Disease Triangle and Pathogens Overview

Disease Triangle, Koch's Postulates, and Disease Development

• Disease occurs when a susceptible host is exposed to a virulent pathogen in a suitable environment.
• Three factors are essential for disease to occur: a susceptible host, a virulent pathogen, and a suitable environment.

Host

• A host is the organism infected by the pathogen.
• Host plants have varying levels of susceptibility:
  • Immune: The host cannot be infected.
  • Susceptible: The host can be infected.
  • Resistant: The host may or may not be infected, but the host produces defense compounds that limit pathogen spread.
• Plant susceptibility is dependent on the stage of development, with some stages being more susceptible than others.
• Different plant species will exhibit different vulnerabilities to pathogens.

Pathogen

• A pathogen is a living organism that infects a host.
• Virulence refers to the pathogen's aggressiveness and the severity of disease.
• The pathogen must be in a stage of active development that allows it to infect the host.

Environment

• The environment must be suitable for the pathogen to infect the host plant.
• Key environmental factors include temperature and moisture.
• Humidity often plays a significant role in controlling disease spread.
• Moisture and temperature conditions favorable to the development of disease typically occur near the soil line.
• Viruses are exceptions because they aren't impacted by climate as much.

Disease Triangle

• The disease triangle describes the relationship between susceptibility (host), pathogen virulence, and environmental factors and the impact disease has on plants.

Koch's Postulates

• Koch's postulates are a set of criteria for determining if a particular microorganism is the cause of a disease:
  • The pathogen must be consistently associated with plants showing the disease.
  • The pathogen can be isolated from the diseased plant and cultivated in a pure culture.
  • When the isolated pathogen is introduced to a healthy and susceptible plant, the same disease occurs.
  • The pathogen can be isolated again from the newly infected plant.

Plant Disease Diagnosis

• Diagnosis involves initial observation with the naked eye, followed by magnification using a hand lens and dissecting microscope.
• All observed features should be recorded, including lesion extent, shape, color, and signs of fruiting structures.

Microscopy

• Examination at fields of x10 and x40.
• Staining with cotton blue may be necessary.
• Heating may be required in some cases.

Incubation

• If fruiting structures aren't evident, then the specimen needs to be placed in a moist environment to promote growth.
• Relative humidity should be close to 100%.
• An examination should be done a day after incubation.

Isolation

• If incubation doesn't produce clear results, then isolating the organism is necessary.
• Small sections of diseased specimens need to be sterilized.
• Sections must be rinsed, blotted dry, and placed onto sterile medium.
• The specimen must then be grown using appropriate media.

Isolation Media

• Media like potato dextrose, sucrose, malt extract agar are useful general media for growing microorganisms.
• Other media is also available but are specific to different types of pathogens.

Disease Development

• Disease development is a five-step process:

  • Inoculation: Introduction of the pathogen to the host plant tissue
  • Incubation: Development stage of the pathogen.
  • Penetration: Entry of the pathogen into the plant tissue using enzymes or openings.
  • Infection: Establishment of a parasitic relationship between the pathogen and the host.
  • Disease: The host plant's response to the pathogen, exhibiting disease symptoms.

• Different methods exist for pathogen entry, including through natural openings and wounds. Also, insects, pruning tools, or driving rain can introduce pathogens.

Symptoms and Signs

• Symptoms: Visible effects of a disease on a plant caused by a pathogen. Changes in physical appearance or function.

• Examples: Blights (sudden death of twigs, foliage, flowers), Cankers (dead places on bark and cortex), Galls (abnormal swellings), Necrosis (death of tissue), Spots (circular or irregular lesions).

• Signs: Visible signs of the pathogen itself, such as fungal fruiting structures or discharges, and mycelium growth.

• Examples: Pycnidia (minute black fungal structures), Rhizomorphs, and Ooze.

Syndrome

• A syndrome describes a group of signs and symptoms indicating a disease.
• Important for diagnosing diseases as knowing only a few symptoms is not usually enough to correctly identify the condition. Case history, recent weather, and location data also helps identify the problem.

Disease IPM (Integrated Pest Management)

• Understanding crops, diseases, and insects is essential for disease control programs.
• Understanding when certain diseases or issues may appear during a season helps develop an IPM plan.
• Some diseases need to be managed while others can be tolerated.
• Proper timing in IPM is critical for successful disease management.