disease development in plant

1. Disease Development in Plants
• A plant becomes diseased when it is attacked by any pathogen and the environmental conditions for the disease and
pathogen development are favourable.
• Host plant, pathogen and environment constitute the main three components for the development of a disease and their
interactions are visualized as a triangle generally called as disease triangle and each side of triangle represents one of
the three components.
• The length of each side is proportional to the sum total of the characteristics of each component that favour disease
• If the three components of the disease triangle could be quantified, the area of the triangle would represent the amount
of disease
• If any of the three components is zero i.e absent, there will be no disease.
Total of conditions favouring susceptibility
Host
The Disease Triangle
Disease

2. The Infection Process
Pathogenesis or Disease Cycle
 Chain of more or less distinct events occurring in succession and leading to development and
perpetuation of a disease and pathogen is called pathogenesis or disease cycle. It is also called
infection chain
 The disease cycle is close to the life cycle of the pathogen but, it refers primarily to the appearance,
development and perpetuation of disease as a function of pathogen rather than to the pathogen it
self.
 The disease cycle involves changes and its symptoms in the plant as well as changes in the
pathogen and spans within a growing season and from one growing season to another

3. The disease cycle or infection chain can be of two types:
i) Continuous infection chain: In this type of disease cycle, the pathogen remains continuously in active form
from one host to another. Most viral and some fungal diseases come under this category
ii) Intermittent infection chain: In this type of disease cycle, the pathogens survive in dormant state or as
active saprophytes to maintain the continuity of chain after the harvest of crop. Most of the fungal, bacterial
and nematode diseases fall under this category.Diseases with intermittent infection chain are further of two
types:
a) Monocyclic diseases: In such diseases only one cycle of the disease is completed in one season and
there is no repetition of disease cycle within a season. Such diseases are also called simple interest
diseases. Eg., loose smut of wheat and covered smut of barley etc.
b) Polycyclic diseases: In such diseases many cycles of a disease are repeated within the same crop
season on the host through spore – infection- spore chain. Such pathogens are responsible for epidemics
if the environmental conditions favour the disease. Such diseases are also called compound interest
diseases. Eg., rusts, powdery mildews, late blight of potato, apple scab etc.

4.  Depending upon the host plants involved, infection chain can be homogenous or heterogeneous.
 The infection chain is said to be homogenous when the pathogen survives on only one genus of plants and it is
heterogeneous if many plant species are involved in the disease cycle.
 The main events in the disease cycle are as follows:
 Inoculation
 Penetration
 Establishment of infection
 Invasion
 Colonization (Growth and reproduction of the pathogen)
 Dissemination of the pathogen
 Survival of the pathogen

5. Inoculation
Dissemination of primary
inoculum
Infection
Invasion
Dissemination of pathogen
(secondary inoculum)
Symptom
development
Growth and
reproduction of
pathogen
Colonization
Attachment
Primary inoculum
Dormant period
Host recognition
Penetration
Overwintering stage
DISEASE CYCLE
Stages in the development of a disease cycle

6. A. Inoculation
• Inoculation is the initial contact of a pathogen with a site of plant where infection is possible
• The pathogen(s) that lands on the host plant or is otherwise brought into contact with the plant is called
inoculum.
• Inoculum is any part of a pathogen that can initiate infection. In fungi, the inoculum may be spores,
sclerotia or fragments of the mycelium.
• In bacteria, mollecutes, protozoa, viruses and viroids the inoculum is always whole individuals of respective
organisms.
• In nematodes, the inoculum may be adult nematodes, juveniles or eggs while, in parasitic higher plants the
inoculum may be plant fragments or seeds.
• One unit of inoculum of any pathogen is called a propagule
• Inoculum potential of any pathogen mainly determines the success of infection by any propagule.
• Inoculum potential may be defined as “the resultant of action of environment, the vigour of the pathogen to
establish an infection, the susceptibility of a host and the amount of inoculum present”.
• In other words “it is the energy of growth of a parasite available for infection of a host at a surface of the
host organ to be affected

7. a) Types of Inoculum
The inoculum is of two types:
i) Primary inoculum: An inoculum that survives dormant in the winter or summer and causes original
infections in spring or autumn on any crop plant is called the primary inoculum and the infections it causes
are called primary infections. Generally, more the primary inoculum, more severe is the disease
ii) Secondary inoculum: An inoculum produced from primary infections is called a secondary inoculum and
the infections it causes are called secondary infections

8. b) Sources of Inoculum
• Sometimes, the inoculum is present in the plant debris or soil in the field where crop is grown.
• Sometimes, it comes to the field with the seed, transplants, tubers or other propagative organs or it may come from
the sources outside the fields.
• In most of annuals, the disease inoculum survives in the perennial weeds or alternate hosts and every season it is
carried from them to the annuals
• Fungi, bacteria, parasitic higher plants and nematodes generally produce their inoculum on the surface of infected
plants or their inoculum reaches the plant surface when the infected tissue breaks
• Viruses, viroids, mollecutes, fastidious bacteria and protozoa produce their inoculum within the plants and the
inoculum never reaches the surface naturally.
• So in these cases, the inoculum may be transmitted from one plant to another through some kind of vector
c) Landing or arrival of inoculum
• The inoculum of most pathogens is carried to host plants passively by wind, water and insects.
• Airborne inoculum reaches the host sometimes by gravity or by washing with the rains
• Some types of inoculum like zoospores and nematodes may be attracted towards the sugars or amino acids diffusing
out from the host roots

9. B. Pre Penetration Phenomenon
a) Attachment of Pathogen to Host
• Pathogens like mollecutes, fastidious bacteria, protozoa and most viruses are placed directly into the cells
of plants by their vectors and in most cases they are immediately surrounded by cytoplasm, cytoplasmic
membranes and cell walls
• Almost all fungi, bacteria and parasitic higher plants are first brought into contact or are attached with
external surface of the plant organs before they can penetrate and colonize the host.
• The act of attachment often seems necessary for the subsequent transmission of signals for germ tube
extension and production of infection structure
• Attachment takes place through the adhesion of spores, bacteria and seeds through adhesive materials that
vary significantly in composition.
• Vector transmitted pathogens are usually carried to their hosts with an extremely high efficiency
• Only a tiny fraction of potential inoculum actually lands on the susceptible host plants while, most of it
lands on the things or weeds which can not get infected and goes waste.

