1. Introduction to Plant Pathology
AND environmental impact

2. 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

4. 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.

5. 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

6. 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

7. Environmental Factors
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 General events are:
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 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 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 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 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 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
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. Evolution of the plant–bacterial pathogen
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
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
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
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
> 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
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
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
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
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.

30. 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)

31. Nematodes Microscopic roundworms Up to 4mm long Clear or transparent
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 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.)
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
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
Moisture
Temperature
Dispersal agents
Soil pH
Other

36. 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

37. 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

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

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

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

41. 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

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

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

49. 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

50. The End!
Any Questions?