prasitic plant disease

1. Parasitism and Plant Disease
Parasitism and Plant Disease
I D fi i i
Important Definitions:
• Pathogen – a disease causing agent
– Pathogenicity – Ability for an organism to interfere with one or
more essential functions of another organism – causing disease
Virulence The degree of pathogenicity of a pathogen
– Virulence – The degree of pathogenicity of a pathogen
• Parasite – Organisms that live on or in another organism
and obtains its food from the later
and obtains its food from the later
– Plant Parasite – Organism that becomes intimately associated
with the a plant and multiplies/grows at the expense of the
with the a plant and multiplies/grows at the expense of the
plant
• Saprophyte – Organisms that live on dead matter and
p p y g
secrete enzymes to break down material for energy

2. Parasitism and Plant Disease
Parasitism and Plant Disease
Pathogen Spectrum
Pathogen Spectrum
Non‐ Obligate Parasites
Facultative Facultative
Saprophytes Parasites
Obligate Parasites Obligate Saprophytes
(Biotrophs) (Necrotrophs)

3. Parasitism and Plant Disease
Parasitism and Plant Disease
Parasites
• Tend to have narrow host
Saprophytes
• Tend to have broad host
range – except viruses
• Does not need to kill plant
cells to complete lifecycle
range
• Kills plant tissues to acquire
cells to complete lifecycle
• Intimate relationship with
plant – continuous
nutrients for growth and
complete lifecycle
d i
p
absorption of nutrients
• Grows inter‐ and intra‐
ll l
• Secrete enzymes and toxins
• Grows inter‐cellularly
cellulary
• Infected plants tend to be
stunted poor vigor
• “Werewolves”
stunted, poor vigor
• “Vampires”

4. Parasitism and Plant Disease
Parasitism and Plant Disease
Disease Triangle
Disease Triangle
Disease occurrence is an
interactive event!
All sides must favor disease for
the disease process to take
interactive event! the disease process to take
place!
Amount of
Disease
Total of conditions favoring susceptibility
Total of conditions favoring susceptibility
Host

5. Parasitism and Plant Disease
Parasitism and Plant Disease
Disease Triangle
Factors affecting the
Disease Triangle
g
Pathogen:
1 Virulence
1. Virulence
2. Population present
3 Life‐stage of propagule
3. Life‐stage of propagule
4. Vectors present

6. Parasitism and Plant Disease
Parasitism and Plant Disease
Disease Triangle
Disease Triangle
Factors affecting the
g
Host:
1 Level of resistance
1. Level of resistance
2. Growth stage of plant
3 Genetic uniformity of
3. Genetic uniformity of
neighboring plants
4. Plant Vigor
4. Plant Vigor
5. Plant Density
6 Structure of Plant
6. Structure of Plant

7. Parasitism and Plant Disease
Parasitism and Plant Disease
Disease Triangle
Disease Triangle
Factors affecting the
g
Environment:
1 Conditions that favor disease
1. Conditions that favor disease
2. Conditions that promote plant
growth
growth
3. Conditions that affect dispersal
Includes levels of humidity,
temperature wind speed and
temperature, wind speed, and
periods of rain

8. Parasitism and Plant Disease
Parasitism and Plant Disease
Disease Triangle
Disease Triangle
Interpretation of the triangle:
‐ Length of each side is
proportional to the sum total
f th h t i ti f h
Amount Amount
Amount
of the characteristics of each
component that favors disease
of
Disease
of
Disease
Amount
of
Disease
‐If one side is zero, no disease
‐If one side is very large,
di t ti l i l
Total of conditions favoring
susceptibility
Host
Disease
disease potential is large
‐ Quantification of the triangle’s
area would represent the Host
area would represent the
amount of disease

9. Plant Disease Development
Plant Disease Development
Diseases are a series of
distinct events that occur
in succession leading to
the perpetuation of the
disease and pathogen
This chain of events is called
the disease cycle!
y
‐Closely related to the
lifecycle of the pathogen
y p g

10. Plant Disease Development
Plant Disease Development
Infection
Infection
Invasion
Colonization
Host
Recognition
Colonization
Pathogen
h d/
Penetration The Disease Cycle Growth and/or
Reproduction
Penetration y
Symptom
Development
Attachment
Dissemination of 2° p
Production of
Incubation
Dissemination
Dissemination of 2
Inoculum
Production of
Dormant Stage
1° Inoculum
Dormant Period

11. Plant Disease Development
P i E
Plant Disease Development
Primary Events:
1. Inoculation
2 Penetration
3
4
2. Penetration
3. Establishment of
Infection The
2
4
5
4. Invasion
5. Growth and reproduction
of the pathogen
Disease
Cycle
5
6
of the pathogen
(Colonization)
6. Dissemination of the
Cycle
1
6
pathogen
7. Survival in the absence of
the host
1
7
the host

12. Plant Disease Development
1 I l i
Plant Disease Development
1. Inoculation
‐ Initial contact of
pathogen with a
Inoculum Sources
‐ Survival in perennial
pathogen with a
susceptible plant site
‐ Innoculum: pathogen
brought into infection
‐ Survival in perennial
plants, weeds,
contaminated soil, soil
debris, seeds,
brought into infection
court
‐ May be spores, sclerotia,
mycelial fragments
, ,
transplants, and vectors
Arrival of inoculum
mycelial fragments,
bacteria, or viruses
‐ Primary inoculum –
primary infection
Arrival of inoculum
1. Passively (i.e. wind)
2. Chemotaxis – organisms
ithi th il
primary infection
‐ Secondary inoculum –
secondary infection
within the soil are
attracted to plant roots
3. Vector‐transmitted (i.e.
insects)
insects)

