Introduction to plant pathology

1. Introduction to Plant
Pathology
AND environmental impact

2. Disease = disturbance from plant
pathogen or environmental factor that
interferes with plant physiology
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Causes changes in plant appearance or
yield loss
Disease results from:
Direct damage to cells
Toxins, growth regulators, or other
byproducts that affect metabolism
Use of nutrients and water or interference
with their uptake

4. Mazz’s Disease Pyramid
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The interaction of components
of plant disease can be
expanded to include time and
humans.
Time is often considered as the
fourth component of plant
disease development.
The four components together
can quantify the amount of
disease.
The human equation can affect
the three components of the
disease triangle and should be
considered as a fifth component
in disease development.

5. Host Factors
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All plants can be considered hosts
Degree of genetic uniformity – crop plants – inbred lines
Age – affects disease development depending on plant-
pathogen interaction
There are different levels of susceptibility, which include:
Immune - cannot be infected.
Susceptible - can be infected.
Resistant - may or may not be infected, and is the plant able to
prevent the pathogen from killing it. ie. defense compounds

6. Pathogen Factors
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Amount of inoculum
Pathogen genetics
Virulence of the pathogen
Type of reproduction:
Monocyclic
polycyclic
Ecology and mode of spread
Air
Soil
Seed
Vector dependency

7. Environmental Factors
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Moisture
Temperature
Effect of human culture practice
Monoculture
Amount of inoculum: seed quality,
disease residues, rotation, alternate
host
Introduction of new pathogens

8. Disease Development
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Every infectious disease requires a series of sequential
events in order for disease to develop.
Specific characteristics are unique for each disease.
General events are:
dispersal of the pathogen to the host
penetration and infection of the host
invasion and colonization of the host
reproduction of the pathogen
pathogen dispersal
pathogen survival between growing seasons and/or
in the absence of a host

9. Fungi
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Diverse and widespread
Filamentous (hyphae) form a network of
mycelium (lots of hyphae)
Recognized by reproductive structures
(mushrooms, rusts, conks, etc.)
Most of the 100,000 spp. are saprophytes
Live on dead organic matter
Approximately 8,000 species attack plants
Plant pathogens

10. Fungal Diseases
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Reproduction by sexual and asexual means
Spread through a variety of methods
wind/water blown spores
rhizomorphs
Sclerotia (overwintering)
Include organisms from Kingdom Protista, that are now
classified outside the Kingdom Fungi:
Downy mildews
Pythium
Phytophthora
Clubroots

11. Symptoms
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Initially, similar to drought & starvation:
Plants appear off-color
Weakened & susceptible to attack
Wilting and dieback occur later
Younger plants usually killed rapidly
Older plants decline over time (years)
Roots have brownish streaks

13. Bacteria
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Prokaryotic microscopic organisms
Free living single cells, or
Filamentous colonies
Reproduce via binary fission
2 daughter cells are identical to mother cell
Don’t usually produce resistant resting spores
Need host or growth medium to survive
For rapid spread, plant infecting bacteria usually
require:
Warmth
Moist conditions

14. Bacterial Diseases
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Less common than fungal or viral diseases
They can be either:
parasites, saprophytes (live off dead material), or autotrophs
(photosynthesis or Chemosynthesis)
Symptoms include:
Cankers, Wilts, Shoot Blights, Leaf Spots,
Scabs, Soft Rots, & Galls
Generally, cannot invade healthy tissue; need wound or
opening to infect.
Control methods usually cultural in nature (don’t use
antibiotics on large scale)

16. Bacterial Diseases
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Bacterial galls: In some cases, toxic materials are
produced that cause plant tissues of roots, stems or
leaves to grow abnormally as in crown gall.
Bacterial leaf spot disease: The bacteria usually enter
through leaf stomata.
Symptoms include water-soaking, slimy texture, fishy
or rotten odor, confined initially between leaf veins
resulting in discrete spots that have straight sides and
appear angular.

