Summary

• Success of pathogen:survival structures, saprotrophic ability,host-specialization, phylogenetic distance, similar ecology of hosts

• Severity of epidemic: density of hosts, environment, genetics of host

• Resistance in host: present/absent, metapopulation structure

Summary-2

• RED QUEEN HYPOTHESIS• Genetic variability/genetic structure.

Population size• Generation time• Increased virulence linked to trade offs

Summary-3

• SIGNS• SYMPTOMS

And of course… fungi

• Fungi: saprophytic, symbionts, and pathogens

• Polyphyletic group in evolutionary terms– Basidiomycetes

Ascomycetes

Zygomycets

Animals

Plants

Red algae

Brown algae

Myxomycetes

Diversity of fungi, but all have ideal structure for plant infection:– hypha/cord/rhizomorph/infection peg/appressorium– Sexual vs. asexual reproduction: can do both– Do not photosynthesize– Chitin in cell wall– Exogenous digestion– Indefinite growth– Phenotypic plasticity and pleomorphisms

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Fungi do not photosynthesize

• Biotrophic: mycorrhyzae, rusts

• Endophites: clavicipetaceae,

• Necrotrophic; most pathogens

• Saprobes: primary (involved in litter decomposition)

Septa

Pores

Pores

CELLS

Thanks to their web-like indefinite Thanks to their web-like indefinite growth in soil and plant substrates and growth in soil and plant substrates and their way of digesting nutrients fungi their way of digesting nutrients fungi

play a critical role in recycling play a critical role in recycling nutrients which can then be reutilized nutrients which can then be reutilized

by plants by plants

•Fungi like this one will actually decay the woody matter and physically free space for new generations of trees, besides recycling the nutrients

The weblike structure of fungi, usually The weblike structure of fungi, usually immersed in the soil or in plant matter is immersed in the soil or in plant matter is

involved in an essential symbiosis that greatly involved in an essential symbiosis that greatly enhances the ability of plants to growenhances the ability of plants to grow

•piant

•fungus

The visible part of root tips of most The visible part of root tips of most trees is actually a mantle of fungal trees is actually a mantle of fungal hyphae fused with the plant tissuehyphae fused with the plant tissue

What is the deal of this mutualism?

• Fungus absorbs nutrients for plants• Plant gives fungus carbohydrates it produces via

photosynthesis

There are thousands of mycorrhzial fungal species, There are thousands of mycorrhzial fungal species, and only at times do they produce the classical fruit and only at times do they produce the classical fruit

body (e.g.mushrooms) above groundbody (e.g.mushrooms) above ground

•In absence of fruit body: how can we identify them?

DNA can be extracted from any part of an organism, DNA can be extracted from any part of an organism, like the web-like hyphae emanating from this root like the web-like hyphae emanating from this root

tiptip

•DNA sequence identified these threads as Tricholoma matsutake

Fungi… again!

• ASCOASCOMYCETES

• BASIDIOBASIDIOMYCETES

• OOMYCETES (fungus-like, water molds)

ASCOASCOMYCETES

• Yeasts (fermentation, human mycoses)Yeasts (fermentation, human mycoses)

• Truffles, morelsTruffles, morels

• Penicillia (penicillin), Fusaria (potent toxins, damping off of seedlings), molds

Ascus is the sack in which the spores are contained

Asci can be placed on a disk (apothecium), many apothecia can be together in a fruitbody

Morel fruitbody

Asci can be carried inside a flask (perithecium)

Nectria

Ploidy is mostly

n

BASIDIOBASIDIOMYCETES

• Mushrooms. mycorrhizal Mushrooms. mycorrhizal

• Wood decay organismsWood decay organisms

• Rusts, Smuts

• Yeasts and damping off

Toadstools and huitacochle are both basidiomycetes

Basidium means “club”, it carries the basidiospores (dispersion

propagules) naked

Most of their life, they aren+n (dikaryons), some rareones are diploid

Oomycetes

• Belong to the kingdom Stramenopila, used to be called Chromista

• Phytophthora, Pythium, Saprolegnia

H20

Oomycetes are not fungi

• Cellulose in cell wall• Ploidy is 2n• Result of sexual activity is oospore

(2n)• Meiosis, somatogamy, caryogamy

all occur at the same time

• Water adapted biology, flagellate phase

• No septa, holocoenocytic hyphae

• Chitin in cell wall• Ploidy is n, or n+n• Result of sexual activity is a spore

n• Meiosis, somatogamy,caryogamy

are usually interupted by vegetative (somatic phase)

• Better adapted for aerial transmission

• Septate hyphae

PhytophthoraPhytophthora

• Some important plant pathogens, with very well known history– Phytophthora infestans and the Irish potato

famine– Phytopthora cinnamomi and the Jarrah dieback

in Australia

The Irish Potato FamineThe Irish Potato Famine

• From 1845 to 1850• Phytophthora

infestans• Resulted in the death

of 750,000 • Emigration of over 2

million, mainly to the United States.

Phytophthora: “Phytophthora: “plant plant destructor”destructor”

• Best known pathogen whose long-distance transport linked to agriculture. – Infected root-stocks – Infested soil– Infected plants

70 species of 70 species of PhytophthoraPhytophthora

• 60 until a few years ago, research accelerated, especially by molecular analyses

• Differentiated on basis of:– Type of sexual intercourse– Type of sexual activity– Number of hosts– Ideal temperature– Type of biology– Evolutionary history (Waterhouse-Cooke)

Hyphae, sporangia, and zoospores of Hyphae, sporangia, and zoospores of P. ramorumP. ramorum

Zoospore

Most of their lifecyclethey are 2n

Have cellulose in cellwall

Not fungi!!, but looklike them because of convergent evolution

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

Effects of diseases on host mortality, growth and reproduction

• Young plants killed before reaching reproductive age

• Affect reproductive output

• Directly affect flowers and fruits

WGR

Complexity of forest diseases

• At the individual tree level: 3 dimensional

• At the landscape level” host diversity, microclimates, etc.

• At the temporal level

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

Progression of cankersProgression of cankers

Older canker with dry seepOlder canker with dry seep

HypoxylonHypoxylon, a secondary , a secondary sapwood decayer will appearsapwood decayer will appear

Root disease center in true fir caused by Root disease center in true fir caused by H. annosumH. annosum

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

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

POPULATION DYNAMICS

Species interactions and diversity

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

Diseases and succession

• Soil feedbacks; normally it’s 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

Janzen-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