1. Fungal relationships with plants
Obligate and facultative parasitism/pathogenicity

2. Why is plant pathology important?
Food quantity
Food quality
Agrinomic practices – tillage, pesticide use
Diversity and stability of ecosystems
Beauty

3. Better understanding of disease processes – examples of use
Disease control through life cycle management
Puccinia graminis tritici on wheat and barberry
Orchard management practice in Venturia inaequalis
Optimizing pesticide application
Disease forecasting
Pesticide design
Breeding for resistance

4. Fungal pathogenicity on plants
Plant pathogens can be biotrophic (rusts and smuts) or hemibiotrophic/necrotrophic (opportunistic)
Necrotrophs can display high levels of host specificity, e. g. Magnaporthe grisea
Increasing specialization
Increasing host range
Biotroph
Obligate parasite
Necrotroph
“Strong”
Facultative parasite
Saprotroph
“Weak”
Facultative parasite

5. Symptom types -- necrosis
Blumeriella on plum
Alternaria solani – early blight of potato

6. Symptom types – wilt Ophiostoma novo-ulmi
Panama disease: Fusarium oxysporum f.sp. cubense

7. Symptom types – hormone induced
Hypertrophy auxin
Taphrina deformans
Synchytrium endobioticum

8. Symptom types – hormone induced
Etiolation
Bakanae of rice
Fusarium moniliforme = Gibberella fujikuroi
gibberellins

9. Symptom types – abscission
Hemileia vastatrix
rust.lbl.gov

10. Symptom types – sterilization
Ustilago maydis
Claviceps purpurea – ergot

11. 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

12. Life Cycle of Puccinia graminis
SUMMER
aecia on barberry
(n+n)
urediniospores
(n+n)
airborne
spermatia (n)
insect transported
to receptive hyphae (n)
heterothallic II
aeciospores
(n+n)
airborne O
uredinia on grass
from infection by
aeciospores or
urediniospores F A L
spermagonia on barberry from
infection by basidiospores
basidiospore
(n)
airborne
meiosis
III
SPRING
telia on grass IV
teliospore on straw
(n+n)
karyogamy
(2n)
teliospore (2n)
germinating on
straw with
promycelium and
basidiospores (n)
WINTER

13. Puccinia graminis f.sp. tritici life cycle
host
ploidy
Overwintering and transport of urediospores

14. 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

15. 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

16. Stage 0 and I produced on “alternate” host
Stage 0: Spermogonia bearing spermatia (n) and
receptive hyphae (n)
helios.bto.ed.ac.uk/bto/microbes/biotroph.htm
fertilization of the receptive hyphae by spermatia initiates
the dikaryon and the formation of aecia

17. Puccinia plasmogamy

18. Stage II: Uredinia bearing urediniospores (n+n)
helios.bto.ed.ac.uk/bto/microbes/biotroph.htm
reinfect primary host
amplifies disease within primary host
uredinia can eventually develop into telia

19. Teliospore: site of karyogamy
technically part of the basidium
Stage III: Telia bearing teliospores (n+n2n)
final stage on primary host
overwinters as dikaryon

20. Facultative parasitism: Magnaporthe grisea infection

21. Magnaporthe grisea / Oryza sativa
Some non pathogenic M. grisea strains can grow in host plants if wound inoculated

22. Host resistance and basic compatibility
Most plants are not attacked by the vast majority of potential pathogens
Preformed defenses
Potential pathogens secrete chemicals during growth that can be detected

23. What is a pathogenicity gene?
A gene whose product contributes to successful fungal establishment in the host
Examples
Hydrolytic enzymes (especially for necrotrophs)
Compatibility determinants (especially for biotrophs)
Defense avoidance/detoxification

24. What is a resistance gene?
A gene whose product enables the host to detect a pathogen and/or mount a defense
The fungal product that is detected
does not have to be directly involved in pathogenesis
is defined as being produced by an avirulence gene

25. Heath – host pathogen interactions
Ann Bot 80, 713

26. Fungal pathogenicity on plants
Fungal pathogens of plants include opportunists, necrotrophs and biotrophs
Resistance is seen at several levels
Non-host resistance –
Widespread, early onset, effective
Passive – attachment/germination
Active – initial colonization, e. g. wall apposition
Hypersensitivity
Durable

27. Varietal resistance Superimposed on basic compatibility
Often based on a single resistance gene
Typically not durable

28. Gene for gene interactions
Host R r
Pathogen A resist susc
a susc susc
basic compatibility overcomes nonhost defense
pressure on host to detect pathogen leads to (temporary) resistance
pressure on pathogen to overcome/evade resistance

29. Breeding for resistance
Identify likely targets, disrupt, look for attenuation of pathogenicity
Cross commercial susceptible strains to wild relatives, backcross to retain yield and desireable characters in resistant strain
8-10 years; resistance may last 3-5 years
Pyramid strategies
Horizontal resistance