1. University of Florida - IFAS
Plant Disease
General Concepts
Signs
Symptoms
Etiology
Epidemiology
Pathogenicity
Virulence
Photos used from various UF/IFAS Extension Publications or provided by UF/IFAS faculty and staff, unless otherwise stated.
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2. University of Florida - IFAS
Signs vs. Symptoms
Sign of Disease
observation of the organism causing the disease (objective observation)
Symptom of Disease
observation of how the host is manifesting infection by a pathogen and disease development due to a pathogen
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3. University of Florida - IFAS
Signs vs. Symptoms
Symptoms of Guinea worm disease or dracunculiasis
Symptoms of Ganoderma Butt Rot
Dracunculus medinensis - nematode
Ganoderma zonatum - fungus
Photos from The Carter Center
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4. University of Florida - IFAS
Bacteria oozing from leaf
Signs of Disease
Rust spores on leaf
Rust spores on leaf
Mycelia (cottony growth)
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5. University of Florida - IFAS
Fungal sclerotia inside stem
Fungal mycelia and sclerotia
Signs of Disease
Powdery mildew mycelia
Fungal mycelia
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6. University of Florida - IFAS
Pycnidia (fungal structures with spores) on orange skin
Pycnidia (fungal structures with spores) on branch
Signs of Disease
Fungal mycelia on roots (microscopic view)
Pythium oospores on roots (microscopic view)
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7. University of Florida - IFAS
Plant
Organs
And
Functions
Disease
Symptoms
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8. University of Florida - IFAS
Types of Symptoms
spot – small, distinct lesion on leaf, fruit . . .
blight – spots that have coalesced or merged
together; more tissue being affected
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9. University of Florida - IFAS
Types of Symptoms
rot – tissue is breaking down (fruit, roots);
usually mushy, but can be dry
wilt – plant droops due to water stress; can be
systemic (xylem) or due to root rot
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10. University of Florida - IFAS
Types of Symptoms
canker – sunken lesions; usually on stems or
woody tissue; but can occur on fruit
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11. University of Florida - IFAS
Types of Symptoms
gall – masses of undifferentiated growth; usually
on stems or woody tissue (branches) but
can be on roots
S. Browning, University of Nebraska, Lincoln
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12. University of Florida - IFAS
Types of Symptoms
patches, decline – terms often used in
association with grasses (turf, grain crops)
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13. Symptoms Caused by Bacteria University of Florida - IFAS
leaf spots and blights – water soaked, greasy
soft rots of fruits
wilts (systemic – xylem)
cankers
gall (overgrowths/cell proliferation)
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14. Symptoms Caused by Fungi/Oomycota University of Florida - IFAS
leaf spots and blights (including rust and
powdery mildew)
soft or dry rots of fruits, bulbs . . .
root rots
wilts (systemic – xylem)
overgrowths/cell proliferation –
clubroot, galls, warts, witches’-broom
scabs, cankers, patches and decline
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15. Symptoms Caused by Viruses University of Florida - IFAS
dwarfing or stunting to some degree
mosaics – light green, yellow or white
areas intermingled with green – leaves
or fruits
ring spots – chlorotic or necrotic rings –
leaves, fruits or stems
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16. Symptoms Caused by Viruses University of Florida - IFAS
Dwarfing
Mosaic
Ringspot
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17. Plant Pathology Terms Pathogenicity: the pathogen either does
or does not cause a disease; a question
with yes or no answer
Virulence: relative capacity of pathogen to
cause disease; range from minimal damage
to dead plant
Etiolgy: determining the cause of disease 17

