DISEASES AND TREES

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DISEASES AND TREES

• What exactly is a disease? It is the outcome of
  an interaction between a plant and the
  environment, resulting in an altered physiology
  of the host
• Sustained interactionbiotic
• Single event abiotic

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What is a pathogen?

• Strictly speaking a pathogen is the causal agent
  of disease
• Bacteria
• Viruses
• Nematodes
• Stramenopiles
• Algae
• Phytoplasmas
• Higher plants

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And of course fungi

• Fungi saprophytic, symbionts, and pathogens
• Polyphyletic group in evolutionary terms
• Basidiomycetes
• Ascomycetes
• Zygomycets
• Animals
• Plants
• Red algae
• Brown algae
• Myxomycetes

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Fungi again!

• Filamentous somatic (vegetative body)
• High surface, good for extrogenous digestion
• Good infection structures, infection peg,
  appressoria, rhizomorphs
• Chitin in cell wall
• Nuclear ploidy very unique
• Reproduction by spores asexual mode very well
  represented
• Small nuclei, but with a lot of plasticity

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Hyphae, sporangia, and zoospores of P. ramorum
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Fungi do not photosynthesize

• Biotrophic mycorrhyzae, rusts
• Endophites clavicipetaceae,
• Necrotrophic most pathogens
• Saprobes primary (involved in litter
  decomposition)

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Some pathogen roles in natural plant communities

• Selection of individuals best suited for the site
• Maintenance of genetic diversity and stability in
  host plant populations
• Establishment or maintenance of host geographic
  ranges
• Natural succession
• Regulation of stand density, structure, and
  composition

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DISEASE!!

• Symptoms vs. signs e.g. chlorosis vs. fruit-body
• The disease triangle

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host-pathogen-environment

• Susceptibility of individuals or of portions of
  individuals
• Genetic variability
• Basic compatibility (susceptibility) between host
  and pathogen
• Ability to withstand physiological alterations

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Genetic resistance in host
Length of lesion (mm) Proportion of stem girdled ()
Nicasio\ 42.5a 0.71a
China Camp 40.5a 0.74a
San Diego 27.8b 0.41b
Ojai 25.0b 0.47b
Interior live oak (Maricopa) 14.1b 0.33b
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host-pathogen-environment

• Basic compatibility with host (virulence)
• Ability to maintain diversity sex vs. no sex
• Size of genetic pool
• Agressiveness (pathogenicity) towards hosts
• Ability to survive without host

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Chlamydospores of P. ramorum
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West Coast
Europe
P. lateralis
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host-pathogen-environment

• Temperatures
• Shading
• Relative humidity
• Free standing water
• pH and any potentially predisposing factors
• Nutrient status

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Colony diameter (mm) at 13 days
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Presence of free water
Between 6 and 12 hours required for infection of
bay leaves
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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

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

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Effects of diseases on host mortality, growth and
reproduction

• Young plants killed before reaching reproductive
  age
• Affect reproductive output
• Directly affect flowers and fruits

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Complexity of forest diseases

• At the individual tree level 3 dimensional
• At the landscape level host diversity,
  microclimates, etc.
• At the temporal level

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Complexity of forest diseases

• Primary vs. secondary
• Introduced vs. native
• Air-dispersed vs. splash-dispersed, vs. animal
  vectored
• Root disease vs. stem. vs. wilt, foliar
• Systemic or localized

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Progression of cankers
Hypoxylon, a secondary sapwood decayer will
appear
Older canker with dry seep
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Stem canker on coast live oak
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Cankers by P. ramorum at 3 months from time of
inoculation on two coast live oaks
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Root disease center in true fir caused by H.
annosum
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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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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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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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POPULATION DYNAMICS

• Species interactions and diversity

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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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Diseases and succession

• Soil feedbacks normally its negative. Plants
  growing in their own soil repeatedly have higher
  mortality rate. This is the main reason for
  agricultural rotations and in natural systems
  ensures a trajectory towards maintaining
  diversity
• Phellinus weirii takes out Douglas fir and
  hemlock leaving room for alder

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Janzel-Connol

• Regeneration near parents more at riak 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

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The red queen hypothesis

• Coevolutionary arm race
• Dependent on
• Generation time has a direct effect on rates of
  evolutionary change
• Genetic variability available
• Rates of outcrossing (Hardy-weinberg equilibrium)
• Metapopulation structure

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Frequency-, or density dependent, or balancing
selection

• New alleles, if beneficial because linked to a
  trait linked to fitness will be positively
  selected for.
• Example two races of pathogen are present, but
  only one resistant host variety, suggests second
  pathogen race has arrived recently

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Diseases as strong forces in plant evolution

• Selection pressure
• Co-evolutionary processes
• Conceptual processes potentially leading to a
  balance between different ecosystem components
• How to measure it parallel evolution of host and
  pathogen

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• Rapid generation time of pathogens. Reticulated
  evolution very likely. Pathogens will be selected
  for INCREASED virulence
• In the short/medium term with long lived trees a
  pathogen is likely to increase its virulence
• In long term, selection pressure should result in
  widespread resistance among the host

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More details on

• How to differentiate linear from reticulate
  evolution comparative studies on topology of
  phylogenetic trees will show potential for
  horizontal transfers. Phylogenetic analysis
  neeeded to confirm horizontal transmission

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Geneaology of S DNA insertion into P ISG
confirms horizontal transfer.Time of
cross-over uncertain
NA S
NA P
EU S
890 bp CIgt0.9
EU F
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Phylogenetic relationships within the
Heterobasidion complex
Fir-Spruce
Pine Europe
Pine N.Am.
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HOST-SPECIFICITY

• Biological species
• Reproductively isolated
• Measurable differential size of structures
• Gene-for-gene defense model
• Sympatric speciation Heterobasidion, Armillaria,
  Sphaeropsis, Phellinus, Fusarium forma speciales

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Phylogenetic relationships within the
Heterobasidion complex
Fir-Spruce
Pine Europe
Pine N.Am.
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SEX

• Ability to recombine and adapt
• Definition of population and metapopulation
• Different evolutionary model
• Why sex? Clonal reproductive approach can be very
  effective among pathogens

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The scale of disease

• Dispersal gradients dependent on propagule size,
  resilience, ability to dessicate, NOTE not
  linear
• Important interaction with environment, habitat,
  and niche availability. Examples Heterobasidion
  in Western Alps, Matsutake mushrooms that offer
  example of habitat tracking
• Scale of dispersal (implicitely correlated to
  metapopulation structure)---two examples
  Heterobasidion in California, and Coriolopsis in
  Panama

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From Garbelotto and Chapela, Evolution and
biogeography of matsutakes
Biodiversity within species as significant as
between species
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S-P ratio in stumps is highly dependent on
distance from true fir and hemlock stands
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.
San Diego
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White mangroves Corioloposis caperata
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Distances between study sites
White mangroves Corioloposis caperata
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Forest fragmentation can lead to loss of gene
flow among previously contiguous populations.
The negative repercussions of such genetic
isolation should most severely affect highly
specialized organisms such as some
plant-parasitic fungi.
AFLP study on single spores
Coriolopsis caperata on Laguncularia racemosa
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From the population level to the individual

• Autoinfection vs. alloinfection
• Primary spreadby spores
• Secondary spreadvegetative, clonal spread, same
  genotype . Completely different scales
• Coriolus
• Heterobasidion
• Armillaria
• Phellinus

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Recognition of self vs. non self

• Intersterility genes maintain species gene pool.
  Homogenic system
• Mating genes recognition of other to allow for
  recombination. Heterogenic system
• Somatic compatibility protection of the
  individual.