Transcript Introduction to Plant Pathology

Introduction to Plant
Pathology
AND environmental impact
Disease = disturbance from plant
pathogen or environmental factor that
interferes with plant physiology
• 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
Mazz’s Disease Pyramid
• 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.
Host Factors
• 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
Pathogen Factors
•
•
•
•
Amount of inoculum
Pathogen genetics
Virulence of the pathogen
Type of reproduction:
– Monocyclic
– polycyclic
• Ecology and mode of spread
–
–
–
–
Air
Soil
Seed
Vector dependency
Environmental Factors
• Moisture
• Temperature
• Effect of human culture practice
– Monoculture
– Amount of inoculum: seed quality,
disease residues, rotation, alternate
host
– Introduction of new pathogens
Disease Development
•
Every infectious disease requires a series of sequential
events in order for disease to develop.
•
Specific characteristics are unique for each disease.
•
1.
2.
3.
4.
5.
6.
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
Fungi
• 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
Fungal Diseases
• 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
Symptoms
• 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
Bacteria
• 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
Bacterial Diseases
• 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)
Bacterial Diseases
• 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.
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.
Gene on gene off action!
(b)
Bacterium injects
effector protein
through type III
secretion system
(TTSS)
TTSS will interfere
with defense signaling
or response.
Gene on gene off action!
(c) Plant responds to
infection by generation of
immune receptors encoding
for nucleotide-binding
(NB), MAP kinase, leucinerich-repeat (LRR)
R-proteins that recognizes
effector protein and
triggers an acute defense
response usually involving
hypersensitive response
(HR) and programmed cell
death
Disease Development
• 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.
Bacterial infections
• 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
Viruses
• 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
Viral Diseases
• > 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.
•
Plant Viral Reproduction
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
•
Plant Viral Reproduction
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.
• Barley yellow dwarf
virus
Remember, most plant
viruses are transmitted by
an intermediate
Virus Disease Symptoms
The symptoms of most virus
diseases can be put into
four categories:
1.
Lack of chlorophyll
formation in normally green
organs
2.
Stunting or other growth
inhibition
3.
Distortions
4.
Necrotic areas or lesions
Movement of pathogens from cell
to cell
• 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.
•
Movement of pathogens from cell
to cell
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)
Nematodes
• 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)
Nematode Diseases
• 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
Shoot Nematodes
(Aphelenchoides spp.)
• 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.
Root Nematodes
• 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
Environmental and cultural factors
affecting buildup of fungal and
bacterial plant pathogens
•
•
•
•
•
Moisture
Temperature
Dispersal agents
Soil pH
Other
Moisture
• 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
Moisture
• 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
Rainy, cloudy conditions = important for
spread and growth of many diseases
Temperature
• Affects growth rates
• Some pathogens adapted to certain
temp. ranges
• Refrigeration = important for
management
Dispersal Agents
• Bacteria, fungi are limited in mobility,
need to be moved by:
• Water
• Wind
• People, machinery
• Insects, other animals
Soil pH
• specific requirements for many soilborne pathogens
Other
Widespread planting of genetically
homogeneous crops can favor epidemic
Management of Plant Disease –
Strategies
• Eliminate or reduce initial inoculum, or
delay its introduction (preventive)
• Slow the rate of increase, shorten
exposure to favorable conditions
Management of Plant Disease
•
•
•
•
•
•
•
•
•
Sanitation
Fungicides
Host plant resistance
Crop rotation
Cultural practices
Temperature
Biological control
Organic amendments
Improved plant health and nutrition
Sanitation (aimed at excluding pest)
•
•
•
•
•
Avoid infested sites
Clean soil, planting material, tools, etc.
Inspection and quarantine
Remove infected debris
Tissue culture can provide disease-free
planting material
Fungicides
• 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
Host Plant Resistance
• 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
Crop Rotation
• Useful vs soil-borne diseases
• Residues of some plants (e.g., cabbage
family) may be toxic to some pathogens
Cultural Practices to Minimize
Spread of Disease
• Favorable irrigation practices (drip vs
overhead)
Moisture
• Timing of Planting
management
• Wider row spacings
• Eradicate alternate hosts for viruses
Important to minimize
water and humidity to
limit disease spread
Temperature
• Heat for soil sterilization
• Hot water treatment of planting
material
• Solarization
• Refrigeration to slow disease progress
in harvested material
Management of Plant Disease
•
•
•
•
•
•
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
The End!
Any Questions?