Inoculum
Inoculum that survives the winter or summer and causes the original
infections in the spring or in the autumn is called primary inoculum and the
infections it causes are called primary infections . Inoculum produced from
primary infections is called secondary inoculum and that , in turn , causes
secondary infections . Generally , the more abundant the primary inoculum
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and the closer it is to the crop , the more severe the disease and the losses
that result .
Sources of Inoculum
Inoculum sometimes is present right in the plant debris or soil in the
field where the crop is grown ; other times it comes into the field with the
seed , transplants , tubers , or other propagtive oranges , or it may come
from sources outside the field . Outside sources of inoculum may be nearby
plants or fields or fields many miles away . In many plant diseases ,
especially those of annual crops , the inoculum survives in perennial weeds
or alternate hosts , and every season it is carried from them to the annual
and other plants . Fungi , bacteria , parasitic higher plants , and nematodes
either produce their inoculum on the surface of infected plants .
Recognition between Host and Pathogen
It is still unclear how pathogens recognize their hosts and vice-versa .
It is assumed that when a pathogen comes in contact with a host cell , an
early event takes place that triggers a fairly rapid response in each
organism that either allows or impedes further growth of the pathogen and
development of disease . The nature of the " early event " is not known
with certainty in any host-parasite combination , but it may be one of many
biochemical substances , structures , and pathways . These substances ,
structures , and pathways may include specific elicitor molecules produced
by one organism that induce formation of specific products by the other
organism ; production of host-specific toxins that react with specific
receptor sites of the host ; erease of hormones that affect growth and
development of the other organism ; enzymes that may alter the cell wall
or membrane : substances that trigger activation of latent enzymatic
pathways ; chelating substances that alter membrane permeability and
affect the balance of ions ; polysaccharides that interfere with the passage
of water and nutrients ; and proteins or glycoproteins that react with cell
wall or membrane polysaccarides .
Relationships between Disease Cycles and Epidemics
Some pathogens complete only one , or even part of one , disease
cycle in one year and are called monocyclic , or single-cycle , pathogens .
Diseases with monocyclic pathogens include the smuts in which the fungus
produces spores at the end of the season ( these spores serve as primary –
and the only – inoculum for the following year ) , many tree rusts , which
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require two alternate hosts and at least one year to complete one disease
cycle , and many soil-borne diseases , for example , root rots and vascular
wilts . In root rots and vascular wilts , the pathogens survive the winter or
summer in decaying stems and roots or in the soil , infect plants during the
growth season , and at the end of the growth season , produce new spores in
the infected stems and roots . These spores remain in the soil and serve as
primary inoculum the following growth season . In monocyclic pathogens
the primary inoculum is the only inoculum available for the entire season ,
since there is no secondary inoculum and no secondary infection . The
amount of inoculum produced at the end of the season , however , is greater
than that present at the start of the season , and so in –monocylic diseases
the amount of inoculum may increase steadily from year to year .
In most diseases ,however , the pathogen goes through more than one
generation per growth season , and such pathogens are called polycyclic ,
or multi-cycle , pathogens . Polycyclic pathogens can complete many ( 230 ) disease cycles per year , and with each cycle the amount of inoculum
is multiplied manifold . Polycyclic pathogens are disseminated primarily by
air or air-borne vectors ( insects ) and are responsible for the kinds of
diseases that cause most of the explosive epidemics on most crops , for
example , the downy mildews , late blight of potato , powdery mildews ,
leaf spots and blights , grainrusts , and aphid-borne viruses . In polycyclic
fungal pathogens , the primary inoculum generally consists of the sexual (
perfect ) spores or , in fungi that lack the sexual stage , some other hardy
structure of the fungus such as sclerotia , pseudosclerotia , or mycelium in
infected tissue . The number of sexual spores or other hardy structures that
survive and cause infection is usually small , but once primary infection
takes place , large numbers of asexual spores ( secondary inoculum ) are
produced at each infectious site , and these spores can themselves cause
new ( secondary ) infections that produce morew asexual spores for more
infections .
In some diseases of trees , for example , fungal vascular wilts ,
mycoplasmal yellows , and viral infections , the infecting pathogen may not
complete a disease cycle within one year but instead may take several years
before the inoculum it produces it produces can be disseminated and
initiate new infections . Such pathogens are called polyetic ( multi-year ) .
Whether the pathogen involved in a particular disease is monocyclic ,
polycyclic , or polyetic has great epidemiological consequences since it
affects the amount of disease caused by the specific pathogen within a
given period of time .
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