Parasitism and Plant Disease

Parasitism and Plant Disease
Important Dfiii 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
• Parasite – Organisms that live on or in another organism
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
plant
• Saprophyte – Organisms that live on dead matter and
p p y g
secrete enzymes to break down material for energy

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

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

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

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

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

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

Parasitism and Plant Disease
Disease Triangle
Interpretation of the triangle:
‐ Length of each side is
proportional to the sum total
of the characteristics ti of each
component that favors disease
Amount
Amount
of
of
Disease
Disease
‐If one side is zero, no disease
‐If one side is very large,
disease potential ti lis large
‐ Quantification of the triangle’s
area would represent the
amount of disease
Amount
of
Disease
Total of conditions favoring
susceptibility
Host

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!
‐Closely related to the
lifecycle of the pathogen

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

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

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

Plant Disease Development
2. Penetration 2.4 Recognition between host and
2.1 Attachment to host
‐ Fungi and bacteria produce
gelatinous substances to help
them stick to leaf surface
22Spore 2.2 germination
‐ Mainly Fungi –spore germinates
forming germ tube and moves
along host to find opening/weak
point
23Appressorium 2.3 Formation
‐ Fungi only – Formation of
appressorium and “softening
enzymes” and prepares p for
penetration into plant
pathogen
‐ Triggers plant resistance genes –if
present –promotes /prevents
infection
2.5 Penetration
‐ Direct – Formation of appressorium,
and penetration peg
‐ Indirect
‐wounds ‐ wind/growth cracks,
wind blown sand, lesions
caused by other pathogens,
vectors
‐natural openings ‐ Stomata,
hydrathodes, lenticels

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

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

Plant Disease Development
3. Establishment of Infection
‐ Infection: Process by which
pathogens establish contact with
susceptible cells and procure
nutrients ti t
‐ Successful infections result in
symptoms –visibly iibl detectable dt tbl
changes in the plant
‐ Obligate fungal parasites form
haustoria ‐ intracellular “vampires”
‐ Pth Pathogen releases enzymes, toxins,
and growth regulators
‐ Plant reacts with defense df
mechanisms
Photos from: Read, N.D. , Kellock, L.J.,
Knight, H., Trewavas, A.J. (1992b). Contact
sensing during infection by fungal
pathogens.

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

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
‐ Only fungi and nematodes
can actively move
‐ All other pathogens rely rapid
reproduction and being
passively moved

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

Plant Disease Development
6. Dissemination of pathogen
Agrios, 1997

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

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

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