17. Gene on gene off action!
• Evolution of the plant–
bacterial pathogen
interaction.
(a)
Plants have evolved
receptors that could
recognize P-AMPs and
triggers basal defense.

18. Gene on gene off action!
(b)
Bacterium injects
effector protein
through type III
secretion system
(TTSS)
TTSS will interfere with
defense signaling or
response.

19. Gene on gene off action!
(c) Plant responds to infection
by generation of immune
receptors encoding for
nucleotide-binding (NB),
MAP kinase, leucine-rich-
repeat (LRR)
R-proteins that recognizes
effector protein and triggers
an acute defense response
usually involving
hypersensitive response (HR)
and programmed cell death

20. Disease Development
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Infections occur through leaf scars and wounds. These give
rise to small cankers in which the bacteria survive the winter.
Rain or water splash, and pruning tools spread the bacterium.
Bacteria overwinter in active cankers, in infected buds, and on
the surface of infected and healthy trees and weeds.
The bacterium reproduces best between 21ºC and 25ºC.
Generally disease seems to be more severe after cold winters
and prolonged spring rains.

21. Bacterial infections
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The infected head tissue
often takes on a tan color
Becomes moist and mushy
Develops a foul odour.
The leaf undergoes HR
response
Results in classic spotting of
leaves.
Reduces photosynthesis and
cell respiration of plant
material

22. Viruses
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Viruses are "submicroscopic" entities that infect
individual host plant cells.
Viruses are obligate parasites: They can only replicate
themselves within a host's cell.
In the virus infected plant, production of chlorophyll may
cease (chlorosis, necrosis)
Cells may either grow and divide rapidly or may grow
very slowly and be unable to divide

24. Viral Diseases
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> 400 viruses infect plants; few are economically
important pathogens
The infection remains forever
Viruses are transmitted from plant to plant by
living factors: insects, mites, fungi and
nematodes
Or non-living factors: rubbing, abrasion or other
mechanical means (including grafting or other
forms of vegetative propagation)
Occasionally transmitted in seed.

25. Plant Viral Reproduction
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1. Attachment--this
requires specialized
envelope proteins. These
proteins make viruses
specific for different cells.
2. Penetration--viral
particles enter the cell, the
caspid is removed and
genetic material enters the
nucleus.
3. Replication--the virus
uses the host replication
machinery to make many
copies of itself

26. Plant Viral Reproduction
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4. Viral protein production--the
virus uses the host’s translation
machinery -copies of the viral
proteins - capsid and new
envelope proteins. Envelope
proteins move to the plasma
membrane thanks to protein
secretion performed by the host.
5. Assembly--genetic material is
packaged into the new caspids.
6 Release--the caspids move to
the cell membrane, get wrapped
in their envelope proteins and
move on to infect a neighboring
cell.

27. Remember, most plant
viruses are transmitted by an
intermediate
• Barley yellow dwarf
virus

28. Virus Disease Symptoms
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The symptoms of most virus
diseases can be put into four
categories:
Lack of chlorophyll formation
in normally green organs
Stunting or other growth
inhibition
Distortions
Necrotic areas or lesions

29. Movement of pathogens from cell to
cell
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Fungi, Bacteria, and Viruses all
move through the plant in the
same when following a
successful penetration.
Movement proteins (MP) are
proteins dedicated to enlarging
the pore size of plasmodesmata
and actively transporting the
pathogen into the adjacent cell.
Thereby allowing local and
systemic spread of pathogen in
plants.

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So, from the entry point (1)
the pathogen moves from cell
to cell via the plasmodesmata
(2).
As a pathogen travels it also
reproduces. Some of the
pathogen can exit the
infected plant by stomata and
infect nearby plants (3).
If the pathogen gets to the
bundle sheath it can rapidly
be transported through the
plant by the xylem and
phloem (4)
Movement of pathogens from cell to
cell

31. Nematodes
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Microscopic roundworms
Barely visible with naked eye
No segments
Up to 4mm long
Clear or transparent
Feed with stylet
Pierce plants (pests)
Kill arthropods (beneficials)

