What Is a Plant Disease?
Margery Daughtrey
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How do you know a
plant is sick?
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How do you know a
plant is sick?
Healthy Plant
•  Green
•  Full size
•  Leaves and stems free
from spots or pustules or
coatings
•  Leaves are turgid
•  Flowers are normal
•  Plant has a normal
shape
•  Roots are white and full
Diseased plant
•  Grayish to Brown
•  Stunted
•  Spots or pustules or
white or gray coatings
on leaves or stems
•  Wilt
•  Less seed, pollen, etc.
•  Plant has distorted
leaves etc.
•  Roots are stunted, rotted
and discolored
Healthy Plant
•  Green
•  Full size
•  Leaves and stems free
from spots or pustules or
coatings
•  Leaves are turgid
•  Flowers are normal
•  Roots are white and full
Symptoms and Signs
of Plant Disease
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Signs
2 Causes of Disease:
BIOTIC
•  Means “living”
(these are the contagious
diseases)
ABIOTIC
•  Means “non-living”
(such as stress factors:
Temperature
Nutrition
pH
Air pollution
(these aren’t contagious!)
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Abiotic or Biotic?
Abiotic or Biotic?
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Abiotic Disease
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Abiotic Disease
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Biotic Disease
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Biotic Disease
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“The Disease Triangle”
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Biotic Disease
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Biotic Disease
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Plant Pathogens
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PLANT PATHOGENS
include members of
these groups:
Fungi
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Water Molds
Water Molds
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Bacteria
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Bacteria
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Fungi
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Viruses and Viroids
Viruses
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Nematodes
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Nematodes
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2 Kinds of Infection
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2 Kinds of Infection
SYSTEMIC
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LOCALIZED
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SYSTEMIC
LOCALIZED
•  Virus diseases
•  Vascular wilts
(fungal or bacterial)
•  Leaf spots
•  Stem cankers
2 Kinds of Pathogens
SAPROPHYTES vs.
OBLIGATE
PARASITES
•  Can survive on plant
debris
•  Must have a living plant
host
e.g.
•  Root rotting fungi
•  Many leaf spotting fungi
e.g.
•  Powdery Mildew
•  Downy Mildew
•  Rust Fungi
Biotic Disease
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STRATEGY #1
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STRATEGY #2
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STRATEGY #3
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