LECTURE 12

Microbial Pathogens of Plants and Insects

  • Isaria fumosorosea - Bagrada bug
  • Beauveria bassiana - Bagrada bug
  • Metarhizium brunneum - Bagrada bug

ILOs (Intended Learning Outcomes)

  • Awareness of common bacterial, viral, and fungal pathogens of plants and insects.
  • Appreciate the economic impact of such diseases.
  • Understand how fungi can be used as biocontrol agents to prevent insect infestation of crops.
  • Understand the mode of action of BT toxin.

Insect Pests and Crop Loss

  • Insect pests: 18%
  • Microbial diseases: 70-80% caused by fungi, 16%
  • Weeds: 34%
  • Major causes of crop loss since 21st century.

Field Crops and Associated Diseases

  • Alfalfa: 34 diseases
  • Rice: 35 diseases
  • Barley: 44 diseases
  • Rye: 35 diseases
  • Maize: 71 diseases
  • Cotton: 51 diseases
  • Tobacco: 66 diseases
  • Potato: >15 diseases

(Multiple other crops listed with corresponding disease counts)

The Disease Triangle

  • Components:
    • Susceptible host
    • Favorable environment
    • Pathogen present
  • No disease occurs without one of these components.

Economic Cost of Disease

  • The Irish Potato Famine (1845-1849):
    • Reliance on one potato variety.
    • Arrival of Phytophthora infestans (late blight).
    • Results: crop loss, starvation (>1 million), mass emigration; 1 in 8 of Irish population perished.

Late Blight of Potato

  • Pathogen: Phytophthora infestans.
  • Favored by cool temperatures and 'free' moisture.

Infection Susceptibility in Plants

  • Every part of the plant is susceptible to infection.
  • Pathogens disrupt essential functions:
    • Formation of vitamins and hormones
    • Reproduction and storage of essential nutrients
    • Photosynthesis and vascular system functionality.

Process of Causing Disease in Plants

  • Pathogens must:
    • Recognize the host
    • Penetrate host barriers
    • Suppress host defense systems
    • Move through host tissues (invasion)
    • Utilize host components for growth and reproduction.

Enzymatic Actions of Pathogens

  • Enzymes (cellulases, xylanases, pectinases) help pathogens break down plant structures, facilitate movement within host tissues, and impair cell functions, leading to cell death.

Toxins Produced by Pathogens

  • Function: Interfere with host cell functions, disrupt metabolism, and cause cell death.
  • Can act as growth regulators to modify plant hormonal activities, promoting pathogen invasion.

Plant Viral Diseases

  • Require a vector (usually insects) for transmission.
  • Symptoms: leaf spotting, chlorosis, stunted growth.
  • Examples: Citrus tristeza, Tobacco mosaic, Curly top of sugar beets.

Tulip Mania

  • 17th-century phenomenon driven by the Tulip Breaking virus, leading to exorbitant prices for infected bulbs.

Citrus Tristeza Virus (CTV)

  • Description: + sense ssRNA virus with a genome size of 9.2kb.
  • Symptoms: clear veins, chlorosis, cupping of leaves, and tree dieback; spread by brown citrus aphid.
  • Environmental conditions affect severity.

Tobacco Mosaic Virus (TMV)

  • + ssRNA virus with helical symmetry.
  • Symptoms: chlorosis and mosaic mottling on leaves; can cause lower crop yields but rarely death.

Plant Bacterial Diseases

  • Soft rots (caused by Erwinia carotovora).
    • Virulence factors: pectinases, proteases, cellulases.
  • Wilts (e.g. Pierce's disease caused by Xylella fastidiosa).
  • Blights: rapid tissue destruction (e.g., Fire Blight caused by Erwinia amylovora).

Crown Gall Disease

  • Cause: Agrobacterium tumefaciens integrates T-DNA into the plant genome, leading to growth alteration (auxin, cytokinin production).
  • Affects various crops but usually does not cause total plant death.

Bacterial Diseases of Insects

  • Bacillus thuringiensis: gram-positive rod, produces Bt toxin.
  • Used as a biocontrol agent since the 1950s.
  • Insect-specific action; disrupts gut function causing starvation.

Genetically Engineered Crops

  • Since 1996, crops like Bt corn, Bt potato, Bt cotton, and Bt soybean have been developed to express Bt toxin for insect resistance.

Entomopathogenic Fungi

  • Fungi like Metarhizium and Beauveria used commercially to control insect pests.
  • They invade and kill specific insect hosts, serving as biopesticides.

Nematode-Trapping Fungi

  • Form traps under nitrogen-deficient conditions to control nematodes, used as biocontrol agents in various applications.

Key Takeaways

  • Understanding these pathogens helps mitigate their economic impacts and improve crop resilience through biocontrol methods and genetic engineering.