Fungal Fungal relationships relationships with plantswith plantsObligate and facultative Obligate and facultative parasitism/pathogenicityparasitism/pathogenicity

Why is plant pathology Why is plant pathology important?important?

Food quantityFood quantity Food qualityFood quality Agrinomic practices – tillage, Agrinomic practices – tillage,

pesticide usepesticide use Diversity and stability of ecosystemsDiversity and stability of ecosystems BeautyBeauty

Better understanding of Better understanding of disease processes – disease processes –

examples of useexamples of use Disease control through life cycle Disease control through life cycle

managementmanagement Puccinia graminis triticiPuccinia graminis tritici on wheat and on wheat and

barberrybarberry Orchard management practice in Orchard management practice in Venturia Venturia

inaequalisinaequalis Optimizing pesticide applicationOptimizing pesticide application

Disease forecastingDisease forecasting Pesticide designPesticide design Breeding for resistanceBreeding for resistance

Fungal pathogenicity on Fungal pathogenicity on plantsplants

Plant pathogens can be biotrophic Plant pathogens can be biotrophic (rusts and smuts) or (rusts and smuts) or hemibiotrophic/necrotrophic hemibiotrophic/necrotrophic (opportunistic)(opportunistic)

Necrotrophs can display high levels Necrotrophs can display high levels of host specificity, e. g. of host specificity, e. g. Magnaporthe Magnaporthe griseagrisea

Saprotroph“Weak”Facultative parasite

Necrotroph“Strong”Facultative parasite

BiotrophObligate parasite

Increasing specialization Increasing host range

Symptom types -- Symptom types -- necrosisnecrosis

Blumeriella on plum Alternaria solani – early blight of potato

Symptom types – wiltSymptom types – wilt

Ophiostoma novo-ulmi

Panama disease: Fusarium oxysporum f.sp. cubense

Symptom types – hormone Symptom types – hormone inducedinduced

Taphrina deformans

Hypertrophy Hypertrophy auxin

Synchytrium endobioticum

http://www.maff.gov.uk/planth/pestnote/pwd.htm

Symptom types – hormone Symptom types – hormone inducedinduced

Fusarium moniliforme = Gibberella fujikuroi gibberellins

EtiolationBakanae of rice

Symptom types – Symptom types – abscission abscission

Hemileia vastatrix

rust.lbl.gov

Symptom types – Symptom types – sterilization sterilization

Claviceps purpurea – ergot

Ustilago maydis

Obligate parasites – e. g. Uredinales

Rust fungi may have as many as five different spore-producing stages in their life cycles

Heteroecism – e.g. wheat stem rust- two taxonomically different host plants in order to

complete life cycle- ‘alternate’ host: stages (haploid)- primary host: stages (diploid)

Autoecism – e.g. bean rust- entire life cycle completed on a single host species

Microcyclic rusts ≤ 3 spore types

spermatia (n)insect transportedto receptive hyphae (n)heterothallic

aecia on barberry(n+n)

aeciospores(n+n)airborne

SUMMERurediniospores(n+n)airborne

FALL

uredinia on grassfrom infection byaeciospores orurediniospores

telia on grass

teliospore on straw(n+n)

WINTER

karyogamy(2n)

meiosis

teliospore (2n)germinating onstraw withpromycelium and basidiospores (n)

SPRING

basidiospore(n)airborne

spermagonia on barberry frominfection by basidiospores

Life Cycle of Puccinia graminis

IV

I

O

II

III

Puccinia graminisPuccinia graminis f.sp. f.sp. triticitritici life cycle life cycle

host ploidyOverwintering and transport of urediospores

Stage IV Basidia bearing basidiospores (n)

• in the spring teliospore germinates a promycelium• diploid nucleus migrates into the promycelium and

undergoes meiosis• four haploid nuclei migrate into developing sterigmata

& are incorporated into basidiospores• basidiospores reinfect alternate host

teliospore germinates, gives rise to a short germ tube of determinate growth known as the promycelium.

