CHAPTER 2 PLANT PATHOLOGY

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CHAPTER 2PLANT PATHOLOGY

• PARASITISM AND DISEASE DEVELOPMENT

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PARASITISM AND DISEASE DEVELOPMENT

• The pathogens that attack plants belong to the
  same groups of organisms that attack people
• Except some diseases carried by vectors, no
  species of pathogen that attacks plants are know
  to affect people
• Infectious diseases are those that result from
  infection by a pathogen and can be spread to
  other plants

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Parasitism and Pathogenicity

• An organism that live on a derives food from an
  other organism is called a parasite
• Parasites remove nutrients and water from the
  host
• When it becomes detrimental to the host it
  becomes associated with pathogenicity
• Occasionally it is mutually beneficial such as
  with mycorrhizae or root nodule bacteria, (called
  symbiosis) (sort of like a successful marriage)

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PARASITISM AND PATHOGENICITY

• In most plant disease the damage is greater than
  the removal of nutrients etc.
• This is caused by toxic secretions from the
  pathogen or the host
• This may show up as disintegration, cell
  collapse, wilting, abscission, abnormal cell
  division or degeneration of components such as
  chlorophyll

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PARASITISM AND PATHOGENICITY

• Pathogenicity is the ability of the pathogen to
  interfere with one or more of the essential
  functions of a plant
• Fungi, bacteria, mollicutes, parasitic higher
  plants, nematodes, protozoa, viruses and viroids
  are the most successful pathogens
• Those pathogens that require a host to survive
  are called biotrophs, obligate parasites

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PARASITISM AND PATHOGENICITY

• Others that can live on living or dead hosts or
  growing media are called nonobligate parasites
• Those that live on live mostly on live host but
  can survive on dead material are called
  semitrophs or facultative saprophytes

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The Disease Triangle
Amount Of Disease
Environment Total of conditions favoring disease
Pathogen Total of virulence, abundance, etc
Total of conditions favoring
susceptibility Host
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Stages in the development of disease the disease
cycle

• Inoculation
• The coming in contact of a pathogen with a plant
• The pathogens that land on the plant are inoculum
• Primary inoculum overwinters and invades in the
  spring or fall, cause primary infection
• Inoculum produced from primary are called
  secondary inoculumgtsecondary infection

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Sources of inoculum

• Inoculum is sometimes present in the field debris
• Comes with the seed, transplants, or other
  propagative materials
• May come from other fields
• May overwinter on weeds
• Carried by vector, i.e. insects, animals, man

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Arrival of inoculum

• Most is carried by wind, water, insects
• Only a small number actually lands on susceptible
  plants
• Vector born pathogens are much more efficient
  since the vectors have an attraction for plants

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Prepenetration Phenomena

• All pathogens are capable of immediate infection
• Fungal spores and seeds of parasitic plants must
  first germinate
• They require high humidity and a film of water on
  the plant
• Some produce a germ tube, others produce other
  spores such as zoospores or basidiospores

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Prepenetration Phenomena

• Certain pathogens are stimulated by exudates of
  plants susceptible to that particular pathogen
• When soils containing antagonistic microorganisms
  repress other fungi the phenomenon is called
  fungistasis
• These soils are called suppressive soils

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Prepenetration Phenomena

• After spores germinate, the resulting germ tube,
  or zoospores must move.
• The number, length and rate of growth are
  regulated by physical conditions
• Chemical stimuli associated with wounds etc
  stimulate growth
• Parasitic plants germinate producing a radicle
  (root) or a plant forms and then penetrates host

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Hatching of Nematode Eggs

• Nematode eggs also require certain conditions and
  stimuli to hatch
• The juvenile immediately molts soon after the egg
  is laid
• When the egg hatches, the juvenile is already in
  the second stage
• It either finds and penetrates a plant or
  undergoes additional molts
• They are attracted to the plants by CO2 or amino
  acids

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Attachment of Pathogens to Host

