BIOLOGICAL CONTROL OF SOIL BORNE and AERIAL PATHOGENS OF CROP PLANTS
• List of soil borne and aerial plant pathogens and their diseases
• Mechanisms to control plant pathogens
• Major microbialbiocontrolagents
3. The terms “biological control” and its abbreviated synonym
“biocontrol” have been used in different fields of biology, most
notably entomology and plant pathology.
Control of plant pathogens and diseases caused by them
through antagonistic microorganisms is termed as BIOLOGICAL
CONTROL. The microorganisms used are BCAs.
Examples Trichoderma, Penicillium,Sporidesmia, Streptomyces
,Bacillus, Pseudomonas .
According to Baker and Cook’s (1974) “ Biological control is
the reduction of inoculum or disease producing activity of a pathogen
accomplished by or through one or more organisms and manipulation
of environment ”
4. It involves sustained manifestation of interactions among the
plant, the pathogen, the biocontrol agent, the microbial
community on and around the plant and physical
From the plant’s perspective, biological control can be
considered a net positive result arising from a variety of
specific and non-specific interactions.
5. Plant pathogens are those organisms which cause plant diseases
and persists in the soil matrix and residues.
Soil borne diseases – Soil-borne diseases are caused by
microorganisms that survive and move about in the soil. Most
cannot be seen by the eye and go undetected until the plant
Aerial phytopathogens - The micro-organisms are spread
through air and attack the plants causing diseases. E.g.
Blight, rust, powdery mildew.
8. Mutualism is an association between two or more species where
both species derive benefit. Sometimes, it is an obligatory
lifelong interaction involving close physical and biochemical
contact, such as those between plants and mycorrhizal fungi.
Protocooperation is a form of mutualism, but the organisms
involved do not depend exclusively on each other for survival.
Many of the microbes isolated and classified as BCAs can be
considered facultative mutualists involved in protocooperation,
because survival rarely depends on any specific host and disease
suppression will vary depending on the prevailing environmental
9. Commensalism is a symbiotic interaction between two living
organisms, where one organism benefits and the other is neither
harmed nor benefited. Most plant-associated microbes are
assumed to be commensals with regards to the host plant,
because their presence, individually or in total, rarely results in
overtly positive or negative consequences to the plant. And,
while their presence may present a variety of challenges to an
infecting pathogen, an absence of measurable decrease in
pathogen infection or disease severity is indicative of
10. Neutralism describes the biological interactions when the
population density of one species has absolutely no effect on
the other. Related to biological control, an inability to associate
the population dynamics of pathogen with that of another
organism would indicate neutralism.
Antagonism between organisms results in a negative outcome
for one or both. Competition within and between species results
in decreased growth, activity and/or fecundity of the interacting
organisms. Biocontrol can occur when non-pathogens compete
with pathogens for nutrients in and around the host plant.
11. Parasitism is a symbiosis in which two phylogenetically
unrelated organisms coexist over a prolonged period of time.
In this type of association, one organism, usually the
physically smaller of the two (called the parasite) benefits and
the other (called the host) is harmed to some measurable
extent. The activities of various hyperparasites, i.e., those
agents that parasitize plant pathogens, can result in
12. Alternative method of disease control.
Can be used where other methods are not applicable.
Non toxic to man and environment.
Act on selective organism.
BCAs are self sustaining and easy adaptability.
Diversified mode of action.
Less possibility of development of resistance in pathogens.
Cost effective and long term effect.
14. Antibiosis occurs during interactions involving low-molecular-
weight diffusible compounds or antibiotics produced by
biocontrol agents that inhibit the growth of other
Antibiotics are microbial toxins that can act at low
concentrations, poison or kill other microorganisms. Most
microbes produce and secrete one or more compounds with
Antibiotics have been shown to be particularly effective at
suppressing plant pathogens and the diseases they cause.
17. Siderophores- low molecular weight(500-100 Da)
compounds-produced by fungi and bacteria-bind with Fe3+-
transports into the cell
Iron –biologically important- cytochrome and cofactors
Produced in iron limiting environment – alkaline soils
They are small peptide molecules containing side chains and
functional groups that can provide a high affinity set of
ligands to coordinate ferric ions
20. SYSTEMIC ACQUIRED RESISTANCE
Systemic acquired resistance (SAR), is mediated by salicylic acid
(SA), a compound which is frequently produced following
pathogen infection and typically leads to the expression of
pathogenesis-related (PR) proteins. These PR proteins include a
variety of enzymes some of which may act directly to lyse
invading cells, reinforce cell wall boundaries to resist infections,
or induce localized cell death.
