L.20-G.Biology
Mycology
D.Ibtihal Muiz
PLANT DISEASE EPIDEMIOLOGY
When a pathogen spreads to and affects many individuals within a
population over a relatively large area and within a relatively short time ,
the phenomenon is called an epidemic . An epidemic and has been defined
as any increase of disease in a population . The study of epidemics ,
sometimes called epiphytotics , occur annually on most crops in many parts
of the world . Most epidemics are more less localized and cause minor to
moderate losses because they are kept in check either naturally or by
chemical sprays and other control measures .
The Elements of an Epidemic
Plant disease epidemics develop as result of the timely combination
of the same elements that result in plant disease : susceptible host plant , a
virulent pathogen , and favorable environmental conditions over a fairly
long period of time . In addition , however , through their activities humans
may unwittingly help to initiate and develop epidemics , or instead may
effectively stop the initiation and development of epidemics under
situations in which they would almost certainly certainly occur without
human intervention .
Thus , the chance of an epidemic increases when the susceptibility of
the host and virulence of the pathogen are greater , as the environmental
conditions approach the optimum level for pathogen growth , reproduction
, and spread , and as the duration of all favorable combinations is prolonged
, provided no human intervention occurs to reduce or stop the epidemic .
For the purpose of describing the interaction of the components of
plant disease epidemics , the disease triangle that was discussed in Chapter
2 and used to describe the interaction of the components of plant disease
can be expanded to include time and humans .
The amount of each of the three components of plant disease , as
well as their effect on each other and , therefore , on the development of
disease , are affected by a fourth
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component : time . Both the specific point in time at which a particular
event in disease development occurs and the length of time during which
the event takes place affect the amount of disease . The interaction of the
four components can be visualized as a tetrahedron , or pyramid , in which
each plane represents one of the components . This figure is referred to as
the disease tetrahedron or disease pyramid ( Figure 8 – 1 ) . The effect of
time on disease development becomes apparent when one considers the
importance of the time of year ( that is , the climatic conditions and stage of
growth when host and pathogen may coexist ) , the duration and frequency
or favorable temperature and rains , the time of appearance of the vector ,
the duration of the cycle of a particular disease , the earliness or maturity of
the host , etc . If the four components of the disease tetrahedron could be
quantified , the volume of the tetrahedron would be proportional to the
amount of disease on a plant or in a plant population .
Disease development in cultivated plants is also greatly influenced by
a fifth component : humans . Humans affect the kind of plants grows in a
given area , their degree of resistance , the numbers planted , time of
planting , and density of the plants . By the resistance of the particular
plants they cultivate , humans also determine which pathogens and
pathogen races will predominate .
In the schematic diagram in Figure 8 - 2 , host pathogen , and environment
are each represented by one of the sides of the triangle , time is represented
as the perpendicular line arising from the center of the triangle and humans
as the peak of the tetrahedron whose base is the triangle and height is the
length of time . In this way , humans interact with and influence each of the
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other four components of an epidemic and thereby increase or decrease the
magnitude of the epidemic . Sometimes , of course , humans themselves
can be affected to a greater or lesser extent by plant disease epidemics .
Host Factors that Affect Development of Epidemics
Several internal and external factors of particular host plants play an
important role in the development of epidemics involving those hosts .
Levels of genetic Resistance or Susceptibility of the Host
Obviously , host plants carrying high levels of ( vertical ) resistance
do not allow a pathogen to become established in them and thus no
epidemic can develop , unless and until a new pathogen race appears that
can attack that resistance and the host then becomes susceptible . Host
plants carrying lower levels of ( horizontal ) resistance will probably
become infected , but the rate at which the disease and the epidemic will
develop depends on the level of resistance and the environmental
conditions . Susceptible host plants lacking genes for resistance against the
pathogen provide the ideal substrate for establishment and development of
new infections and , in the presence of a virulent pathogen and favorable
environment , favor the development of disease epidemics . plant parts are
actually quite resistant to infection while still very young , become more
susceptible later in their growth , and then become resistant again before
they are fully expanded ( Figure 8 - 3 , Ib ) . In other diseases , such as
infections of blossoms or fruit by Botrytis , Penicillium ,Monilinia , and
Glomerella , and in all postharvest infections , plant parts ( that is , fruit )
are resistant during growth and the arly adult period but become susceptible
near ripening ( Figure 8 - 3 , II ) . In still other diseases , such as potato late
blight ( caused by Phytopthora infestans )and tomato early blight ( caused
by Alternaria solani ) , a stage of juvenile susceptibility during the growth
period of the plant is followed by a period or relative resistance in the early
adult stage and then susceptibility after maturity ( Figure 8-3 , III) .
Apparently then , depending on the particular host-pathogen
combination , the age of host plant at the time of arrival of the pathogen
may affect considerably the development of infection and of an epidemic .
Pathogen Factors That Affect Development of Epidemics
1. Levels of Virulence
Virulent pathogens capable of rapidly infecting the host ensure
faster production of larger amounts of inoculum than
pathogens of lesser virulence .
