Biological methods:

Mechanisms of biological control :

1. Competition:

• Most of the biocontrol agents are fast growing and they compete with plant pathogens for space, organic nutrients and minerals
• Pseudomonas fluorescens (known as pseudobactins or pyoveridins) helps in the control of soft rot bacterium, Erwinia caratovora.

2. Antibiosis :

• Antagonism mediated by specific or non-specific metabolites of microbial origin, by lytic agents, enzymes, volatile compounds or other toxic substances is known as antibiosis.

a. Antibiotics:

• Antibiotics are generally considered to be organic compounds of low molecular weight produced by microbes.
• Gliocladium virens produces gliotoxin that was responsible for the death of Rhizoctonia solani on potato tubers.
• Colonization of pea seeds by Trichoderma viride resulted in the accumulation of significant amount of the antibiotic viridin in the seeds, thus controlling Pythium ultimum.

b. Bacteriocins:

• These are antibiotic like compounds with bactericidal specificity closely related to the bacteriocin producer.
• The control of crown gall (caused by Agrobacterium tumefaciens) by the related Agrobacterium radiobacter strain K 84 is by the production of bacteriocin, Agrocin K84.

c.  Volatile compounds:

• Antibiosis mediated by volatile compounds has been observed in the management of soil borne pathogens, viz., Pythium ultimum, Rhizoctonia solani and Verticillium dahlia, by Enterobacter cloacae.

3. Hyperparasitism:

• Direct parasitism or lysis and death of the pathogen by another micro-organism when the pathogen is in parasitic phase is known as hyperparasitism.
• Ex: T. harzianum parasitize and lyse the mycelia of Rhizoctonia and Sclerotium.

Biocontrol agents for the management of plant pathogens

Important fungal biocontrol agents:

• harzianum, T. viride, T. virens (Gliocladium virens) are used as biocontrol agents against soil borne diseases, such as, root rots, seedling rots, collar rots, damping off and wilts caused by the species of Pythium, Fusarium, Rhizoctonia, Macrophomina, Sclerotium, Verticillium, etc.

Plant growth promoting Rhizobacteria (PGPR):

• Many bacterial species, viz., Bacillus subtilis, Pseudomonas fluorescens, etc., are usually used for the management of plant pathogenic microbes.
• Bacillus has ecological advantages as it produces endospores that are tolerant to extreme environmental conditions.