The document discusses various biological control mechanisms such as antagonism, parasitism, hyperparasitism, and antibiotic-mediated suppression. It highlights the roles of specific organisms like Trichoderma spp., Pseudomonas, and Bacillus cereus in suppressing plant pathogens through competitive interactions, enzyme production, and antibiotic secretion. Additionally, it explains the significance of specificity in interactions and the effectiveness of these biological control strategies in agricultural contexts.

1. MECHANISMS OF BIOLOGICAL CONTROL
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2. Antagonism
❖ Mechanisms by which an organism suppresses the normal growth and activity of a
pathogen or the diseases they cause.
❖ Effect of antagonism depends upon the interspecies contact.
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4. ❖ Parasitism is the direct competitive interaction between two organisms in which one organism
is gaining nutrients from the other.
❖ Trichoderma spp parasitize fungal plant pathogens.
❖ The parasite extends hyphal branches towards target host, coils around and attaches to it with
appressorium-like bodies, and punctures its mycelium.
❖ These events require specific interactions between the parasite and fungal host, including the
detection of chemical gradients and mycelial surface features.
Contd…….
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Parasitism

5. ❖ Specificity: Trichoderma coils around Pythium ultimum hyphae but not plastic threads of
a similar diameter.
❖ Digestion of host cell walls - enzyme - proteases, chitinases, and glucanases
Antifungal
Synergistic with antibiotics
❖ Pseudomonads - antibiotic 2,4-diacetylphloroglucinol (DAPG) – induce host defences
❖ Trait of Colonizing root - Suppress pathogen activity in the rhizosphere of wheat through
competition for organic nutrients (Raaijmakers and Weller 2001).
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6. Hyperparasitism
❖ If the host is also a parasite (a plant pathogen), the interaction is defined as hyperparasitism.
Eg: 1. Coniothyrium minitans - attacks sclerotia
2. Pythium oligandrum - attacks living hyphae
3. Acremonium alternatum, Acrodontium crateriforme, Ampelomyces quisqualis,
Cladosporium oxysporum, and Gliocladium virens - parasitize powdery mildew pathogens.
Four major classes of hyperparasites
❖ Obligate bacterial pathogens,
❖ Hypoviruses,
❖ Facultative parasites, and
❖ Predators.
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7. Obligate bacterial pathogens
❖ Pasteuria penetrans is an obligate bacterial pathogen of root-knot nematodes.
Hypoviruses
❖ Virus infects Cryphonectria parasitica, a fungus causing chestnut blight, which causes
hypovirulence, a reduction in disease-producing capacity of the pathogen.
❖ Controlled the chestnut blight in many places (Milgroom and Cortesi 2004).
❖ The interaction of virus, fungus, tree, and environment success or failure
of hypovirulence.
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8. ❖
❖ Generalized feeders
❖ Non- specific
❖ Less predictable in the level of disease control.
❖ Eg: Trichoderma
1. Fresh bark used in composts - not directly attack the plant pathogen,(Rhizoctonia solani)
2. Decomposing bark- concentration of cellulose decreases activates the chitinase
genes - parasitize R. solani (Benhamou and Chet, 1997).
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Predators

9. ❖ Antibiotics are microbial toxins that can, at low concentrations, poison or kill other
microorganisms.
❖ Effective at suppressing plant pathogens and the diseases they cause.
❖ Antibiotics must be produced in sufficient quantities near the pathogen to result in a
biocontrol effect.
❖ Bacillus cereus strain (UW85) produce both zwittermycin (Silo-Suh et al. 1994) and
kanosamine (Milner et al. 1996).
❖ Pseudomonas putida (WCS358r) strains genetically engineered to produce phenazine and
DAPG - suppress plant diseases in field-grown wheat (Bakker et al. 2002).
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Antibiotic-mediated suppression

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11. Some of antibiotics produced by BCAs
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12. ❖ Many microorganisms produce and release lytic enzymes - hydrolyze a wide variety of
polymeric compounds, including chitin, proteins, cellulose, hemicellulose, and DNA.
❖ Eg: Control of Sclerotium rolfsii by Serratia marcescens appeared to be mediated by
chitinase expression.
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Lytic enzymes

13. ❖ P. fluorescens CHA0 produces antibiotics, siderophores and HCN.
❖ Eg: P. fluorescens - HCN production – suppress black rot of tobacco (Thielaviopsis
basicola) by blocking cytochrome oxidase pathway.
❖ Enterobacter cloacae – ammonia – suppress Pythium ultimum-induced damping-off of
cotton (Howell et al., 1988)
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Un-regulated waste products

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