10. • Disruption of adhesion by non toxic synthetic compounds results in the failure of spores to infect the leaves.
• The adhesion of spores to the plant surface may either involve a very specific interaction of spores with a host
surface via some chemical or physical signals.
• It is now well known fact that fungal cell wall plays an important role in the adhesion of fungi to the host
surface in addition to its structural role
• The propagules of pathogens have mucilaginous substances consisting of water soluble polysaccharides,
glycoproteins, lipids and fibriller materials on their surface which when get moistened become sticky and help
the pathogen to adhere the plant surface
• In some fungi causing vascular wilts, spores fail to adhere after hydration but become adhesive after they are
allowed to respire and synthesize new proteins
• In powdery mildew fungi, which do not require free water for infection, adhesion is accomplished by release
of enzyme cutinase from the spore which makes the plant and spore areas of attachment more hydrophilic
and cements the spore to the plant surface.
b) Spore / Seed Germination / Hatching of Nematode eggs and Perception of Host Surface
• Almost all pathogens in their vegetative state are capable of initiating infection immediately
• But, fungal spores, seeds of parasitic higher plants and nematode eggs first need to germinate before causing
infection in the host plant

11. i) Spore germination
• Spores germinate by producing a typical mycelium that infects and grows into host plants or they produce a
short germ tube that produces a specialized infectious structure called haustorium
• A germ tube is a very specialized structure distinct from the fungal mycelium, often growing for a very short
distance before it differentiates into an appresorium
• Germ tube extension and differentiation takes place when appropriate physical and chemical signals like
surface hardness, hydrophobicity, surface topography and plant signals are present.
• In order to germinate, spores require a favourable temperature and also moisture in the form of rain, dew or
a film of water on the plant surface or at least high relative humidity.
• Most spores can germinate immediately after their maturation and release but others (resting spores) require
a dormancy period of varying durations before they can germinate.
• Some fungal spores germinate by producing other spores eg. Sporangia produce zoospores and teliospores
produce basidiospores.
• Contact of spore with the host surface, hydration and absorption of low molecular weight ionic material from
the host surface and availability of nutrients play important roles in spore germination.

12. • Spore germination is often favoured by nutrients diffusing from the plant surface. More the nutrients (sugars
and amino acids) exuded from the plant, the more and faster the spores germinate
• Spores also have some mechanisms that prevent their germination until they sense these stimulations or
there are too many spores in their vicinity
• In some cases, spore germination is stimulated only by exudates of plan`ts susceptible to that particular
pathogen
• In some other cases, spore germination may be inhibited to a lesser extent by the materials released by the
plant into surrounding water, by the substances contained within the spore itself ie. especially when the
spores are highly concentrated (“quorum sensing”) or by the saprophytic microflora present or near the plant
surface.
• the soil fungi coexist with some antgonistic microorganisms and the spore are unable to germinate in such
soils. This phenomenon is called fungistasis and soils are called suppressive soils. In such soils, fungistasis is
overcome by some root exudates of host plants growing nearby and spores germinate there.
• As soon as the spore is stimulated, it mobilizes its stored food reserves like lipids, polyoles and carbohydrates
and synthesizes the cell membrane and cell wall towards the germ tube formation and extension.
• Germ tube also perceives the host surface and if the appropriate external signals are not received, the germ
tube remains undifferentiated and stops growing after the nutrients are exhausted.

13. • After the attachment of infection propagule to the host surface, as the spores germinate, germ tubes also
produce mucilaginous materials that allow them to adhere to the cuticular surface of host either along their
entire length or only at the tip of germ tube
• In the regions of contact with germ tube, the structure of host cuticle and cell walls often appear altered,
presumably as a result of degradative enzymes contained in the mucilaginous sheath
• After the spores germinate, the resulting germ tube must grow, or the motile secondary spore (zoospore) must
move towards a site on plant surface where successful penetration can take place
• The number, length and rate of germ tube or the number of motile spores may be affected by physical
conditions like temperature and moisture, by kind and amount of exudates the plant surface produces at its
surface and by saprophytic microflora
• The growth of germ tubes in the direction of successful penetration site seems to be regulated by several
factors:
i) Greater humidity and chemical stimuli associated with openings like stomata, wounds and lenticels
ii) Thigmotropic (contact) responses to the topography of leaf surface resulting in growth of germ tubes at
right angles to cuticular ridges surrounding stomata and thus eventually reaching a stoma (pit)
iii) Nutritional responses to germ tubes towards greater concentrations of sugars and amino acids present
along the roots.

14. • The direction of movement of motile spores is also regulated by similar factors namely, chemical stimuli
emanating from stomata, wounds or the zone of root elongation; physical stimuli related to structure of open
stomata; and nutrient gradient present in wounds and root exudates
ii) Seed germination
• Seeds of parasitic higher plants germinate by producing a radicle which either penetrates the host plant directly
or first produces a small plant that subsequently penetrates the host plant by means of specialized feeding
organs called haustoria
• The conditions which affect spore germination and growth of germ tubes in fungi, also are applicable to seeds
too.
iii) Hatching of nematode eggs
• Nematode eggs also require conditions of favourable temperature and moisture to become activated and
hatched
• In most nematodes, the egg contains first juvenile stage before or soon after the egg is laid. This juvenile
immediately undergoes a molt and gives rise to second juvenile stage, which remains dormant in the egg for
various periods of time
• When the egg hatches, it is actually second juvenile that emerges which either finds a host and penetrates or it
undergoes further molts
• Once the nematodes are in close proximity of plant roots, they are attracted by certain chemical factors
associated with root growth especially CO2 and amino acids. These factors may diffuse through the soil and can
attract the nematodes from several centimeters away .

15. c) Formation and maturation of appresorium or infection peg
• Appresorium is the swelling produced at the tip of germ tube (after its growth upto a short distance) which
contains lipids, polysaccharides, proteins and enormous glycerol. The turgor pressure inside an appresorium is
as high as 40 times the pressure of a typical car tyre
• Just after their production, appresoria adhere tightly to the leaf surface and secrete extracellular enzymes as
well as generate physical force to bring about penetration of host plant cuticle by the fungus
• Apprresoria must be adhered to host surface tightly enough to withstand the invasive physical force applied
by the fungus and to resist chemical action of enzymes secreted by the fungus
• Appresoria in certain cases produce a narrow penetration hypha also called infection peg or penetration peg
from the base of appresorium and use primarilly physical force to puncture the plant cuticle with that
penetration peg.
• High turgor pressure in the appresorium pushes the penetration hypha thruogh the host cuticle and layers of
epidermal cell walls.