32. Nematode Diseases
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Plant pathogenic nematodes = pests
Infect roots & bulbs (below-ground)
Foliar nematodes (above-ground)
Also vectors of plant viruses
As they feed, they weaken & stress plants – also
predispose to other problems
Causes bulb & root decline, and root knots
Spread by splashing water, and infested soil & plant parts

33. Shoot Nematodes
(Aphelenchoides spp.)
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Foliar nematodes feed inside leaves between major
veins causing chlorosis and necrosis.
Injury is most often seen at the base of older foliage.
When plants with a net-like pattern of veins become
infested with foliar nematodes, the tissues collapse in
wedge-shaped areas and then change color.

34. Root Nematodes
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Moisture and nutrient stress symptoms and
general stunting are common (by killing
meristem tissue)
Root lesion nematodes (Pratylenchus spp.)
Burrowing nematodes (Radopholus similis)
destroy root cortex tissues as they feed
Root-knot nematodes (Meloidogyne spp.) inject
growth-regulating substances into root tissues
as they feed, stimulating growths called galls or
knots

35. Environmental and cultural factors
affecting buildup of fungal and bacterial
plant pathogens
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Moisture
Temperature
Dispersal agents
Soil pH
Other

36. Moisture
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Activates resting stages
Affects germination of spores and
penetration into host
Water on leaves
Humidity
Splashing water distributes inoculum
Leaf wetness = best indicator but difficult
to measure

37. Moisture
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Activates resting stages
Affects germination of spores and penetration into
host
Water on leaves
Humidity
Splashing water distributes inoculum
Leaf wetness = best indicator but difficult to measureRainy, cloudy conditions = important for spread
and growth of many diseases

38. Temperature
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Affects growth rates
Some pathogens adapted to certain temp.
ranges
Refrigeration = important for management

39. Dispersal Agents
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Bacteria, fungi are limited in mobility, need
to be moved by:
Water
Wind
People, machinery
Insects, other animals

40. Soil pH
• specific requirements for many soil-borne
pathogens
Other
Widespread planting of genetically
homogeneous crops can favor epidemic

41. Management of Plant Disease –
Strategies
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Eliminate or reduce initial inoculum, or
delay its introduction (preventive)
Slow the rate of increase, shorten
exposure to favorable conditions

42. Management of Plant Disease
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Sanitation
Fungicides
Host plant resistance
Crop rotation
Cultural practices
Temperature
Biological control
Organic amendments
Improved plant health and nutrition

43. Sanitation (aimed at excluding pest)
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Avoid infested sites
Clean soil, planting material, tools, etc.
Inspection and quarantine
Remove infected debris
Tissue culture can provide disease-free
planting material

44. Fungicides
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Bactericides, if target is bacteria
Dusts, sprays, fumigants, etc.
Foliar, soil, seed, wound, or post-plant
application
Preventative – slows rate of increase
Insecticides may also be useful for
managing insect vectors

45. Host Plant Resistance
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Caution: pathogens can have multiple isolates
Vertical resistance – against some genotypes of
a pathogen
Horizontal resistance – not limited to certain
genotypes, across all isolates
Host genetic diversity is important to slow
epidemics

46. Crop Rotation
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Useful vs soil-borne diseases
Residues of some plants (e.g., cabbage
family) may be toxic to some pathogens

47. Cultural Practices to Minimize
Spread of Disease
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Favorable irrigation practices (drip vs
overhead)
Timing of Planting
Wider row spacings
Eradicate alternate hosts for viruses
Important to minimize
water and humidity to
limit disease spread
Moisture
management

48. Temperature
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Heat for soil sterilization
Hot water treatment of planting material
Solarization
Refrigeration to slow disease progress in
harvested material

49. Management of Plant Disease
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Sanitation
Fungicides
Host plant resistance
Crop rotation
Cultural practices
Temperature
Biological control – Rhizobacteria may interfere with
colonization of plant roots by fungi and bacteria
Organic amendments (avoid diseased plants in mulch,
etc.)
Improved plant health and nutrition

50. The End!
Any Questions?