Summary of fourth lesson

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Summary of fourth lesson

• ASCOMYCETES, BASIDIOMYCETES, OOMYCETES
• DISEASE TRIANGLE humans
• Dominant/CO-Dominant/ Genotype

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Disease triangle

• Effect of humans

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Human activities affecting disease incidence in
forests

• Introduction of exotic pathogens
• Planting trees in inappropriate sites
• Changing stand density, age structure,
  composition, fire frequency
• Wound creation
• Pollution, etc.

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Effects of fire exclusion
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DISEASE plant microbe interaction

• 1-Basic compatibility need to be present
• 2- Chemotaxis, thighmotropy
• 3- Avirulence in pathogen matched by resistance
  in host according to the gene for gene model
• 4-Pathogenicity factors such as toxins and
  enzymes important in the infection process

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1- Basic compatibility

• Size of infectious propagules
• Timing of susceptibility in host and production
  of infectious structures

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2- Finding the host

• Chemotaxis pathogen has receptor that detects
  food base in oomycetes zoospores will all swim
  towards host
• Thigmotropy recognizing morphological structures
  that indicate presence of host prelude to
  production of infective structures such as
  infection pegs and appressoria

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3- Infecting the host

• Pathogen will produce array of enzymes to infect
  host cells
• Upon identification of infection, host will
  produce array of antimicrobial compounds , or
  will kill some of its cells to halt infection
  process (hypersensitive response)

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3- Infecting the host

• Plant that are resistant, must be able to react
  (dominant R resistant allele)
• Plants that cannot react (r allele) are always
  sensitive
• Pathogens that are not noticed by plant can
  infect (recessive avirulence allele)
• Pathogens that are noticed may be stopped
  (dominant A avurulence allele)

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3- Infecting the host

• RA no disease
• Radisease
• radisease
• rAdisease
• There will be a strong selection in favor of R
  alleles but R comes at a cost

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4- Causing disease

• Correlated to ability of pathogen to invade plant
  cell, pathogenicity is usually a dominant trait

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Categories of wild plant diseases

• Seed decay
• Seedling diseases
• Foliage diseases
• Systemic infections
• Parasitic plants
• Cankers, wilts , and diebacks
• Root and butt rots
• Floral diseases

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Seed diseases

• Up to 88 mortality in tropical Uganda
• More significant when seed production is episodic

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Stress cone crop
BS on DF
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Seedling diseases

• Specific diseases, but also diseases of adult
  trees can affect seedlings
• Pythium, Phytophthora, Rhizoctonia, Fusarium are
  the three most important ones
• Pre- vs. post-emergence
• Impact up to 65 mortality in black cherry.
  These diseases build up in litter
• Shady and moist environment is very conducive to
  these diseases

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Foliar diseases

• In general they reduce photosynthetic ability by
  reducing leaf area. At times this reduction is
  actually beneficial
• Problem is accentuated in the case of small
  plants and in the case other health issues are
  superimposed
• Often, e.g. with anthracnose,needle cast and
  rust diseases leaves are point of entry for twig
  and branch infection with permanent damage
  inflicted

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Systemic infections

• Viral?
• Phytoplasmas
• Peronospora and smuts can lead to over 50
  mortality
• Endophytism usually considered beneficial

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Grass endophytes

• Clavicipetaceae and grasses, e.g. tall fescue
• Mutualism antiherbivory, protection from
  drought, increased productivity
• Classic example of coevolutionary development
  Epichloe infects flowers of sexually
  reproducing fescue, Neotyphodium is vertically
  transmitted in species whose sexual reproductive
  ability has been aborted

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Parasitic plants

• True (Phoradendron) and dwarf mistletoe
  (Arceuthobium)
• Effects
• Up to 65 reduction in growth (Douglas-fir)
• 3-4 fold mortality rate increase
• Reduced seed and cone production
• Problem accentuated in multistoried uneven aged
  forests

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Cankers, wilts, and die-backs

• Includes extremely aggressive, often easy to
  import tree diseases pine pitch canker, Dutch
  elm disease, Chestnut blight, White pine blister
  rust
• Lethal in most cases, generally narrow host range
  with the exception of Sudden Oak Death

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Root diseases

• Extremely common, probably represent the most
  economically damaging type of diseases
• Effects tree mortality (direct and indirect),
  cull, effect on forest structure, effect on
  composition, stand density, growth rate
• Heterobasidion, Armillaria, Phellinus weirii,
  Phytophthora cinnamomi

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Removing food base causes infection of roots of
other trees
Hyphae in plant tissue or soil (short-lived)
Melanin-covered rhizomorphs will allow for fungus
to move to new food Sources (Armillaria mellea)
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Effects of fire exclusion
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Floral diseases

• Pollinator vectored smut on silene offers an
  example of well known dynamic interaction in
  which pathogen drives genetic variability of
  hosts and is affected by environmental condition
• Puccinia monoica produces pseudoflowers that
  mimic real flowers. Effects reduction in seed
  production, reduction in pollinators visits

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Density-dependence

• Most diseases show positive density dependence
• Negative dependence likely to be linked to
  limited inoculum e.g. vectors limited
• If pathogen is host-specific overall density may
  not be best parameter, but density of susceptible
  host/race
• In some cases opposite may be true especially if
  alternate hosts are taken into account

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Counterweights to numerical effects

• Compensatory response of survival can exceed
  negative effect of pathogen
• carry over effects?
• NEGATIVE progeny of infected individuals less
  fit
• POSITIVE progeny more resistant (shown with
  herbivory)

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Disease and competition

• Competition normally is conducive to increased
  rates of disease limited resources weaken hosts,
  contagion is easier
• Pathogens can actually cryptically drive
  competition, by disproportionally affecting one
  species and favoring another

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Janzen-Connol

• Regeneration near parents more at risk of
  becoming infected by disease because of proximity
  to mother (Botryosphaeria, Phytophthora spp.).
  Maintains spatial heterogeneity in tropical
  forests
• Effects are difficult to measure if there is
  little host diversity, not enough
  host-specificity on the pathogen side, and if
  periodic disturbances play an important role in
  the life of the ecosystem