16. d) Recognition between Host and Pathogen
• It is assumed that when a pathogen comes in contact with the host cell, an early event takes place that
triggers a fairly rapid response in each organism which either allows or impedes further growth of the
pathogen and development of disease
• The nature of this ‘early event’ is not known exactly but, it may be one of many biochemical substances,
structures or pathways. These may be specific host signal compounds or structures, or pathogen elicitor
molecules and either of two may induce specific actions or formation of specific products by the other
organism.
• Host components acting as signals for recognition by and activation of pathogens are numerous:----
i) Fatty acids of plant cuticle which activate production of cutinase enzyme by the pathogen for breaking down
the host cutin
ii) Galacturonan molecules of host pectin which stimulate the production of pectin lyase enzymes by the fungus
or bacterium
iii) Certain phenolic compounds like strigol which stimulates the activation and germination of some pathogen
propagules
iv) Isoflavons and other phenolics, amino acids and sugars released from plant wounds that activate a series of
genes in the pathogen leading to infection
• A host may also send certain clues for recognition by some of its pathogens through its surface characteristics
like furrows, hardness or release of certain ions such as calcium.

17. • Pathogen components that act as elicitors of recognition by the host plant subsequent
activation of host defense is poorly understood
i) Elicitor molecules may be released from attacking pathogens before or during entry into
the host and they may have a narrow host range
ii) Some elicitors may be components of cell surface of pathogen (eg. β- glucans, chitin or
chitosan) that are released by the action of host enzymes (β- glucanase and / or chitinase)
and have broad host range
iii) Some elicitors may be synthesized and released by the pathogen after it enters the host in
response to host signals. These include harpin proteins of bacteria which induce
hypersensitive response, certain hydroxylipids, peptides and carbohydrates which induce
host defense reactions like production of phytoalexins
• Elicitors are considered as determinants of pathogen avirulence because they elicit the
hypersensitive response and various other components of defense response.

18. • When the initial recognition signal received by the pathogen favours growth and development, disease may
be induced and if signal suppresses pathogen growth and activity, disease may be aborted.
• If the initial recognition elicitor received by the host triggers a defense reaction, pathogen growth and
activity may be slowed or stopped and disease may not develop and if elicitor either suppresses or bypasses
the defense reaction of host, disease may develop.

19. C. Penetration
• After the host pathogen recognition is compatible, the infection propagule (in the form of germ tube or
appresorium or infection peg) penetrates the host tissues
• Pathogens penetrate plant surfaces by direct penetration of cell walls, through natural openings or through
wounds
• Fungi penetrate the host surfaces by any of the above methods
• Bacteria mostly enter through wounds and less frequently through natural openings and never directly
through unbroken cell walls
• Viruses, viroids, mollecutes, fastidious bacteria and protozoa enter through wounds made by vectors and in
some cases through wounds made by tools and other means
• Parasitic higher plants enter their hosts by direct penetration
• Nematodes enter plants by direct penetration and sometimes through natural openings
a) Direct Penetration through Intact Plant Surface
• It is the most common type of penetration by fungi, oomycetes and nemtodes and only type of penetration
by parasitic higher plants. None of other pathogens can enter the plants by direct penetration
• Among the fungi, hemibiotrophs (non obligate parasites) penetrate through a fine hypha produced by a spore
or mycelium while, the obligate parasites do so through a penetration peg produced by an appresorium

20. • The fine hypha formed at the point of contact of germ tube and plant surface grows towards the plant surface
and pierces the cuticle and cell wall through mechanical forces and enzymatic action on cell wall substances
• Most fungi, form an appresorium at the end of germ tube. The appresorium is bulbous or cylindrical with a
flat surface in contact with the surface of host plant
• A penetration peg grows from the flat surface of appresorium towards the host and pierces the cuticle and
cell wall
• The penetration peg grows into a fine hypha generally much smaller in diameter than a normal hypha of the
same fungus and regains its original diameter after its entry into the host
• Most of the fungi penetrate the cuticle and cell wall but the apple scab fungus penetrates only the cuticle and
stays between the cuticle and cell wall
• Parasitic higher plants also form an appresorium and penetration peg at the point of contact of radicle with
the host plant and penetration is same as in case of fungi.
• Direct penetration in the nematodes is accomplished by repeated back and forth thrusts of their stylets. Such
thrusts finally create a small opening in the cell wall, the nematode then inserts its stylet into the cell or the
entire nematode enters the cell

21. b) Penetration through wounds
• All bacteria, most fungi, some viruses and all viroids can enter the plants through various types of wounds
• Some viruses, all mollecutes, fastidious bacteria and protozoa enter the plants through wounds made by their
vectors
spore
Superficial
mycelium spore Subcuticular mycelium
Direct subcuticular only
Direct with appresorium (A), penetration peg
(PP) and Intracellular mycelium (IM)
Direct with intercellular mycelium Direct with intercellular mycelium with haustoria
Direcrt with haustoria
Direct with haustoria
spore Germ tube
A
PP
IM
Direct penetration
Penetration through
wounds
Through wounds Through natural cracks between main and lateral
roots
Fungus kills and macerates cells ahead of its advance

22. • The wounds used by fungi or bacteria may be fresh or old and may consist of macerated or killed tissue. They
grow briefly into such tissue before they advance to the healthy tissues
• Maceration or death of the tissue may be result of environmental factors (wind breakage or hail), animal
feeding (insects and large animals), cultural practices by humans (pruning, transplanting and harvesting), self
inflicting injuries (leaf scars) and wounds or lesions caused by other pathogens
• Bacteria and fungi penetrating through wounds germinate or multiply in the wound sap in a film of water or
collected in the wound
• Subsequently, the pathogen invades adjacent plant cells or it secretes enzymes and toxins that kill and
macerate the nearby cells
• The penetration of viruses, mollecutes, fastidious bacteria and protozoa through wounds depends on the
deposition of these pathogens by their vectors in fresh wounds created by them at the time of inoculation
c) Penetration through Natural Openings
• Many fungi and bacteria enter the plants through stomata, some enter through hydathodes, nectrathodes
and lenticels
• Stomata are more numerous on the lower leaf surface and are open in the day time while closed mostly in
the night
• Bacteria present in the film of water over stoma can enter or swim through the stoma through water soaking
and reach substomatal cavity where they can multiply and start infection