• Some pathogens such as viruses, fastidious
  bacteria, and protozoa are placed directly into
  plants by their vector
• Most fungi, bacteria and parasitic plants are
  usually brought into contact with the surface of
  the host. This is done by a mucilaginous
  substance like glue

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Attachment of Pathogens to Host

• This glue is made up of polysaccharides,
  glycoprotein's, and fribrillar materials
• As spores and seeds germinate they also produce
  theses substances
• The areas of contact appear to degrade
• Presumed to be enzymes

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Host Recognition

• It is not known with any certainty what is the
  trigger but it may be one of many bio-chemical
  substances, structures, or pathways
• This may include host specific signals
• Host signals are many

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Host Recognition

• They may include
• Fatty acids of the plant cuticle that activates
  the production of the cutenase enzyme.
• Galacturonan molecules of pectin which stimulate
  pectin lyase enzymes
• Phenolic compounds such asstrigol which
  stimulates germination of propagules

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Host Recognition

• Phenolics and sugars released from wounds which
  stimulates certain pathogens
• The whole stimulation recognition process is
  still poorly understood
• When the initial recognition signal received by
  the pathogen favors growth, disease may be
  induced. A defense reaction may also be produced

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penetration

• Pathogens penetrate plant surfaces through
  natural openings or wounds
• Some fungi penetrate only one way others, several
  ways
• Bacteria enter mostly through wounds sometimes
  through natural openings but never directly
• Viruses, viroids, mollicutes fastidious bacteria
  and protozoa enter by vectors

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Direct penetration through intact plant surfaces

• The most common type of penetration by fungi and
  nematodes and the only type by parasitic plants
• Fungi use a fine hyphae produced by a spore or
  mycelium or penetration peg produced by an
  appressorium
• This is formed at the point of contact with the
  host

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Direct penetration through intact plant surfaces

• The penetration is through mechanical force and
  softening of the cell walls by an enzyme
• Most fungi form an appressorium at the end of the
  germ tube
• Then a penetration peg emerges from the flat
  surface of the appressorium and pierces the
  cuticle and cell wall

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Direct penetration through intact plant surfaces

• The peg grows into a fine hyphae then reaches a
  normal diameter once it is inside the cell
• As in the case of apple scab, the fungus
  penetrates only the cuticle and stays between the
  cuticle and the cell wall
• Parasitic plants also penetrate the same way

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Direct penetration through intact plant surfaces

• Nematodes penetrate by repeated back and forth
  thrusts of its stylet
• When it creates an opening it inserts its stylet
  or the entire nematode enters

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Penetration through wounds

• Bacteria, most fungi, some viruses, all viroids
  enter through wounds
• Viruses mollicutes, fastidious vascular bacteria,
  and protozoa enter through wounds made by
  vectors (insects, birds, pruning tools etc)
• How are wounds formed?
• Some pathogens can be only carried by specific
  vectors

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Penetration through natural openings

• Many fungi and bacteria enter through
• Stomata
• Hydathodes
• Nectarthodes
• Lenticels

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Penetration through stomata

• Most stomata are present in large numbers on the
  underside of the leaf
• They are open during the day but closed at night
  bacteria present in a film of water can easily
  swim into an open stomata
• Fungi can germinate on a wet surface and enter a
  stomata
• Often, and appressorium is formed over a stomata,
  and the peg then enters

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Hydathodes

• Open pores on the margins and tips of leaves
• Connected to veins
• Secrete droplets of liquids containing nutrients
  some bacteria but few fungi enter here
• Some enter nectarthodes which are similar to
  hydathodes but on blossoms

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Lenticels

• Openings on fruits, stems, tubers, filled with
  loosely connected cells to allow passage of air
• Relatively few fungi and bacterial enter this way
• A lees efficient , secondary pathway

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Infection

• The process by which pathogens establish contact
  with the host
• During infection, the pathogen grows or
  multiplies, colonizes the host plant
• Infection results in the appearance of symptoms
• Some infections remain latent and show up when
  conditions are more favorable