Promycelium: site of meiosis & formation of sterigmata and basidiospores

Stage 0: Spermogonia bearing spermatia (n) and receptive hyphae (n)

• fertilization of the receptive hyphae by spermatia initiates the dikaryon and the formation of aecia

Stage 0 and I produced on “alternate” host

helios.bto.ed.ac.uk/bto/microbes/biotroph.htm

Puccinia plasmogamyPuccinia plasmogamy

www.apsnet.org/education/LessonsPlantPath/StemRustWheat

Stage II: Uredinia bearing urediniospores (n+n)

• reinfect primary host• amplifies disease within primary host• uredinia can eventually develop into telia

helios.bto.ed.ac.uk/bto/microbes/biotroph.htm

Teliospore: site of karyogamytechnically part of the basidium

Stage III: Telia bearing teliospores (n+n2n)

• final stage on primary host• overwinters as dikaryon

Facultative parasitismFacultative parasitism: : Magnaporthe griseaMagnaporthe grisea

infectioninfection

Magnaporthe griseaMagnaporthe grisea / / Oryza Oryza sativasativa

Some non pathogenic M. grisea strains can grow in host plants if wound inoculated

Host resistance and basic Host resistance and basic compatibilitycompatibility

Most plants are not attacked by the Most plants are not attacked by the vast majority of potential pathogensvast majority of potential pathogens

Preformed defensesPreformed defenses Potential pathogens secrete Potential pathogens secrete

chemicals during growth that can be chemicals during growth that can be detected detected

What is a pathogenicity What is a pathogenicity gene?gene?

A gene whose product contributes to A gene whose product contributes to successful fungal establishment in successful fungal establishment in the hostthe host

ExamplesExamples Hydrolytic enzymes (especially for Hydrolytic enzymes (especially for

necrotrophs)necrotrophs) Compatibility determinants (especially Compatibility determinants (especially

for biotrophs)for biotrophs) Defense avoidance/detoxificationDefense avoidance/detoxification

What is a resistance What is a resistance gene?gene?

A gene whose product enables the A gene whose product enables the host to detect a pathogen and/or host to detect a pathogen and/or mount a defensemount a defense

The fungal product that is detected The fungal product that is detected does not have to be directly involved in does not have to be directly involved in

pathogenesispathogenesis is defined as being produced by an is defined as being produced by an

avirulence geneavirulence gene

Heath – host pathogen Heath – host pathogen interactionsinteractions

Ann Bot 80, 713Ann Bot 80, 713

Fungal pathogenicity on Fungal pathogenicity on plantsplants

Fungal pathogens of plants include Fungal pathogens of plants include opportunists, necrotrophs and biotrophsopportunists, necrotrophs and biotrophs

Resistance is seen at several levels Resistance is seen at several levels Non-host resistance – Non-host resistance –

Widespread, early onset, effective Widespread, early onset, effective Passive – attachment/germinationPassive – attachment/germination Active – initial colonization, e. g. wall Active – initial colonization, e. g. wall

appositionapposition HypersensitivityHypersensitivity

DurableDurable

Varietal resistanceVarietal resistance

Superimposed on basic compatibilitySuperimposed on basic compatibility Often based on a single resistance Often based on a single resistance

genegene Typically not durableTypically not durable

Gene for gene Gene for gene interactionsinteractions

HostHost RR rr PathogenPathogen AA resistresist suscsusc aa suscsusc suscsusc basic compatibility overcomes nonhost basic compatibility overcomes nonhost

defensedefense pressure on host to detect pathogen leads pressure on host to detect pathogen leads

to (temporary) resistanceto (temporary) resistance pressure on pathogen to overcome/evade pressure on pathogen to overcome/evade

resistanceresistance

Breeding for resistanceBreeding for resistance

Identify likely targets, disrupt, look for Identify likely targets, disrupt, look for attenuation of pathogenicityattenuation of pathogenicity

Cross commercial susceptible strains to Cross commercial susceptible strains to wild relatives, backcross to retain yield wild relatives, backcross to retain yield and desireable characters in resistant and desireable characters in resistant strainstrain 8-10 years; resistance may last 3-5 years8-10 years; resistance may last 3-5 years

Pyramid strategiesPyramid strategies Horizontal resistanceHorizontal resistance