23. • Fungal spores germinate on the plant surface and germ tube may then grow through the stoma.
• Frequently, the germ tube forms an appresorium which fits tightly into stoma and a fine hypha growing from it
enters into the stoma.
• In the substomatal cavity the hypha enlarges and from there it grows into one or several hyphae that actually
invade host cells directly or by means of haustoria
• Some fungi can apparently penetrate even closed stomata while, others penetrate only through open stsomata
• Powdery mildew fungi can grow over open stomata without entering them
• Hydothodes are more or less permanently open pores at the margins and tips of leaves which are connected
to veins and secrete droplets of liquid called guttation drops containing various nutrients
• Some bacteria use these hydathodes as a means of entry into leaves but few fungi seem to enter plants
through these openings
• Some bacteria also enter blossoms through similar openings called nectrathodes or nectries.
Penetration through
natural openings
Through stomata Through lenticels
Guttation or water
droplets
Through hydathodes

24. • Lenticels are openings on fruits, stems, and tubers that are filled with loosely connected cells that allow the
passage of air
• Lenticels are open during growing season but still, very few fungi and bacteria enter through them and grow
and advance mostly intercellularly
• Most pathogens entering through lenticels can also enter through wounds thus making lenticels a less
common passage for pathogen entry
• Penetration does not always lead to infection. Many organisms actually penetrate cells of plants that are not
susceptible to these organisms and that do not become diseased. These organisms cannot proceed further and
die without causing disease
D. Infection
• Infection is the process in which pathogens establish contact with susceptible cells or tissues of host and
procure nutrients from them.
• After infection, pathogens grow, multiply or both within the plant tissues and invade and colonize the plant to
a lesser or greater extent.
• Successful infections result in the appearance of symptoms on the host
• Some infections remain latent and do not produce symptoms immediately but, with change in environmental
conditions and stage of maturity the plants exhibit some kind of symptoms

25. • All the visible and otherwise detectable changes in the infected plants constitute the symptoms which may
appear as soon as 2 to 4 days after infection as in case of some fungal, bacterial or viral diseases of
herbaceous plants or as late as 2 to 3 years as in case of viral, mollecute and other diseases of trees. In most
of diseases the symptoms appear from a few days to few weeks time after inoculation
• Time lapse between inoculation and development of symptoms is called incubation period
• Incubation period in different diseases varies with different host – pathogen combinations, stage of
development of host and environmental conditions especially temperature and relative humidity
• During infection, some pathogens obtain nutrients from living cells, often without killing the cells or at least
not for a long time; some others kill cells and utilize their contents and still others kill and disorganize the
surrounding tissues
• During infection, pathogens release a number of biologically active substances like enzymes, toxin and growth
regulators that may affect the structural integrity of the host cells or their physiological process
• In response to infection, host reacts with a variety of defense mechanisms, which result in varying degrees of
protection of plant from the pathogen

26. • Many conditions are to be satisfied for a successful infection to occur after the penetration of the pathogen
i) The plant variety must be susceptible to the particular pathogen
ii) The stage at which penetration occurs should be the susceptible growth stage of the host
iii) The pathogen must be in pathogenic stage which can infect immediately after penetration without
undergoing any dormancy
iv) The temperature and moisture conditions in the environment must favour the growth and multiplication of
the pathogen
• When these conditions are optimum, the pathogen can invade the host plant upto its maximum potential
even in the presence of plant defenses and ultimately the disease develops
E. Invasion
• Various pathogens invade hosts in different ways and to different ways and to different extents
a) Fungi
• Some fungi like apple scab fungus and rose black spot fungus produce mycelium grow only in the area
between cuticle and epidermis (Subcuticular)
• Some other fungi like powdery mildews produce mycelium only on the surface of plant (superficial)and send
their haustoria into epidermal cells

27. • Most fungi spread into all the tissues of the plant organs (root, stem and leaf) they infect either by growing
intercellularly or intracellularly and producing their mycelium there
• Vascular wilts causing fungi invade the xylem vessels of the plants
b) Bactereia
• Bacteria invade tissues intercellularly, although when parts of the plant cell walls dissolve, bacteria also grow
intracellularly
• Bacteria causing vascular wilts like fungi also invade xylem vessels

28. c) Nematodes
• Most nematodes invade tissue intercellularly, but some can invade intracellularly as well
• Many nematodes do not invade cells or tissues at all but feed by piercing epidermal cells with their stylets

29. d) Viruses, viroids, mollecutes, fastidious bacteria and protozoa
• These pathogens invade tissues by moving from cell to cell ontracellularly
• Viruses and viroids invade all types of living plants cells
• Mollecutes and protozoa invade phloem sieve tubes and perhaps a few adjacent phloem parenchyma cells
• Most fastidious bacteria invade xylem vessels and a few invade only phloem sieve tubes
e) Local invasion /infections
• Infections caused by fungi, bacteria, nematodes, some viruses and parasitic higher plants are local i.e. they
involve a single cell, a few cells or a small area of the plant
• These infections may remain localized throughout the growing season or they may enlarge slightly or very
slowly
f) Systemic invasion / infections
• Infections caused by fastidious bacteria, mollecutes and protozoa and natural infections caused by viruses and
viroids are systemic i.e. the pathogen, from one initial point of infection spreads and invades most or all
susceptible cells and tissues throughout the plant
• Vascular wilt fungi and bacteria invade xylem vessels internally, but they are usually confined to few vessels in
the roots, stems or top of infected plants initially and invade most or all xylem vessels of the plant in the final
stages of infection

30. • Some downy mildew fungi, most of the rust and smut fungi invade their hosts systemically, however, in most
cases the older mycelium degenerates and disappears and only younger mycelium survives in the actively
growing tissues
F. Growth and Reproduction of the Pathogen and their Colonization
a) Growth
• Many fungi and parasitic higher plants invade and infect tissues by growing on or into them from initial point
of inoculation
• These pathogens continue to grow and branch out within the infected host indefinitely so that the pathogen
spreads into more and more plant tissues until the spread of infection is stopped or plant is dead
• In some fungal infections, younger hyphae continue to grow into new healthy tissues and the old ones in the
already infected tissues die out and disappear. So, there are several points where separate units of mycelium
are active
• Fungi causing vascular wilts often invade plants by producing and releasing spores within the vessels and as
the spores are carried in the xylem sap stream, they invade vessels far away from the mycelium, germinate
there and produce a mycelium which invades more vessels
• Other pathogens namely bacteria, mollecutes, viruses, viroids, nematodes and protozoa do not increase in
size with time
• These pathogens invade and infect new tissues within the plant by reproducing at a rapid rate and increasing
their number tremendously in infected tissues.

