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Biological Control of Plant Diseases

Biological Control of Plant Diseases
Several microorganisms have shown potentialities as biological control agents against important plant diseases caused by soil-borne pathogens. They are Pseudomonas fluorescens against the take-all disease of wheat caused by Gaeumannomyces graminis, Erwinia caratovora infection of wheat, Thielaviopsis basicola infection of tobacco and damping off caused by Pythium in cotton; Bacillus subtilis against Fusarium roseum wilt of com: Trichoderma harzianum against Alternaria spp. infection of radish;

Penicillium oxalicum against root rot of peas; non-pathogenic Fusarium oxysporum against Fusarium wilt of cucumber; Trichoderma viride against Verticillium wilt of tomatoes; Cytophaga sp. against damping off of conifer seedlings; Bacillus sp., Penicillium sp., and Alcaligenes sp. against crown gall disease of cherry seedlings caused by Agrobacterium tumefaciens; Chaetomium globosum against damping off of sugar beets; Pseudomonas putida against Fusarium solani wilt of beans; Bacillus sp. and Pseudomonas sp. against Fusarium oxysporum wilt of carnations.

Although research on biological control of plant diseases has been car­ried out for a number of years, it is relevant to mention some examples of success in this regard.

One example is the use of Peniophora gigantea to control Heterobasidium annosum infection of pine trees. In the U.K. and the U.S.A., felled pine tree stumps are coated with P. gigantea which prevents the colonization of stumps by H. annosum, thereby restricting the survival and multiplication of the pathogen. The second example is the use of Agrobacterium radiobacter in the control of crown-gall disease caused by A. tumefaciens. Considerable damage to stone fruit, pome fruit, other rosaceous plants, walnut, grapevine, chrysanthemum and dahlia is caused by crown-gall infection (plant cancer). This disease has been successfully controlled in Australia by using the strain 84 of the non-pathogenic A. radiobacter, which produces an unusual antibiotic called "Agrocin 84" which belongs to a new group of antibiotics known as nucleotide bacteriocins. A bacteriocin is a substance produced by certain strains of bacteria and active on other strains of the same or closely related species

A large conjugative plasmid called Ti plasmid (tumour-inducing cir­cular DNA molecule) controls crown-gall infection. This plasmid can be transferred to another cell by conjugation and in this process a non-pathogen can become a pathogen. The plasmid directs the synthesis of chemicals called opines-nopaline, agrocinopine A, octopine and agropine. Strains of A. tumefaciens which produce the first two opines are known as 'nopaline strains' and those which produce the other opines are known as 'Octopine strains'. All octopine strains are not controlled' by agrocin 84 but nopaline strains are susceptible to the bacteriocin and fortunately crown-gall on stone fruit is caused by nopaline strains of A. tumefaciens and hence can be controlled.

The role of rhizobacteria in the control of soil-borne diseases of plants has been discussed in the chapter on The Rhizosphere.

 
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