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Infection

• All changes in appearance make up the symptoms
• Symptoms may show up in 2-4 days or as long as
  2-3 years
• The time interval between inoculation and
  symptoms is called the incubation period
• In most plants the period is from a few days to a
  few weeks

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Infection

• During infection some pathogens
• Obtain nutrients without killing the cell
• Kill cells and use contents
• Kill cells and disorganize surrounding tissue
• Release enzymes, toxins, growth regulators
• The host reacts with defense mechanisms

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Invasion

• Various pathogens invade hosts in different ways
• Some like apple scab grow between the cuticle and
  the epidermis
• Powdery mildew grow on the surface and send
  haustoria into cells
• Some grow into and throughout the plant

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Invasion

• Into the cells is called intracellular
• Between the cells is called intrecellular
• Vascular wilts invade the xylem vessels
• Bacteria invade intercellularly the intracellular
• Most nematodes are intercellular some
  intracellular and some dont invade at all but
  feed from the surface

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Invasion

• Viruses, viroids, mollicutes, fastidious bacteria
  and protozoa invade by moving from cell to cell
• Viruses and viroids invade all types of cells
• Mollicutes, and protozoa invade phloem sieve
  tubes and some parenchyma
• Most fastidious bacteria invade xylem vessels and
  a few invade phloem sieve tubes

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Invasion

• Many infections caused by fungi, bacteria,
  nematodes, viruses, and parasitic plants remain
  local throughout the season or spread
• All infections caused by fastidious bacteria,
  mollicutes, and protozoa are systemic (internal)
• some fungi are also systemic

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Growth and Reproduction of the Pathogen

• Fungi and parasitic plants generally invade by
  growing into them from the initial point of
  inoculation.
• The spread continues until it is stopped or the
  plant is dead
• Fungi causing vascular wilt may produce spores
  that are carried in the sap to other parts of the
  plant

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Growth and Reproduction of the Pathogen

• Bacteria, mollicutes, viruses, viroids,
  nemetodes, and protozoa do not increase much
• These pathogens invade and infect new tissues
  rapidly increasing their numbers
• Plant pathogens reproduce in a variety of ways

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Growth and Reproduction of the Pathogen

• Fungi spores, sexual or asexual
• Parasitic plants seeds on branches
• Bacteria, mollicutes, protozoa fission
• Viruses and viroids replicate in cell like photo
  copy as long as machine works and have paper
• Nematodes eggs

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Growth and Reproduction of the Pathogen

• Most fungi reproduce inside the plant but most
  release spores outside
• Powdery mildew on the outside
• Viruses, viroids, mollicutes, protozoa, and
  fastidious bacteria only reproduce inside
• Fungi can produce millions of spores in a season
• Bacteria divide every 20-30 minutes

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Growth and Reproduction of the Pathogen

• Viruses can produce 100,000 to 10 million
  particles in a single cell
• Nematodes lay 300-500 eggs about half female who
  do the same and so on
• How many cats are produced from a pair during an
  8 year reproductive life span
• 2 to 8 million, same with nematode but during one
  season

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Dissemination

• Nematodes, fungal zoospores, and bacteria move a
  short distance on their own
• Some puff into the air and air carried by the
  wind
• Some parasitic plants shoot their seeds several
  meters
• Air, insects, water, humans also carry

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Dissemination by air

• Fungal spores and seeds of parasitic plants are
  carried by the air
• The spores land or are washed out by rain
• Spores can be carried from several to several
  hundred kilometers (high altitude)
• This can cause an epidemic over several years
• Bacteria , nematodes may be carried by wind,
  water etc

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Dissemination by air

• Bacterial fireblight exudes strands of bacteria
  that break off and air carried in the wind
• Nematodes and spores may blow along with ground
  debris
• Windy rain can carry many pathogens
• Touching plants can transfer pathogens

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Dissemination by water

• Bacteria, nematodes, spores, mycelium parts can
  move by rain or irrigation water
• All Bacteria and many spores are exuded in a
  sticky liquid that depends on dissemination by
  water
• Water is less important than air movement but
  still accounts for much contamination

