Biological Control of Plant Pathogens

Biological Control of Plant Pathogens

• Utilizes natural antagonistic organisms to suppress plant pathogens.

Mechanisms of Biological Control

• Direct Antagonism: Involves hyperparasitism and predation, where a biological control agent (BCA) directly attacks the pathogen.
  • Examples include:
    • Ampelomyces quisqualis: hyperparasitic fungus.
    • Lysobacter enzymogenes: suppresses various plant pathogens.
    • Trichoderma virens: effective against several soil-borne fungi.
• Mixed-path Antagonism: Combines multiple mechanisms, such as:
  • Antibiotics: compounds like 2,4-diacetylphloroglucinol and phenazines produced by bacteria.
  • Lytic Enzymes: enzymes like chitinases and glucanases that degrade pathogen cell walls.
  • Unregulated Waste Products: toxic compounds such as ammonia and hydrogen cyanide affecting pathogens.
  • Physical/Chemical Interference: Involves blockage of soil pores or consumption of germination signals.
• Indirect Antagonism:
  • Competition for resources, including nutritional compounds and physical space.
  • Induction of host resistance via interactions with pathogen-associated molecular patterns.

Hyperparasitism and Predation

• Considered the most direct form of antagonism.
• Classes of Hyperparasites:
  • Obligates, such as Pasteuria penetrans, which targets root-knot nematodes.
  • Hypoviruses which can reduce the virulence of pathogens like Cryphonectria parasitica.

Antibiotic-Mediated Suppression

• Involves the secretion of microbial toxins that inhibit other microorganisms, functioning at low concentrations.
• Important antibiotics include:
  • 2,4-diacetylphloroglucinol: Effective against Pythium spp., causes damping-off.
  • Agrocin 84: Targets Agrobacterium tumefaciens, responsible for crown gall disease.
  • Bacillomycin D: Aflatoxin contamination control against Aspergillus flavus.
  • Xanthobaccin A: Suppresses Aphanomyces cochlioides, also linked to damping-off.
  • Gliotoxin: Targets Rhizoctonia solani, responsible for root rots.

Additional Antibiotics Produced by BCAs

• Iturin A: Effective against Botrytis cinerea and Rhizoctonia solani, responsible for damping-off.
• Phenazines: Combat Gaeumannomyces graminis var.tritici, causing take-all disease.
• Mycosubtilin: Targets Pythium aphanidermatum, associated with damping-off.
• Pyoluteorin: Addresses Pythium ultimum, involved in damping-off.

The Biological World: Balance in Natural Communities

• Homeostasis is the property of living organisms that maintains stable internal conditions.
• Disruption of homeostasis can lead to diseases and cellular malfunction.
• Ecosystem homeostasis reflects a dynamic equilibrium that adapts to changing environmental conditions.

Homeostatic Mechanisms

• Ecosystems utilize self-regulating processes for stability while adapting to survival conditions.
• Key homeostatic mechanisms include:
  • Matter cycling and energy flow
  • Protection of production levels
  • System structure

Attack-Avoidance Behavior

• In mammals and birds, members of the same species engage in harassing behavior, expending energy in interactions that prevent serious harm but regulate population dynamics.

Overcrowding and Its Effects

• Overcrowding in small animals like mice can cause stress that damages the thyroid gland, resulting in severe population declines due to endocrine issues.

Prey-Predator Dynamics

• Abundant prey populations lead to increased predator births, creating cycles of abundance and eventual crashes when resources decline.
• Predators may migrate or risk starvation when prey becomes scarce.

Reproductive Homeostasis

• Reproductive strategies like multiple births are influenced by population density, with crowded conditions favoring single births.

Biodiversity vs Monoculture

• Biodiversity: Greater variety of organisms strengthens community resilience, adaptability, and stability.
• Monoculture: Involves cultivating a single crop species repeatedly, leading to decreased diversity.

Consequences of Monoculture

• Heavy reliance on chemical fertilizers and pesticides increases disease susceptibility and environmental degradation.
• Results include crop failures, reduced soil fertility, increased erosion, loss of genetic diversity, and diminishing habitats for beneficial species.

Biological Control of Plant Pathogens

• Controlling plant diseases is critical for maintaining food production quality and quantity.
• Traditional reliance on chemical pesticides is being challenged due to environmental pollution concerns.

Natural Ecosystem Balance

• Pests exist in a complex equilibrium with plant communities, where both beneficial and harmful interactions occur.
• Disturbance through intensive crop production disrupts this balance, leading to pest population shifts.

Harmful Effects of Pesticides

• Chemical pesticides can lead to:
  • Pest resistance and resurgence
  • Harm to non-target organisms
  • Increased pesticide residues in crops and animals
  • Environmental pollution

Pesticide Usage Statistics

• Global pesticide usage is approximately 3 million tons annually, significantly boosting agricultural productivity.
• Over 98% of insecticides and 95% of herbicides end up beyond their target, affecting non-target species and environmental elements.

Health and Environmental Concerns

• Synthetic pesticides pose risks to humans and the environment, prompting calls for reduced chemical usage and alternative agricultural methods.