31. • Their progenies then are carried passively into new cells and tissues through plasmodesmata (viruses and
viroids), phloem (viruses,viroids, mollecutes, some fastidious bacteria and protozoa) or xylem (some bacteria)
or they may move actively by their own power as in case of protozoa, nematodes and some bacteria.
b) Reproduction
• Fungi reproduce by means of asexual or sexual spores
• Some fungi produce tremendous spores in one growing season while, others produce them in successive
crops. But in any case thousands of spores are produced per square centimeter of infected tissue
• When these spores are released, they constitute the sufficient inoculum to infect all the plants in that
particular area
• Parasitic higher plants reproduce by seeds
• Bacteria and mollecutes reproduce by fission in which one mature individual splits into two equal smaller
individuals
• Under optimum nutritional and environmental conditions, the bacteria divide continuously in the host cells
until the nutrients are exhausted
• Fastidious bacteria and mollecutes appear to reproduce more slowly than normal bacteria, but they spread
throughout the vascular tissue. But still, their number in the infected plants is relatively small. The same is true
for protozoa
• Viruses and viroids are replicated by the host cell
• Viruses and viroids replicate quite fast in the living cells and each infected plant may contain innumerable
number of these pathogens

32. • Nematodes reproduce by means of eggs
• Nematode females lay about 300 to 500 eggs and half of them produce females which again will lay the same
number of eggs
• A nematode species may have more than a dozen of generations per year.
c) Colonization
• Majority of plant pathogenic fungi and oomycetes produce a mycelium only within the plants they infect
• Few fungi and oomcetes produce a mycelium on the surface of their host plants, but most powdery mildew
fungi produce a mycelium only on the surface of their hosts
• The great majority of fungi and oomycetes produce spores on or just below the surface of infected area of the
host and released out into the environment
• Plasmodiophoromycetes and vascular wilt causing fungi produce spores inside the host tissue and are
released only after the disintegration or death of the host tissue
• Parasitic higher plants produce their seeds on aerial branches
• Some nematodes lay their eggs at or near the surface of the host plant
• Bacteria reproduce between the host cells or in xylem or phloem cells of the hosts and come to the surface
only through wounds, cracks, stomata etc.
• Viruses, viroids, protozoa and fastidious bacteria reproduce only inside the cells and often do not reach or
exist at the surface of the host plant

33. G. Dispersal or Dissemination of the Pathogen
• Dispersal means spread of the pathogen propagules from one plant to the another or one area to a new area
for short as well as long distances.
• Dispersal of pathogen is important not only for spread of disease in the population but also for the continuity
of life cycle and evolution of pathogen
• Dispersal of pathogens is accomplished in following two manners:
a) Direct or active or autonomous dispersal e.g. in the soil, by the soil, seed and planting material
b) Indirect or passive dispersal involving the role of water, air, man, inscets, nematodes and other animals
a) Direct or active or autonomous dispersal
• Autonomous dispersal of pathogens is accomplished through the agency of soil, seed and plant organs during
normal agronomic operations. There no role of any external agencies like water, air, wind or vector etc
i) Soil as a means of autonomous dispersal
• Some zoosporic soil inhabiting fungi, oomycetes, nematodes and few bacteria can move on their own within
the soil to very short distances mainly under the conditions of soil moisture and presence of some root
exudates in their close proximity
• In most cases when the soil is contaminated with the plant pathogens, they grow and persist in the soil for
longer periods. Such pathogens are dispersed by the soil when it is carried alongwith plant parts from one
field or area to the other.

34. ii) Seed as a means of autonomous dispersal
• Seeds are dormant structures and rarely a fungus or bacterium survives in or on the seeds in active form.
But some fungi and bacteria are externally seed borne and get adhered to the seed coat and in some cases
they are internally seed borne i.e. they penetrate the seed coat. So, such pathogens are dispersed along with
the dispersal of the seeds. E.g. Xanthomonas campestris pv campestris causing black rot of crucifers and
Ustilago segatum tritici causing loose smut of wheat. In such cases the seeds are said to be infested with the
pathogen
• The dormant structures of pathogens like seeds of parasitic higher plants, sclerotia of different fungi, cereal
cockles or cysts containing nematodes, smut balls etc. can get mixed with the seed lots of crops to be grown
and get dispersed as seed contaminants
• Around 100 plant viruses and few viroids like spindle tuber viroid infect the seeds also either through
systemic infections or through pollen transmission, so these are also dispersed alongwith the seeds. In such
cases the seeds are infected with the pathogens
iii) Plant and plant organs as a means of autonomous dispersal
• Plant and plant parts other than true seeds that are used for propagation of plants , raw garden produce and
plant debris accumulated during the course of cropping get contaminated with the pathogens and can
disseminate them.

35. • The contamination of plants and plant parts occurs in the fields, orchards, plantations or nurseries through
infection by the inoculum already present there or introduced from outside
• Sometimes the infection or contamination of propagating materials like tubers and bulbs occurs during
storage.
• Late blight fungus, citrus canker bacterium (Xanthomonas axonopodis pv citri) and fastidious bacteria are
transmitted by this method
b) Indirect or Passive dispersal
• Passive dispersal of plant pathogens is accomplished through the agency of members of animal kingdom
(man, insects, nematodes, farm and wild animals and birds etc), air and water etc.
i) Dispersal by members of animal kingdom
i a) Man
• Man is the single most important factor affecting dispersal of plant pathogens fro a limited area to ong
distances.
• The day to day activities of man in normal faming practices and his trade and commerce activities help the
dispersal of pathogens and their introduction to new areas
• Vegetative propagation of ornamentals, fruit trees, tuber crops, and plantation crops is a highly efficient
means of introducing a pathogen or disease in new areas.