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Dissemination by Insects, Mites, and other Vectors

• Aphids, leafhoppers, whiteflies are the most
  important vectors of viruses
• Leafhoppers are the most important vector of
  mollicutes, fastidious bacteria, and protozoa
• Specific insects transmit certain bacteria and
  fungi such as Dutch Elm disease and bacterial
  wilt of cucurbits

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Dissemination by Insects, Mites, and other Vectors

• Although some are specific, many pathogens such
  as soft rot, anthracnose, and ergot are carried
  by insects externally as they move from plant to
  plant
• Mites and nematodes can also carry virus
  pathogens internally and bacteria and fungus
  externally
• Animals that walk among plants also carry
• Parasitic plants can carry as they bridge

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Dissemination by Seed, Transplants, Budwood, and
Nursery Stock

• Many pathogens are present on seeds, transplants,
  budwood, or nursery stock and are disseminated
  during propagation
• The grower can infect his own stock and sell it
  out to garden centers, home owners etc.
• Crown gall of euonymus is a good example.
  Certified stock and seed is a must

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Dissemination by Humans

• Humans disseminate pathogens by handling diseased
  stock and then healthy
• Tobacco mosaic is transmitted through cigarettes
  , shoes, handsgttomatoes etc
• Humans may bring pathogens to new areas by
  traveling long distances
• Tools carry pathogens from plant to plant
• Fireblight is a good example

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Dissemination by Humans

• Diseases carried to new areas by humans
• Dutch elm disease fungus
• White pine blister rust fungus
• Citrus canker bacterium
• Powdery and downy mildew fungus of grape (Europe)

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Overwintering/ oversummering of pathogens

• Annuals and perennials die or die back and go
  dormant during the winter and some go dormant in
  the summer
• Pathogens have developed mechanisms to survive
  while their host is in this state
• On perennials, fungi overwinter as mycelium,
  cankers or spores
• Fungi on trees overwinters as mycellium
• or spores on fallen leaves and fruits

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Overwintering/ oversummering of pathogens

• Fungi on trees may also overwinter on buds scales
• Annuals, fungi overwinter on debris mass in the
  soil, on seeds
• Some fungi are permanent inhabitants of the soil
  and survive as saprophytes
• Other fungi live on plants but survive a short
  time in the soil

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Overwintering/ oversummering of pathogens

• Some fungi survive on the host all winter
• Some remain on winter crops in warmer climates
  and move to the same spring crop grown in colder
  climates
• Some fungi live on weeds and alternate between
  annual and perennial hosts
• Bacteria overwinter in much the same way

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Overwintering/ oversummering of pathogens

• Viruses, viroids, mollicutes, fastidious
  bacteria, and protozoa survive on living tissue
  such as perrenials or perennials that die back to
  the soil
• A few viruses survive on their insect vectors
• Nematodes overwinter/summer as eggs or on debris

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Relationships between disease cycles and epidemics

• Pathogens that complete their life cycle in one
  year are called monocyclic
• Fungi such as smut produce spores at the end of
  the season to begin the next season
• Root rot and vascular wilts require two seasons
  and alternate hosts to complete their life cycle

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Relationships between disease cycles and epidemics

• Some pathogens go through many generations in one
  year
• They are called polyclyclic pathogens
• They can complete from 2 to 30 cycles in per year
• Polyclyclic pathogens are usually airborne or
  airborne vectors and can cause explosive epidemics

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Relationships between disease cycles and epidemics

• Downy mildew, late blight of potato, powdery
  mildews, leaf spots and blights, grain rusts and
  insect borne viruses cause explosive epidemics
• The number of spores etc that survive the season
  are small but multiply rapidly once conditions
  are suitable

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Relationships between disease cycles and epidemics

• In some diseases of trees, fungal vascular wilts,
  phytoplasmal declines, and viral infections the
  pathogen may not complete the life cycle in one
  year but may take several years to produce
  inoculum.
• These are called polyetic
• Sine they remain in the host, they produce
  exponentially. (dutch elm disease)

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Nematodes