Biocontrol potential of compost
Biocontrol refers to the disease reduction or decrease in inoculum potential of a pathogen brought about directly or indirectly by other biological agencies. It is an alternative to chemical pesticides and forms an important part of sustainable agriculture. Composts can provide natural biological control of diseases of roots and plant foliage. Though, the control of all diseases may not occur up to a level that allows the elimination of fungicide use but integration of suitable compost into current disease management practice can reduce fungicide use and associated problems. Use of compost for disease suppression is both ecologically and economically beneficial.
The disease suppressive effect of composts depends on the diversity and activity of micro flora inhabiting compost, composting substrates, environment during its production and curing, rate of its application and stability and maturity of compost. Composts made from substrates with high lignin, cellulose, tannins and waxes can sustain their disease suppressive effects for much longer period, compared to those from food wastes, manures and bio-solids, having more readily degradable materials. Fresh organic matter does not support biocontrol, even when inoculated with the best strains. High concentrations of glucose and amino acids in fresh crop residues repress the production of enzymes required for parasitism by Trichoderma spp. Composts must be stabilized well enough and colonized to a degree that microbiostasis prevails. Excessively humified organic matter as peat, cannot support the activity of biocontrol agents. Organic matter with properties in between these to extreme degrees of decomposition level supports biological control.
Members of genera Bacillus, Enterobacter, Pseudomonas and Flavobacterium (F. balustinum), Streptomyces, Pencillium, Trichoderma and Gliocladium virens have been identified as biocontrol agents in composts. Pathogens sensitive to compost suppressiveness capacity include Pythium ultimum, Rhizoctonia solani, Venturia inaequalis, Fusarium oxysporum, Verticillium dahliae, Phytophtora nicotianae, Phytophtora cinnamomi, Cylindrocladium spathiphyll.
The beneficial bacterial species as Pseudomonas, Bacillus and Pantoea recolonise compost rapidly after thermophilic phase of composting and have shown general disease suppression. The inoculation with beneficial organisms should occur after peak heating at a moisture content of 40-50 %.
Some beneficial fungal species as Trichoderma and Penicillium show suppression against Phytophthora, Rhizoctonia and Fusarium. . The beneficial fungal spp. recolonise compost during maturation or curing phase of composting and their colonization depends upon composition of organic matter. Trichoderma spp prefer to colonise compost rich in lignocellulosic substances (woody materials), while Penicillium and Aspergillus spp.colonise substrates low in cellulose but high in sugars (food waste, vegetable waste, manure). Aspergillus and Penicillium spp. are predominant in eradicating sclerotia of pathogenic fungus Sclerotium rolfsii.
Several disease suppressive mechanisms may operate at the same time against different pathogens in compost-amended media. Outside the host, the biocontrol agent may be antagonistic and there by reduce the activity, efficiency and inoculum density of the pathogen through antibiosis, competition and predation/hyper parasitism.
• Salicylic acid, lipopolysaccharides and antibiotic metabolites released by beneficial microorganisms during composting induce systemic disease resistance and thereby disease suppression.
• Siderophores (iron chelating compound secreted by microorganisms) produced by non pathogenic plant growth promoting microflora are natural chelaters that play a major role in nutrient competition among pathogens and beneficial microorganisms at infection sites.
• Phenolic compounds and volatile fatty acids (acetic, propionic, butyric, isobutyric, valeric, isovaleric and caproic acids) produced during decomposition of organic matter are also released by beneficial microorganisms mainly during curing phase of composting and have shown their potential to suppress diseases. Organic matter decomposition by bacteria under anaerobic conditions result in the production of volatile fatty acids.
• The beneficial bacteria Pseudomonas spp. produce the antibiotic 2,4-diacetylphloroglucinol which is considered responsible for the suppression of take-all in wheat, Fusarium wilt in peas, soft rot in potatoes and cyst nematode.
• Gliocladium a beneficial saprophytic fungus produces a broad spectrum antibiotic called gliotoxin, which kills many soil pathogens. Antibiotic production by the Gliocladium is reported effective in the suppression of damping off (caused by Pythium ultimum). The fungus wraps itself around the pathogen and releases enzymes that destroy the pathogen's cuticle, leaving the pathogens susceptible to attack. The beneficial fungus Trichoderma hamatum 382, a fungal biocontrol agent, has been reported to be highly effective in controlling pathogens.
• Amendments of composting mixture with high nitrogen substrates as poultry manures/meat meals etc. result in production of ammonia, nitrous acid or volatile fatty acids that have been implicated in controlling fireblight, Phytophthora dieback, and Fusarium wilt.
• Organic amendments stimulate the growth of beneficial nematodes, springtails and mites, which consume pathogen propagules in soils (sclerotia of pathogenic fungus Rhizoctonia solani) and significantly slow the growth of the pathogenic brown rot fungus. Specific isolates of Trichoderma spp. act as parasitic biocontrol agents, destroying the sclerotia of pathogenic fungus Rhizoctonia.
Compost amendments play a valuable role in reducing disease and increasing yields in organic production systems. Organic soils may support a microbial community that play a role in induced resistance. Apply compost 4-6 weeks before sowing to allow beneficial microorganisms to colonise the soil and consume readily degradable substances that may otherwise support pathogens.
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