Biocontrol Agents and Mechanism Involved in Biological Control
Author: Naresh Kumar

According to the estimate of FAO, the annual losses to crop production due to plant diseases alone are about 12 per cent. The losses are going to increase with change in agricultural practices and our country, which is tropical with warm and humid climate, provides ideal conditions for development and spread of diseases of crops cultivated under good management systems. Therefore, control of plant diseases assumes greater importance. Although several methods are followed for this purpose, disease control by chemicals is the promising one especially under intensive cropping programmes. However, their use poses certain problems of residues left over on crops, which has gained considerable importance in recent years, not only in India but also throughout the world.

The reduction of inoculums density or disease producing activities of a pathogen or parasite in its active or dormant stage by one or more organisms accomplished naturally or through manipulation of the environment, host or by introduction of one or more antagonists.

Biocontrol Agents :

Fungi (Antagonists) Pathogen Controlled
Trichoderma viride Macrophomina phaseolina, Pythium, Sclerotium rolfssi
Trichodema harzianum Fusarium oxysporum f.sp. vasinfectum,Fusarium oxysporum f.sp. melonis, Rhizoctonia solani
Trichoderma haematum Pythium sp., Rhizoctonia solani
Verticillium laccani Uromyces dianthi
Gliocladium virens P. ultimum, Rhizoctonia solani
Latesaria arvalis Sclerotinia sclerotiarum, R. Solani, Pythium sp.
Bacteria :
Agrobacterium radiobacter Agrobacterium tumefaciens
Azotobacter chrococcum Rhizoctonia solani
Bacillus subtilis Fusarium spp., Pythium spp., Rhizoctonia spp., Sclerotium rolfsii, Streptomyces scabies, Verticillium spp.,
Bacillus thuringiensis Alternaria alternata, Hemileia vastatrix
Erwinia herbicola pv. herbicola Erwinia amylovora
Pseudomonas fluorescens Fusarium spp., Macrophomina phaseolina, Pyricularia oryzae, Pythium spp., Rhizoctonia solani
P. cepacia Cercospora sp.
Streptomyces diastaticus Pythium aphanidermatum
S. griseoviridis Alternaria brassicola, Fusarium oxysporum f. sp. dianthi, Rhizoctonia solani
Mechanism involved in biological control :-

1. Parasitism :

The biocontrol agent parasitizes the pathogen by coiling around the hyphae.

Ex – Tricoderma viride parasite derives its nutrition from the

pathogen by puncturing the host hyphae and kills them.

Various bactria and fungi secrete hydrolytic enzymes for the degration of cell wall of pathogen.

2. Lysis :

Bacillus sp. Causes hyphal lysis of gaumanomyces gramini.

The chitinolytic enzymes of Serratia marcescens caused cell wall lysis of Sclerotium rolfsi.

Tricoderma sp. Produces chitineses and B 1-3 glucanases which lysis the cell wall of Rhizoctonia solani.

3. Antibiosis :-

The antibiotic compounds secreted by the biocontrol agent suppress the growth of the pathogen.

Ex – Phenazine-1-carboxylic acid (PCA) produced by Pseudomonas fluorescence play an important role in suppressing the take all disease of wheat, Trichoderma sp. Produces antibiotics Viz- Trichodermin, Gliotoxin and viridian.

4. Competition :-

The biocontrol agent competes for food and essential elements with the pathogen thereby displacing and suppressing the growth of pathogen. Ex.- The competition for nutrients between pythium aphanidermatum, P. ultimum and bacteria suppress the damping off disease in cucumbers.

Fluorescent iron chelator siderophores such as Pseudobactins and pyoverdins produced by P.fluorscence chelate the iron available in the soil, thereby, depriving the pathogen of its Fe requirement

Mode of Application

  • Innundative application- The biocontrol agent is applied directly to the infection court at high population level. For Ex. – Tricoderma used for seed treatment.
  • Augmentative application – The biocontrol agent is applied at one place at lower multiplies and spreads to other plant part and provides protection. For Ex.- PGPR are used for seed bacterization.

    Methods of application
  • Seed Treatment
  • Seedling root dipping treatment
  • Soil application
  • Foliar application

    Advantage of biocontrol agent :-
  • Less costly and cheaper than any other method.
  • Give protection throughout the crop period.
  • Highly effective against specific plant disease.
  • They do not cause toxicity to the plant.
  • Application is safer to the environment.
  • They can be combined with biofertilizer.

Biological control is nothing but ecological management of community of organisms. It involves harnessing disease-suppressive microorganisms to improve plant health. Disease suppression by use of biological agents is the sustained manifestation of interactions among the plant (host), the pathogen, the biocontrol agent (antagonist), the microbial community on and around the plant and the physical environment.

About Author / Additional Info:
I am doing Ph.D (Agriculture) Plant Pathology from S.K.N. Agriculture University, Jobner (Raj.)