36. • Such propagation can disperse the pathogens from field to field, orchard to orchard and from one geographic
area to another
• Role of commercial nurseries is very important. The nurserymen who do not care to follow proper hygienic
methods, distribute contaminated or infected propagative materials to the growers which leads to the
introduction and dispersal of a pathogen into new areas
• E.g. late blight potato, citrus canker and powdery as well as downy mildew of grapes are some common
examples in the history of plant pathology in which man was responsible for dispersal and introduction of
diseases in new areas
• Seed trade is another means of pathogen dispersal in which man plays an important role
• The import and export of contaminated seed without proper precautions i.e. certification and quarantine
result in entry of pathogens from one country to another
• E.g. Karnal bunt of wheat, an endemic disease in India has spread to Maxico and some European countries
when the infected seed was taken there for experimentation.
• Loose smut of wheat, grain smut of barley, ergot of pearl millet and ear cockle of wheat are common
diseases which get dispersed by the infected seed
• Spores and other structures of fungi, bacterial cells in ooze and even viruses can be carried by clothings,
shoes and hands etc .of workers from plant to plant and field to field

37. • Use of contaminated implements , cutting knives etc. help in transmission of many soil borne and other
pathogens and also the pathogens present in propagative material from one plant or field or area to another.
• The rattoon stunting bacterium has no other means of transmission except during harvesting by cutting tools
• Man is responsible for the spread of almost all graft transmissible diseases. Grafting and budding between
diseased and healthy plant is the most effective method of distribution off viruses in fruit trees
• Cultural operations like hoeing, weeding, pruning, harvesting and packing of fruits in orchards are all carried
out by man and these not only transmit the pathogens from one plant to other but, also transport them to
long distances.
i b) Insects
• Insects are the second most important agents of pathogen dissemination next to man
• They can disseminate bacteria, fungi as well as viruses most importantly
• Some bacterial pathogens exclusively depend upon insects for their survival and dispersal e.g. Erwinia
trachiephila (cucurbit wilt) survives in the body of cucumber beetle (Acalymna vittatum) and 12 spotted
cucumber beetle (Diabrotica undecipuntata) during winters and is dispersed through these beetles from one
area to another
• The xylem limited bacterium Xylella fastidiosa is transmitted by its insect vectors in a permanent manner

38. • Insects are important media for dispersal of several fungal pathogens of plants
• Although dispersal of fungi by insects in accidental, but it has the chief role in spreading those diseases in
which pathogen produces some sugary or sticky substances
• E.g. in ergot of pearl millet, the honey dew stage of disease attracts the insects and the fungal conidia get
smeared on their bodies. When such insects sit on the healthy ears, they spread the fungus to those flowers.
• In Dutch elm disease, the spores are dispersed exclusively by insects
• Insects also play important role in the dispersal of fungi and bacteria infecting underground parts of plants in
the soil.
• Insects are also important agency of bringing about diploidization of haploid stage of rusts and other
heterothallic fungi
• the most important role of insects in pathogen dispersal is in virus transmission
• A vast majority of virus diseases are exclusively transmitted by insect vectors
• Most of insect vectors of viruses have sucking mouth parts e.g. aphids, leafhoppers, plant hoppers, white flies
and thrips etc. in addition some mites having sucking mouth parts also transmit plant viruses
• Beetle and caterpillars having biting and chewing mouth parts transmit plant viruses only accidently and
mechanically
• The insects acquire the viruses in their mouth / body after feeding on the infected plants and carry them to
healthy plants when they feed on latter.

39. i c) Nematodes
• Nematodes are soil borne pathogens
• They are associated with many fungal and bacterial diseases of roots but, they mainly predispose and
aggravate the disease
• Once they start infection a plant, they can not carry the fungal or bacterial pathogen to other plants due to
their limited mobility
• the bacterial yellow ear rot of wheat can not develop unless the associted nematode is not present. The
nematode acts as a vector of the bacterium as a surface contaminant in the galls
• Many viruses are also known to be transmitted by nematodes
• Important nematode vectors of plant viruses are Xiphinema, Longidorus, Trichodorus and Paratrichodorus
• There is acquisition period and inoculation feed time for a particular virus in the nematode.
• The nematode can retain the virus in their bodies for varying periods of time but, can not retain it after the
molting
• Once viruliferous the nematode remains viruliferous throughout their lives without undergoing molting
• There is also specificity of transmission. Different strains of same virus are often transmitted by different
species of same nemtode

40. i d) Dispersal by animals
• In addition to man, insects and nematodes other larger animals can also disseminate the plant pathogens
accidently
• Birds feeding on insect vectors can carry the viruses to longer distances which are not generally covered by
vector itself.
• Dispersal of fungal spores on the bird feathers is also possible, but there is no evidence till date
• Birds feed on the berries of phanerogamic plant parasites and disperse their seeds on other trees.
• Stem fragments of dodder are carried by birds for preparing their nests and are dispersed to varying distances
• Rodents and fur feeding animals can also transmit the plant pathogens
• Cattle feeding on contaminated fodder can pass out viable fungal propagules in their dung and can act as
source oof inoculum when used as manure. E.g. sclerotia of many fungi can be seen in cattle dung
ii) Dispersal by fungi and phanerogamic plant parasites
• Fungi growing in soil are suspected to carry bacterial cells externally and put them in roots of susceptible
plants
• The most important interaction is between fungi and soil borne viruses which are transmitted by members of
orders Chytridiales and Plasmodiophorales of fungi
• Important fungal genera acting as virus vectors include Olpidium spp., Polymyxa spp. and Spongospora spp.

41. • Phanerogamic plant parasite dodder is a total stem parasite of crop and fruit plants and roadside trees. It
establishes parasitic relationship with the vascular tissues of the host plants and transmits many viruses and
fastidious bacteria from one plant to another.
• Dodder picks up the pathogen through its haustorium and transmits it through its stem via vascular tissue
• Tobacco rattle virus and citrus greening bacterium are transmitted through dodder.
iii) Dispersal by water
• Water provides for short and long distance dispersal of fungus, bacterial and nematode pathogens
• Water helps in long distance dispersal of fungi and bacteria with the help of air
• Dissemination of wilt causing fungi, bacterial blight of rice and collar rot pathogen that survive in the soil or
plant debris can occur through water flow in the field
• Passive movement of nematode larvae can also be facilitated by flowing water
• One of the most efficient method of water dispersal is through rain drop splashes or water drops falling with
force with sprinkler irrigation on sori, pustules, cankers, bacterial ooze or even the soil surface. These may be
splashed in the water droplets and with the help of air currents enable them to land on healthy plants
• In many fungi, the spores are embedded in the matrix and can not take off unless wetted by water
• When spores are present in air, the rain drops wet them and enable them to land on the healthy plants

