What is a Disease?

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What is a Disease?
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An organism that lives on or in another organism
and obtains food from the second organism is a
parasite.
Pythium on bentgrass
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Rhizobium japonicum

• Fits definition of parasitic bacterium in
  soybeans
• Soybeans benefit from infection (symbiosis)

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What is Pathogenicity and Disease?

• Pathogenicity - is the ability of a pathogen to
  interfere with one or more of the essential plant
  cell functions.
• Disease the process in which a pathogen
  interferes with one or more essential plant cell
  functions

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Pathogens may interfere with plant cell functions
using

• Toxins
• Growth regulators
• Enzymes
• By absorbing water and nutrients from host cells

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Pathogens can also cause disease by

• Blocking xylem or phloem vessels
• By inserting parts of their DNA or RNA into the
  host cell and commandeering the cells
  replication machinery.

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Plant Stresses may cause Abiotic Diseases
Usually caused by to much or to little of
something
Stress factors include temperature,
moisture, pH, mineral nutrients, etc.

Pollutants, pesticides, and salt may also be
causes.
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Examples of stresses not considered causes of
Abiotic Diseases

• Pruning
• Animal feeding
• Lawn mowing
• Lightening

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Infectious Plant Disease Causes

• Fungi
• Prokaryotes
• Viruses
• Viroids

• Nematodes
• Protozoa
• Algae
• Parasitic Seed Plants

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Infectious disease agents are called pathogens.
Pathogens can be classified as Biotrophs, Facu
ltative Parasites Facultative Saprophytes
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Biotrophs do not kill plant cells. They
penetrate the cell wall and establish a
continuous relationship or move from cell to cell.
Cedar Hawthorne Rust
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A biotroph can reproduce and grow in nature on
living hosts

• Powdery mildews
• Downy mildews
• Rusts
• White Rusts
• Some species of Xylella
• Mollicutes

• Viruses
• Viroids
• Phytoparasitic nematodes
• Phytoparasitic Protozoa
• Dwarf mistletoe

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Other Types of Pathogens
Facultative Parasite usually grows and completes
life cycle on dead material, but can do so on
living tissue.
Facultative Saprophyte usually grows and
completes life cycle on living tissue, but can do
so on dead material.
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Nonbiotrophs may secrete toxins or enzymes that
disrupt cells (kill them) before feeding on the
cells or the cells contents.
Necrotrophs live on dead tissue.
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Disease Triangle
Host
Disease
Pathogen
Environment
Disease is not an discrete event. It is dynamic.
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Disease Cycle

• Disease Cycle a series of events that occur
  during a pathogenic relationship of a pathogen
  and host that leads to disease
• A pathogens life cycle may follow events similar
  to the disease cycle, but they are not the same
  thing.

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Disease Cycle

• Inculation
• Penetration
• Infection
• Invasion
• Reproduction
• Dissemination

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Primary Disease Cycle
Monocyclic Disease
Dissemination
Primary Inoculum
Primary Infection
Over wintering
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Secondary Disease Cycle
Polycyclic Disease
Primary Infection
Secondary Infection
Reproduciton and Dissemination
Over wintering
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Signs and Symptoms

• Signs are the physical presence of the pathogen.
• Symptoms are the plants response to disease.

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How do you determine the cause of an unknown
disease?

• Kochs Postulates

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Kochs Postulates

• Identify symptoms and signs associated with all
  diseased plants
• Pathogen must be isolated in culture or
  transferred to susceptible host
• Inoculate plant with pathogen from culture and
  get same symptoms and signs as in 1.
• Isolate same pathogen that was isolated in 2.

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Kochs Postulates

• Identify symptoms and signs associated with all
  diseased plants
• Pathogen must be isolated in culture or
  transferred to susceptible host
• Inoculate plant with pathogen from culture and
  get same symptoms and signs as in 1.
• Isolate same pathogen that was isolated in 2.

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Kochs Postulates

• Identify symptoms and signs associated with all
  diseased plants
• Pathogen must be isolated in culture or
  transferred to susceptible host
• Inoculate plant with pathogen from culture and
  get same symptoms and signs as in 1.
• Isolate same pathogen that was isolated in 2.

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Kochs Postulates

• Identify symptoms and signs associated with all
  diseased plants
• Pathogen must be isolated in culture or
  transferred to susceptible host
• Inoculate plant with pathogen from culture and
  get same symptoms and signs as in 1.
• Isolate same pathogen that was isolated in 2.

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Reading AssignmentChapter 2 and 3