The document discusses biological control of plant diseases. It describes ideal characteristics of biocontrol agents including being non-pathogenic and having a broad spectrum of activity. Common biocontrol agents used are fungi, bacteria, and actinomycetes from the genera Trichoderma, Gliocladium, Pseudomonas, and Bacillus. Mechanisms of biocontrol include direct mechanisms like hyperparasitism and antibiosis, as well as indirect mechanisms like competition and inducing systemic resistance in plants.

1. Biological control of plant diseases
Course in charge
Dr. Y. Nagaraju

2. Ideal characters of antagonist/bioagents
 A biological agent that reduces the number or disease
producing activities of the pathogen as called antagonist or
biocontrol agent
 The important biocontrol agents used in plant disease
management includes fungi, bacteria, actinomycetes,
mycorrhizal fungi, viruses, protozoa, Bdellovibrio etc.,
 Among them fungal and bacterial antagonists especially
species of Trichoderma sp., Gliocladium sp., Pseudomonas sp.,
and Bacillus sp., are most widely used against plant diseases

3.  An ideal biocontrol agent should have the following features or
charecters
1. It should not be pathogenic to plants, human beings, animals,
and beneficial microorganisms
2. It should have broad spectrum of activity in controlling many
types of diseases and must be genetically stable
3. It should have fast growth and sporulation
4. It must be cultured under artificial media
5. The inoculums must be capable of abundant production using
traditional fermentation techniques such as liquid
fermentation

4.  It should have long shelf life
 Its should be effective under different environmental conditions
 It should be compatible with biofertilizers
 It should be least susceptible to seed treating chemicals
 It should not toxic to beneficial microorganisms
 It should be easily formulated and methods of application must
be convenient and compatible with common cultural practices
 It should be easily establish in the soil with high persistence
and survival capacity
 It should be competitive with other microorganisms

5. Mechanisms of biocontrol
 In 1932 Weindling discovered that Trichoderma lignorum
parasitized many soil borne fungi in culture and thus suggested
controlling certain pathogenic fungi by augmenting soil with an
abundance of this parasite
 Mechanisms of action is broadly classified into two types
1.Direct mechanisms
a. Hyperparasitism
b. Antibiosis
II. Indirect mechanisms
a. Competition
b. Induced systematic resistance

6. a. MycoparasitismHyperparasitism
 Direct utilization of pathogens as a source of nutrients
 Mycoparasistsim refers to the association in which a parasitic fungus
(hyperparasite) live as a parasite on another fungus (hypoparasite)
 Hyperparasite produces parasiting hyphae which penetrate into hosts
hyphae to acquire nutrients
Steps involved in mycoparsitism
1.Chemotrophic growth: biocontrol fungi grows towards the target
fungi, through chemical signaling
2. Recognition stage: interaction between biocontrol receptors and that
of the host fungus
3. Attachment and cell wall degradation: hyperparasite produces
cell wall degrading enzymes such as chitinases and glucanases
4. Penetration: hyperparasite produces appressoria like structures
which penetrate the hyphae of host fungus

10. Mode of
antagonism
Plant pathogens Antagonists
(hosts)
Post-infection events
Mycoparasitism:
(parasitizing fungus)
Rotrytis alii Gliocladium roseum Penetration of hypae
Cocchliobolus
sativus
Myrothecium
verrucaria and
Epicoccum
purpurascens
Antibiosis and
penetration
Rhizoctonia solani
and Fomes annosus
Trichoderma viride Coiling, cytoplasm
coagulation
Sclerotium rolfsii T. harzianum Coiling, penetration
and
lysis
Nematophagy:
(process of consuming
nematodes as food)
Heterodera
rostochiensis
Phialospora
heteroderae
Penetration of cysts
and egg killing
Mycophagy (process
of consuming fungi as
food)
Cocchliobolus
sativus
Soil amoebae Perforation in conidia
Gaeumannomyces
graminis var.
tritici
Soil amoebae Penetration and lysis
of hyphae