18. Etiology and Pathogenicity Testing University of Florida - IFAS
Consistent isolation of a pathogen from symptomatic host tissue
Pathogen is grown in pure culture and its characteristics documented
Inoculation of a healthy plant with the pure culture of the pathogen, and inoculated plant must then develop symptoms similar to those observed initially
Recovery of the same pathogen used for inoculation purposes
To confirm that a fungus is the actual cause of a disease, scientists must follow these steps.
This is general scheme; non-culturable plant pathogens have special rules.
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19. Etiology and Pathogenicity Testing
Example: Fusarium Wilt of Queen Palm
The “potential” pathogen isolated consistently from symptomatic tissue was Fusarium.
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20. Etiology and Pathogenicity Testing
Example: Fusarium Wilt of Queen Palm
Isolated Three Fusarium Species
(sometimes from the same tissue piece)
1) F. incarnatum-equiseti species complex (6 isolates)
isolated from Fusarium wilt symptomatic palms in Australia
2) F. oxysporum (43 isolates)
known Fusarium wilt pathogen of palms worldwide
3) F. proliferatum (9 isolates)
known pathogen of palms; can cause wilt symptoms 20

21. University of Florida - IFAS
Control F. oxysporum
It was difficult to complete Koch’s postulates with older, juvenile palms, but we could complete Koch’s postulates with seedlings, as shown here. Of the three Fusarium species isolated from diseased palms, only the Fusarium oxysporum is pathogenic.
F. incarnatum-equiseti F. proliferatum
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22. But, how do pathogens enter the plant?
Plant Pathology Terms
Epidemiology: study of the factors influencing the initiation, development and spread of infectious disease
But, how do pathogens enter the plant? 22

23. How do pathogens enter plant? University of Florida - IFAS
Viruses and Viroids and Fastidious Bacteria
most require vectors; a few mechanical entry
Bacteria – most enter through natural
openings or wounds
Fungi & Oomycetes – enter through natural
openings, wounds; by mechanical pressure
or enzymes they produce; a few by vectors
Nematodes – stylets used to gain entry
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24. How do pathogens enter plant?
Plants have natural openings: stoma or stomata (plural)
Plant Epidermis
Stoma
(plural=stomata)
Stoma
(plural=stomata)
Plant Epidermis 24

25. How do pathogens enter plant?
Leafhopper
(insect vector)
Use mouth parts to penetrate 25

26. How do pathogens enter plant?
Leafhopper
(insect vector)
Use mouth parts to penetrate
Bacteria
on water film, enter
through stoma 26

27. How do pathogens enter plant?
Leafhopper
(insect vector)
Use mouth parts to penetrate
Bacteria
on water film, enter
through stoma
Fungal Spore
entering
through stoma 27

28. How do pathogens enter plant?
Leafhopper
(insect vector)
Use mouth parts to penetrate
Bacteria
on water film, enter
through stoma
Appressorium
fungal structure from spore for direct penetration
Fungal Spore
entering
through stoma 28

29. How do pathogens enter plant?
Leafhopper
(insect vector)
Use mouth parts to penetrate
Bacteria
on water film, enter
through stoma
Appressorium
fungal structure from spore for direct penetration
Fungal Spore
entering directly
through stoma
Fungal Hyphae
can grow between cells or penetrate cells 29

30. Disease Development pathogen infects plant – penetration, can be
pathogen comes in contact with plant
pathogen infects plant – penetration, can be
direct or indirect; with or without vector
incubation period – time between penetration
and first appearance of symptoms
pathogen increases within plant, uses host to
grow and reproduce
symptoms observed continue to increase 30 30

31. Favorable Environment
Disease Development
Environmental conditions influence each and every step in disease development process!!
Susceptible
Host
Favorable Environment
DISEASE
Pathogen 31

32. Disease Development DISEASE Susceptible Host
Nematode
Fungus
W. Deacon, Univ. of Edinburgh
Vector required for some pathogens!
DISEASE
Favorable Environment
Pathogen
Mite
USDA/ARS
Insect
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33. University of Florida - IFAS
How do plant pathogens move from plant to plant if they are not moved by a vector?
Wind dispersal of spores
Splash dispersal of spores – rain, irrigation
Physical movement of soil-borne pathogens that don’t produce spores – ex: soil tillage
Nematodes swim or move with soil
Seed associated – internal or external
Plant associated – cuttings, grafting
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34. University of Florida - IFAS
How do plant pathogens move from plant to plant if they are not moved by a vector?
Time to have some fun!
Cheap, easy way to demonstrate
spore dispersal without
water or spores!
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35. Splash Dispersal of Spores University of Florida - IFAS35