42. iii) Dispersal by air
• Air is not a habitat for any kind of plant pathogens. It only acts as a carrier of propagules of organisms which
produce structures adapted for aerial dispersal
• Air dispersal is the most common method of dissemination of pathogens whether they are soil or seed borne
; or they survive in collateral hosts, alternate hosts or early sown crops
• Seeds of phanerogamic plant parasites are carried efficiently by wind.
• However, viruses are not directly transmitted by air, but the direction of flight of their insect vectors is
determined by wind currents
• Wind is the most important source of dispersal of some important plant pathogenic fungi
• To become air borne, fungal spores must be able to cross the air flow near the host surface. The spore
discharge occurs with sufficient force to enable them cross this boundary and reach the upper air currents
• The spores and other propagules to be dispersed through air should be produced in large numbers and are
light in weight
• Apart from spores, bits of mycelium are also sometimes disseminated by wind
• Air also carries the rain drop splashes containing fungal spores
• The cysts of nematodes present in upper layer of soil are also dispersed by air to long distances
• E.g. in plains of northern India the annual recurrence of cereal rusts is due to urediospores brought by wind
from the source of their survival in north west hills

44. H. Survival of the Pathogen
• Survival means the overwintering or over summering of the pathogen in the absence of susceptible host or
sources for renewal of infection chain of a pathogen in one or the other way
• In the absence of cultivated host, the animate pathogens must find some alternate source of their survival to
maintain the infection chain
• The sources of survival of pathogens can be grouped as below:
i) Infected host as a reservoir of inoculum
ii) Saprophytic survival outside the host
iii) Dormant spores and structures in / on / outside the host
• These survival structures finally constitute the sources of primary inoculum in the infection chain
a) Survival of fungal pathogens
• The fungal pathogens have more elaborate mechanisms of survivalthan other pathogens
i) Infected host as a reservoir of primary inoculum
• Generally the host on which fungal pathogen survives is not only the substrate of the pathogen but, it also
exhibits symptoms of a disease
• If the pathogen is present in the host without producing symptoms is called a carrier of pathogen

45. • The infected host serving as reservoir of active inoculum can be considered in three groups:
 The cultivated host
 Wild host of same family as the host (collateral host)
 Wild hosts of other family (alternate hosts)
• Cultivated hosts are seasonal or perennial
• Perennial cultivated hosts are the good sources for survival of pathogens
• Most diseases of fruit trees maintain the active or dormant primary inoculum throughout the year. E.g. brown ro
fungi of pome and stone fruits survive through cankers and dead twigs, leaves and mummified fruits left hangin
on the tree
• In the absence of main host crop the pathogen can also survive in active form on collateral hosts.
• E.g. in case of powdery mildew of cucurbits, the pathogens (Erysiphe cichoracearum and Sphaerotheca fuliginea
do not produce cleistothecia more commonly in Indian conditions but, they survive in conidial stage in oof seaso
cucrbits growing in shaded places or in cooler hills of north where cleistothecia are also produced
• The cereal rust fungi in India also survive in their active uredial stage on wheat and collateral hosts in the hills of
northwest and south
• Alternate hosts important sources of pathogen survival only when they have a wide host range across many plan
families and can tolerate a wide range of weather conditions. E.g. Fusarium moniliformae, Sclerotium rolfsii and
Rhizoctonia solani

46. ii) Saprophytic survival of fungal pathogens
• In the absence of living host, the facultative parasites are capable of surviving as saprophytes
• Soil and plant debris serve as media for this saprophytic survival
• In some cases like Pythium and Sclerotium spp., the fungi remain active by growing saprophytically on the
dead organic matter or plant debris in the soil. But these fungi are susceptible to antagonists present in
their vicinity in the soil. These fungi grow actively for sometime as saprophytes in the soil and as soon as
they sense the presence of other microbes in their vicinity, they produce their resting structures or spores.
These resting structures or spores again germinate after sometime by producing active mycelium which again
forms resting structures after sometime
• Another category of saprophytic pathogens is more resistant to the soil antagonism and survive in the
rhizosphere for a longer time actively. But if the organic matter already colonized by other microbes is added
to such soil, then their survival ability is reduced to a considerable extent
• Third category of saprophytic survival in soil is of those pathogens which have low competitive survival
ability and survive saprophytically for only a short time. These are root inhabiting fungi (Fusarium,
Verticillium etc)and can survive in active form saprophytically for as long as their roots are not decomposed
completely. After decomposition of their host tissues, they are displaced by other strong saprophytes in the
soil and so they survive as resting structures.

47. iii) Survival in the form of dormant structures
• Among all pathogens, only fungi produce dormant structures and these are the major and most important
sources of their survival.
• The dormant structures of survival can be grouped in the following categories:
 Soil borne fungi:
a) Dormant spores (conidia, chlamydospores, oospores, perithecia etc)
b) Other dormant structures like thickened hyphae and sclerotia
• In Plasmodiophora brassicae (club root of crucifers), the pathogen survives in the form of its resting spores
lying free or in plant debris in the soil. These spores remain viable for upto 10 years, but no dormancy
period is required for them. Other fungi producing such resting spores in clude Spongospora subterranea
and Synchytrium endobioticum
• Oospores in Pythium, Phytophthora, Albugo and downy mildews, cleistothecia in powdery mildews,
chlamydospores in Fusarium and sclerotia in Sclerorinia, Rhizoctonia, Sclerotium and Claviceps are
dormantt structures which can survive in soil in free state or with plant debris. These structures are
generally formed in response to external stimuli i.e. when the substrate is exhausted and is unable to
support active growth of the fungus
• Sometimes, the stromatic mass of fungal hyphae as in case of Cercospora spp also serves as dormant
structure of survival
• Conidia of some fungi imperfectii serve as resting structures as they remain viable in crop debris for a long
time and can help in survival of the fungus

48. • Telia in rust fungi and smut spores on seeds and in leaf tissues are also dormant structure of survival
• Formation of resting structures ensures survival in the in presence of strong antagonism
 Seed borne fungi:
a) Dormant spores on seed coat
b) Dormant mycelium and spores under the seed coat or in the embryo
• The spores of smuts and many other fungi (Ustilago hordei,Sphaerotheca sorghi, Tilletia spp) are present in
dormant state on the seed surface
• In some cases (Ustilago tritici, Ascochyta rabiei) the fungus is present in dormant state within the seed,
often in embryonic tissues
• in some cases (Phomopsis vexans) the pathogen is both externall and internally seed borne.
 Dormant fungal structures on dormant or active host including dead plant parts remaining on the trees
• The dormant structures of fungus can also be present on the host
• In powdery mildew of apple (Podosphaera leucotricha) the fungus survives as dormant mycelium and
encapsulated haustoria in infected dormant terminal and lateral buds and in blossom buds
• In many pathogens (Monilinia sp and Phytophthora sp), the fungus is found in dormant state in mummified
fruits and dead twigs