13. Plant Disease Development
2. Penetration 2.4 Recognition between host and
th
Plant Disease Development
2.1 Attachment to host
‐ Fungi and bacteria produce
gelatinous substances to help
pathogen
‐ Triggers plant resistance genes – if
present –promotes /prevents
infection
gelatinous substances to help
them stick to leaf surface
2 2 Spore germination
infection
2.5 Penetration
Di t F ti f i
2.2 Spore germination
‐ Mainly Fungi – spore germinates
forming germ tube and moves
along host to find opening/weak
‐ Direct – Formation of appressorium,
and penetration peg
‐ Indirect
/
along host to find opening/weak
point
2 3 Appressorium Formation
‐wounds ‐ wind/growth cracks,
wind blown sand, lesions
caused by other pathogens,
2.3 Appressorium Formation
‐ Fungi only – Formation of
appressorium and “softening
enzymes” and prepares for
vectors
‐natural openings ‐ Stomata,
y p p
penetration into plant
p g ,
hydrathodes, lenticels

14. Plant Disease Development
2. Penetration (cont.)
Plant Disease Development
Agrios, 1997

15. Plant Disease Development
Plant Disease Development
2. Penetration (cont.)
Photos from: Read, N.D. , Kellock, L.J.,
Knight, H., Trewavas, A.J. (1992b). Contact
g ( )
sensing during infection by fungal
pathogens.

16. Plant Disease Development
3. Establishment of Infection
Plant Disease Development
‐ Infection: Process by which
pathogens establish contact with
susceptible cells and procure
t i t
nutrients
‐ Successful infections result in
t i ibl d t t bl
symptoms – visibly detectable
changes in the plant
Obli t f l it f
‐ Obligate fungal parasites form
haustoria ‐ intracellular “vampires”
P th l t i
‐ Pathogen releases enzymes, toxins,
and growth regulators
Pl t t ith d f
Photos from: Read, N.D. , Kellock, L.J.,
‐ Plant reacts with defense
mechanisms
Knight, H., Trewavas, A.J. (1992b). Contact
sensing during infection by fungal
pathogens.

17. Plant Disease Development
4 I i
Plant Disease Development
4. Invasion
Pathogens spread through plant
Fungi and bacteria: Spread by
intracellular and intercellular
growth by using enzymes and
h l li d
hormones, localized
Nematodes: Move intercellularly,
localized
localized
Viruses, Viroids, and
xylem/phloem limited bacteria:
xylem/phloem limited bacteria:
Move cell to cell intracellularly,
systemic

18. Plant Disease Development
5 G th d d ti
Plant Disease Development
5. Growth and reproduction
of the pathogen
(Colonization)
( )
‐ Pathogens continue to
spread until the infection is
stopped or the plant is dead
stopped or the plant is dead
‐ Only fungi and nematodes
can actively move
can actively move
‐ All other pathogens rely rapid
reproduction and being
passively moved

19. Plant Disease Development
5. Growth and reproduction of
th th (C l i ti )
Plant Disease Development
the pathogen (Colonization)
‐ Reproduction:
‐ Fungi – spores, inter‐ and
g p ,
intra‐, surface and interior
‐ Bacteria – cell division, inter‐
and intra‐, surface and interior
‐ Viruses – intra‐, inside cells
only
only
‐ Nematodes – inter‐ and
intra‐, surface and interior
,
‐ Parasitic Plants – seeds,
exterior of plant only
Rate varies on pathogen present,
environment, and host

20. Plant Disease Development
Plant Disease Development
6. Dissemination of pathogen
Agrios, 1997

21. Plant Disease Development
7. Survival of pathogen
Plant Disease Development
Bacteria: same way as fungi;
without a host
(Overwintering stage)
Fungi: mycelium in cankers bud
Bacteria: same way as fungi;
infected plants, seeds, tubers,
and plant debris; in the bodies of
Fungi: mycelium in cankers, bud
scales, seeds, tubers, and plant
debris; spores; and sclerotia
insect vectors. Survive better in
large slimy colonies than as small
groups
‐ Soil inhabitants – survive
in soil indefinitely
(saprophytes)
groups.
Viruses: survive only in living
( p p y )
‐ Soil transients – survive in
soil for short period of time
(parasites)
plant tissues; roots of perennial
plants, seeds of some hosts, and
insect vectors.
(parasites)
Parasitic Plants: Seeds and
insect vectors.
Nematodes: Survive as eggs in
h il lif h
vegetative from on host the soil; or lifestages that are
dormant in seeds and bulbs

22. Plant Disease Development
Plant Disease Development
7. Survival of pathogen without a host
Agrios, 1997

23. Plant Disease Development
Disease Epidemics:
Monocyclic: completes 1 disease cycle in a
p
1° inoculum
Monocyclic: completes 1 disease cycle in a
year
‐ 1° inoculum is only inoculum for
entire year
1° infection
entire year
‐ Disease increases year to year as
inoculum builds Over‐seasoning Stage
Polycyclic: 2or more disease cycles in a year
‐ most pathogens
‐ disseminated by air and airborne
1° inoculum 1° infection
‐ disseminated by air, and airborne
vectors
‐ create explosive epidemics – Late
Blight, Powdery Mildew, and Rusts
2° inoculum
2° infection
g , y ,
Polyetic – requiring two or more years to
complete lifecycle (considered
p y (
monocyclic)
‐ typical of many vascular wilt
pathogens – Dutch Elm Disease
Over‐seasoning Stage