Summary of fourth lesson ASCOMYCETES, BASIDIOMYCETES, OOMYCETES DISEASE TRIANGLE+ humans...

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