Transcript DISEASES AND TREES

Summary of fourth lesson
• ASCOMYCETES, BASIDIOMYCETES, OOMYCETES
• DISEASE TRIANGLE+ humans
• Dominant/CO-Dominant/ Genotype
Disease triangle
Effect of humans
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.
Effects of fire exclusion
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
1- Basic compatibility
• Size of infectious propagules
• Timing of susceptibility in host and
production of infectious structures
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
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)
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)
3- Infecting the host
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RA= no disease
Ra=disease
ra=disease
rA=disease
There will be a strong selection in favor of R
alleles but R comes at a cost
4- Causing disease
• Correlated to ability of pathogen to invade
plant cell, pathogenicity is usually a
dominant trait
Categories of wild plant diseases
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Seed decay
Seedling diseases
Foliage diseases
Systemic infections
Parasitic plants
Cankers, wilts , and diebacks
Root and butt rots
Floral diseases
Seed diseases
• Up to 88% mortality in tropical Uganda
• More significant when seed production is
episodic
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
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
Systemic infections
• Viral?
• Phytoplasmas
• Peronospora and smuts can lead to over
50% mortality
• Endophytism: usually considered beneficial
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
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
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
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
Removing food base causes
infection of roots of other trees
Hyphae in plant
tissue or soil (shortlived)
Melanin-covered rhizomorphs will
allow for fungus to move to new food
Sources (Armillaria mellea)
Effects of fire exclusion
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
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
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)
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
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