2. Quantity of Inoculum Near Hosts
The greater the number of pathogen propagules ( spores ,
sclerotia , eggs , etc ) , within or near fields of host plants , the
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more incoulum reaches the hosts and at an earlies time ,
greatly increasing the chances of an epidemic .
3. Type of Reproduction of the Pathogen
All pathogens produce many offspring but some of them ,
such as most fungi , bacteria , and viruses , produce
incomparably more offspring than others .A few fungi and all
nematodes and parasitic plants produce relatively small
numbers of offspring . Even more important is the fact that
some pathogens ( most fungi , bacteria , and viruses ) have
short reproduction cycles and therefore can produce many
reproductive cycles ( generations ) in a single growing season .
These are the polycyclic pathogen that usually cause rusts ,
mildews , and leaf spots , and are responsible for most of the
sudden , catastrophic plant disease epidemic in the world .
4. Ecology of the Pathogen
Some pathogens , such as most fungi and parasitic higher
plants , produce their inoculum ( spores and seeds ,
respectively ) on the surface of the aerial parts of the host .
From there , spores and seeds can be dispersed with ease over
a range of distances and can cause widespread epidemics .
Other pathogens , such as vascular fungi and bacteria ,
mycoplasmas , viruses , and protozoa , reproduce inside the
plant .In this case , spread of the pathogen is rare or impossible
without the help of vectors , and therefore such pathogens can
cause epidemics only when vectors are plentiful and active .
Still other pathogens , such as soilborne fungi , bacteria , and
nematodes , produce their inoculum on infected plant parts in
the soil , within which the inoculum disperses slowly , if at all
, and presents little danger for sudden or widespread epidemics
.
5. Mode of Spread of the Pathogen
The spores of many plant pathogenic fungi , such as those
causing rusts , mildews , and leaf spots , are released into the
air and can be dispersed by air breezes or strong winds over
variable distances up to several miles . These kinds of fungi
are responsible for the most frequent and most widespread
epidemics . In terms of their ability to cause sudden and
widespread epidemics , the next most important group of
pathogens includes those whose inoculum is carried by
airborne vectors . Such pathogens are many of the viruses
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transmitted by aphids and some other insects ; mycoplasmas
and fastidious bacteria transmitted by leafhoppers ,
planthoppers , and psyllids ; and some fungi (such as the cause
of Dutch elm disease ) , bacteria ( such as the cause of
bacterial wilt of cucurbits ) , and even nematodes ( such as the
cause of pine wilt disease ) disseminated primarily by beetles .
Pathogens that are transmitted by wind-blown rain ( primarily
fungi causing diseases like anthracnoses and apple scab , and
most bacteria ) are almost annually responsible for severe but
somewhat localized epidemics within a field , a township , or a
valley . Pathogens carried with the seed or other vegetative
propagative organs ( such as tubers or bulbs ) are often placed
in the midst of susceptible plants , but their ability to cause
epidemics depends on the effectiveness of their subsequent
through the soil , because of the physical restrictions , are
generally unable to cause sudden or widespread epidemics but
often cause local , slow-spreading diseases of considerable
severity .
Measurement of plant Disease
When measuring disease , one is interested in measuring ( I ) the
incidence of the disease , that is , the number or proportion of plant units
diseased ( the number or proportion of plants , leaves , stems , and fruit that
show any symptoms ) ; (2) the severity of the disease , that is , the
proportion of area or amount of plant tissue that is disease ; and (3) the
yield loss caused by the disease , that is , the proportion of the yield that the
grower will not be able to harvest because the disease destroyed it directly
or prevented the plants from producing it .
The Structure of Epidemics
Epidemics develop as a consequence of the interactions of the
populations of their two components , hosts and pathogens , as influenced
by environmental and human interference over time . The interactions of
hosts and pathogens produce the third component , disease .
Each of these primary components of epidemics consists of
subcomponents . The host may be an annual , a perennial , or a tree ; it goes
through certain growth stages ( seedling , tillering , blossoming ) ; it is
propagated by seed , or vegetatively ; it may be resistant or susceptible ; it
may react by producing lesions or a blight .
The subcomponents of the pathogen include pathogenicity ( biotroph,
necrotroph , toxins, mode of penetration ) ; virulence ( varietal
specialization or race ) ; sporulation ( kind and amount of inoculum ) ;
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dispersal ( growth , by wind , water , vectors ) ; and survival ( duration ,
form ) .
The subcomponents of disease include infection ( number of lesions ,
systemic ) ; pathogenesis ( presence and length of incubation period ) ;
lesion formation ( size , rate , toxins ) ; infectiousness ( time and amount of
sporulation , amount of new inoculum ) ; spread ( infection gradient in host
population ) ; multiplication ( length of infection cycle , duration and /or
number of generations per season ) ; and survival ( longevity in months or
years ) .
As we increase our knowledge of the subcomponents of each
epidemic, we also increase our ability to predict the pattern of individual
epidemics and to interfere at the most appropriate stage of the epidemic
with more efficient and more dependable methods of control .
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