summary of fourth lesson ascomycetes, basidiomycetes, oomycetes disease triangle+ humans...
Post on 19-Dec-2015
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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
• 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
• 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
Stress cone crop BS on DF
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 (short-lived)
Melanin-covered rhizomorphs willallow 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