11. b. Antibiosis
 Amensalism is a phenomenon where one population adversely
affects the growth of another population whilst itself being
unaffected by the other population.
 Generally amensalism is accomplished by secretion of
inhibitory substances.
 Antibiosis is a situation where the metabolites secreted by
organism A inhibit organism B, but organism A is not affected.
It may be lethal also.
 Metabolites penetrate the cell wall and inhibit its activity by
chemical toxicity. Generally antimicrobial metabolites are
produced by underground parts of plants, soil microorganisms,
plant residues, etc.,

13.  Substances noxious to certain soil-borne plant pathogens are secreted by
roots of maize, clover, lentil (glycine, phenylalanin) and other legumes, flax
(hydrocyanic acid), pine (volatile mono-and sesquiterpenes) and by other
plant roots.
 Other plant residues are the source of phenolic and non-volatile compounds.
Similarly, antimicrobial substances (antibiotics) produced by
microorganisms (soil bacteria, actinomycetes, fungi) are aldehydes,
alcohols, acetone, organic acid, nonvolatile and volalite compounds which
are toxic to microbes.
 Changes in microbial structures (cell wall, hyphae, conidia, etc.), may occur
when microorganisms lack resistance against the attack by deleterious
agents or unfavorable nutritional conditions.

14.  A chemical substance (i.e. melanin) is present in their cell walls to
resist the lysis. Moreover, cell wall constituents, for example, xylan
or xylose containing hetero polysaccharides, may also protect fungal
cells from lysis.
 The potent antagonists e.g. Trichoderma harzianum and T. viride are
known to secrete cell wall lysing enzymes, α-1, 3 glucanase,
chitinase, and glucanase. However, production of chitinase and α-1,
3-glucanase by T. harzianum inside the attacked sclerotia
of Sclerotium rolfsii
 Voltaile antibiotics Non volatile antibiotics
1. Hydrogen cyanide Polyketides
2. Aldehydes Heterogenous nitrogenous compounds
3. Alcohols
4. Ketones
5. Sulfides

15.  Siderophores: Siderophores are the other extracellular metabolites which
are secreted by bacteria {e.g. Aerobacter aerogenes, Arthrobacter pascens,
Bacillus polymyxa, Pseudomonas cepacia, P. aeruginosa, P. fluorescens,
Serratia, etc.), actinomycetes (e.g. Streptomyces spp.) yeasts (e.g.
Rhodotorula spp.), fungi (Penicillum spp.). and
dinoflagellates (Prorocentrum minimum).
 Siderophores are commonly known as microbial iron chelating compounds
because they have a very high chelating affinity for Fe3+ ions and very low
affinity with Fe2+ ions.
 Siderophores are low molecular weight compounds. After chelating
Fe3+ they transport it into the cells. Kloepper et al. (1980) were the first to
demonstrate the importance of siderophore production by PGPR in
enhancement of plant growth.
 Siderophores after chelating Fe3+ make the soil Fe3+ deficient for other
microorganisms. Consequently growth of other microorganisms is inhibited.
When the siderophore producing PGPR is present in rhizosphere, it supplies
iron to plants. Therefore, plant growth is stimulated.

21. Indirect mechanisms
a. Competition= “Exclusion of pathogens by biological control
agents via competition for space or nutrients”
 Among micro-organisms competition exists for nutrients,
including oxygen and space but not for water potential,
temperature, or pH.
 Amensalism involves the combined action of certain
chemicals such as toxins, antibiotics and lytic enzymes.
 Success in competition for substrate by any particular fungal
species is determined by competitive saprophytic ability and
inoculum potential of that species. Competitive saprophytic
ability is "the summation of physiological characteristics that
make for success in competitive colonization of dead organic
substrates"

23. Difference between induced systemic
resistance and systemic acquired resistance
Systemic Acquired Resistance Induced Systemic Resistance
Salicylic acid dependent Salicylic acid cycle independent
Necrosis reaction present Necrosis reaction absent
Signaling molecule JA Signaling molecule JA, Ethylene
Less elastic More elastic
Against biotrophs Against necrotrophs and insects
Continuous interaction is Not required Continuous interaction is required
PR proteins Defense genes involved are PDF 1.2