49. b) Survival of bacterial pathogens
• Phytopathogenic bacteria do not form resting spores or any other such structures except for Bacillus and
Clostridium species which form resting endospores or Streptomyces scabies which forms conidia
• Most bacteria have very limited soil survival even in the debris of infected host if it is buried deep
• They mostly survive in mild or vigorous form on the host
i) Survival with the seed
• Association of phytobacteria with seeds is known since long
• Bacterial cells are not subject to dormancy and their survival corresponds to seed viability
• Some pathogens die before the seed loses its viability (Xanthomonas axonopodis pv malvacearum
causingcotton blight), while many survive more than the longevity of the seeds as in case of legumes
• Many species of Xanthomonas and Pseudomonas survive in host seeds from 3 to 20 years
• Level of seed infection or contamination varies with crops and environmental conditions
• Even a low level of seed borne inoculum, if successful in initiating the disease ,can lead to serious epidemics
under favourable conditions
• Success of seed borne bacteria depends upon their location in the seeds

50. • Most of the phytopathogenic bacteria are internally seed borne
• Many seeds contain vascular elements in the testa extended upto varying extents. Once the bacteria given
entry into the testa of seeds, they can enter the vascular elements which promotes survival and subsequent
pathogenesis of bacteria
ii) Survival in plant residue
• The survival of phytobacteria through diseased crop residue depends mostly upon the speed with which the
debris decomposes. When the debris is buried deep in the soil or in wet areas, the survival is less due to
early decomposition of debris. These bacteria are weak competitors of soil microorganisms associated with
decomposition of debris
• Coryneform bacteria are not considered soil borne pathogens, but some survive with the debris for a year or
more. E.g. Clavibacter michiganensis sub sp insidiosum survives in dry alfalfa stem left on the soil for 10 years
• Species of Xanthomonas have the poorest ability to survive outside the host. They show a rapidly declining
phase in crop debris depending upon the rate of decomposition of debris
• The species of Pseudomonas behave much in the same manner as the species of Xanthomonas in the plant
debris
• The persistence of Ralstonia solanacearum (bacterial wilt in tomato and potato) in infested debris depends
on the environment since either desiccation or the antagonistic effect of secondary invaders decreases their
population rapidly

51. iii) Survival in soil
• Bacteria reach the soil through plant debris, rain drop splashes, sprinkler irrigation washings or dry flakes of
bacterial ooze displaced by wind
• The bacteria which normally colonize underground parts like tubers, bulbs and roots are released in the soil
after their disintegration
• Most bacteria do not survive well in the soil. Different bacteria have been categorized on the basis of their
affinity and survival with the soil as follows:
 Transient visitors :
• These are the bacterial species whose populations are developed exclusively on the host plants where
maximum number of generations are produced.
• Their populations decline rapidly as soon as these bacteria reach the soil either with plant debris or rain
splashes
• They do not form a source of primary inoculum from the infected soil
• Most phytopathogenic bacteria are poor soil competitors and fall under this category. E.g. most species of
Xanthomonas, non soft rot causing species of Erwinia & Clavibacter michiganensis
 Resident visitors:
• These bacteria also have their maximum generations in the host but their populations decline gradually
when in the soil.
• If their populations enter the soil at sufficient levels, they can survive from season to season, but their
persistence in the soil is dependent on presence of host at least once in a year

52. • E.g. Agrobacterium tumefaciens, Ralstonia solanacearum, Streptomyces, scabies & S.ipomoea are best
examples in this category
 True saprophytes:
• These bacteria are true saprophytes and have permanent soil phase in the life cycle
• Their populations are produced largely in the soils and rhizosphere and they colonize the host plants
opportunistically.
• E.g. fluorescent Pseudomonas causing soft rots, some species of Bacillus and soft rotting Clostridium
iv) Survival in perennial hosts
• Usually some characteristic microflora including some bacteria are present on different plants or their organs
which live in plant exudates. These surface flora are called Epiphytes and are present on roots (rhizoplane),
buds (gemmiplane) and leaves (phylloplane) as well as seed surface.
• These microflora generally exist in a balance and mostly antagonistic to plant pathogenic bacteria, but if the
population of pathogenic bacteria increase in the microflora, they may constitute the source of primary
inoculum.
• E.g. citrus canker bacterium (Xanthomonas axonopodis pv citri) colonizes the underground and exposed roots
and survives there for long
• Pseudomonas syringae pv syringae (bacterial canker of stone fruits) is a good epiphyte and survives on
foliage of weeds growing in the orchards

54. v) Association with insects
• Some bacterial pathogens use insects as their host and source of survival and dispersal
• E.g. potato black leg bacterium (Erwinia carotovra subsp atroseptica) can live in all stages of seed corn
maggot ( Hylemya platura) and persist in the intestinal tract inspite of its ability to survive through tubers and
soil.
c) Survival of plant pathogenic nematodes
• Some nematodes maintain continuity of infection chain through active parasitic living on variety of hosts
while others (majority) survive through their dormant structures like eggs, cysts, galls, cockles formed from
host tissues
• Quiescence is an adaptation for survival of nematodes under adverse conditions. The nematodes become
dormant or anabiotic. In this condition they can live for many years. E.g. in quiescent state, the second stage
larvae of Anguina tritici are known to remain viable for up to 28 years in cockles.
• The root knot nematode has a very wide host range in Solanaceae, Cruciferae and Malvaceae, hence the
nematode can survive on one or the other host of these families grown one after the other in the same field
• Cyst nematodes are host specific and mainly survive through their cyst for many years. E.g. the cysts of
potato cyst nematode (Globodera rostochiensis) continue to contain viable larvae for 7-8 years in the absence
of host

55. d) Survival of phanerogamic plant parasites
• Phaneogamic plants parasites produce seeds, just like other flowering plants and these seeds can remain
dormant for many years. E.g. the seeds of dodder (Cuscuta) fall on the ground and remain viable in dormant
state until a favourable host or season is available
• The parasites attacking perennial hosts survive in active state
e) Perennation of plant viruses
• Plant viruses have no dormant stage and maintain a continuous infection chain.
• They are actively present in crop host or in its absence in some collateral or alternate host