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.

• General events are:1. dispersal of the pathogen to the host2. penetration and infection of the host3. invasion and colonization of the host4. reproduction of the pathogen5. pathogen dispersal6. 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, 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

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.

Remember, most plant viruses are transmitted

by an intermediate

• Barley yellow dwarf virus

Virus Disease SymptomsThe 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.

• 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

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

measureRainy, 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 soil-

borne pathogens

OtherWidespread 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)• Timing of Planting• Wider row spacings• Eradicate alternate hosts for viruses

Important to minimize water and humidity to limit disease spread

Moisture management

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?