21. INDUCED SYSTEMIC RESISTANCE
It is mediated by jasmonic acid (JA) and/or ethylene,
which are produced following applications of some
nonpathogenic rhizobacteria. Interestingly, the SA- and JA-
dependent defense pathways can be mutually antagonistic, and
some bacterial pathogens take advantage of this to overcome
the SAR. For example, pathogenic strains of Pseudomonas
syringae produce coronatine, which is similar to JA, to overcome
the SA-mediated pathway (He et al. 2004).
Bacterial strain Plant species
mycoides strain Bac
chitinase and β-
subtilis GB03 and
Arabidopsis 2,3-butanediol ISR
CHA0 Tobacco Siderophore SAR
Arabidopsis Antibiotics (DAPG) ISR
WCS374 Radish Lipopolysaccharide ISR
Iron regulated factor
24. COMPETITION for food base, essential nutirents, iron and many
other growth factors- root exudates
Competition also occur for ecological niche
Ectomycorrhizal fungi compete for space and nutirients fungal
Successful competition- also require antagonizing the pathogen
by means of killing or poisoning.
Example : Pseudomonas flourescens prevents Bacterial blotch by
competing with Pseudomonas tolaasii.
26. Colonization : the microorganisms will grow and persist in the
Motility, osmotolerance and cell surface characteristics- contributes
to colonization ability.
PGPR are uniquely equipped to sense the chemoattarctants.
Lipopolysaccharides – helps in root adhesion.
High bacterial growth rate,vitamin B1 production and exudation of
Site specific recombinases- regulatory process of DNA
Root exudates and root mucilage (carbon source)- biofilm fromation.
28. BIOFILM- MULTICELLULAR AGGREGATES- in
Adhere to each other –polysaccharides, protiens and DNA
Resistance to antimicrobial agents,UV rays, dessication etc.
They act as physical barriers for pathogenic bacteria
29. Attack or feed directly on target organisms or produce some
kind of toxin that kill the target organism and then feeds on the
Obligate bacterial pathogens,hypoviruses, facultative parasites
Mycoparasitism is exhibited by fungal biocontrol agents such as
Trichoderma, Gliocaldium etc
Lytic enzymes and toxins are also produced.
Predation occurs during Nutrient limiting condition.
Example Trichoderma harzianum BioTrek used as seed
treatment against pathogenic fungus.
31. Highly effective biocontrol strain must be obtained or produced
Be able compete and persist
Be able to colonize and proliferate
Be non pathogenic to host and environment
Inexpensive production and formulation of agent must be
Production must be with excellent shelf life.
Delivery and application must permit full expression of the
33. PGPRs are the soil bacteria that colonize plant root.
Mechanism of action
1. Antibiotic production
2. Hydrolytic enzyme production
3. Induced systemic resistance
4. Exopolysaccharide production
5. Siderophore production
34. Use Pseudomonas strains are commonly uses to treat seeds or
roots of plants before planting. They can also be used to treat
tubers and bulbs.
Advantages, limitations and ways to improve efficacy-
grow rapidly in vitro and can be mass produced at relatively low
cost. good compatibility with several fungicides.
The major weakness of pseudomonads as biocontrol agents is
their inability to produce resting spores (as do many Bacillus spp.),
which complicates formulation of the bacteria for commercial use.
The identification of new formulations, which can prolong their
shelf-life, can dramatically increase their use in practice.
39. Aerobic, gram-negative bacteria, ubiquitous in agricultural
soils, and are well adapted to growing in the rhizosphere.
Pseudomonads possess many traits that make them well
suited as biocontrol and growth-promoting agents
Some species : P. fluorescens
40. The most known species hosting biocontrol agents are
They are Gram positive rod-shaped bacteria that can
form a protective endospore that can tolerate extreme
41. Trichoderma is a genus of asexually reproducing fungi that
are often the most frequently isolated soil fungi
Colonize woody and herbaceous plant materials.
The use of Trichoderma as microbial control was recognized
in early 1930’s.
42. Biocontrol agents of plant diseases and nematodes and a few
of them have been developed and registered as bio-
Grow rapidly on culture.
They reproduce themselves by Chlamydospores hence are a
fastest way to control fungi.
43. T. atroviride
44. Action against pathogenic fungi
Attachment to the host hyphae by coiling : Lectin-carbohydrate
Penetrate the host cell walls by secreting lytic enzymes :
Chitinases, Proteases and Glucanases.
Rhizoctonia, Fusarium, Alternaria, Ustilago, Venturia and
Colletotrichum , as well as fungus like organisms such as the
Oomycetes Pythium and Phytophthora which lack chitin in their
Accumulation of antimicrobial compounds.
50. Although the actual use of commercially available BCA is rather
limited, it is expected that these and other such products will find
wide acceptance in the not too distant future.
The main constraints in the slow progress in use of these
products is the apprehension of farming community about both its
efficiency and safety.
It is definite and has been emphasized time and again that
future of plant disease management can not rest on the shoulders of
hazardous chemicals and therefore biological control has to play
significant role in days to come.