Biological control of plant diseases
How microbes like Trichoderma, Pseudomonas and Bacillus suppress plant pathogens, suppressive soils, classic successes, limits, and a seed treatment sum.
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Most plant pathogens live in a crowded world. In the soil around roots and on leaf surfaces, they compete with billions of other microbes, some of which produce antibiotics, steal their food, or attack them directly. Biological control of plant diseases uses these natural antagonists, deliberately applied or encouraged, to reduce disease. It complements resistant varieties and fungicides, and is especially valuable against soil-borne pathogens that are hard to reach with chemicals.
Biocontrol agents, their mechanisms and the classic examples are standard exam topics in plant pathology and in integrated disease management.
Definitions
Biological control of plant pathogens is usually defined, following K. F. Baker and R. J. Cook (1974), as the reduction of inoculum or disease-producing activity of a pathogen by one or more organisms other than people. In practice it includes:
- Introducing antagonists, as seed treatments, soil applications, sprays or post-harvest dips.
- Encouraging resident antagonists through organic amendments, rotations and other practices that build suppressive soils.
- Cross-protection with mild virus strains and hypovirulence.
How biocontrol agents work
- Antibiosis: the agent produces antibiotics or other toxic compounds. Pseudomonas fluorescens strains make 2,4-diacetylphloroglucinol (DAPG) and phenazines; Bacillus species make lipopeptides such as iturins and surfactins.
- Competition: for space, nutrients and especially iron. Many beneficial pseudomonads release siderophores that grab iron so tightly that pathogens are starved of it.
- Mycoparasitism: one fungus parasitises another. Trichoderma hyphae coil around those of Rhizoctonia or Sclerotinia and dissolve their walls with chitinases and glucanases; Coniothyrium minitans destroys the sclerotia of Sclerotinia.
- Induced systemic resistance (ISR): root-colonising bacteria and fungi prime the whole plant's defences, through jasmonic acid and ethylene signalling, so it responds faster to attack. A related state, systemic acquired resistance (SAR), follows a localised infection and depends on salicylic acid.
- Hypovirulence: viruses that weaken a pathogen.
- Cross-protection: infecting a plant with a mild virus strain protects it against severe strains of the same virus.
Major biocontrol agents
| Agent | Type | Target diseases |
|---|---|---|
| Trichoderma (T. harzianum, T. asperellum and others) | fungus | damping-off, root rots, wilts, Sclerotinia, Rhizoctonia; seed and soil treatment |
| Pseudomonas fluorescens and related species | bacterium | root diseases, take-all, some foliar diseases; ISR |
| Bacillus subtilis, B. amyloliquefaciens | bacterium | foliar and soil diseases; spore-forming, so formulations keep well |
| Agrobacterium radiobacter strain K84 | bacterium | crown gall of stone fruits and roses |
| Coniothyrium minitans | fungus | Sclerotinia white mould |
| Ampelomyces quisqualis | fungus | powdery mildews |
| Non-pathogenic Fusarium oxysporum | fungus | Fusarium wilts |
| Antagonistic yeasts | yeasts | post-harvest rots of fruit |
Classic successes
- Crown gall: in 1972, A. Kerr in Australia showed that Agrobacterium radiobacter K84, which produces the antibiotic agrocin 84, protects roots of stone fruit nursery stock from crown gall. It was one of the first commercial biocontrol agents.
- Chestnut blight: in Europe, naturally spreading viruses that cause hypovirulence in the chestnut blight fungus allowed European chestnut trees to recover, unlike in North America.
- Take-all decline: in wheat fields grown continuously, take-all disease often peaks and then declines, because populations of DAPG-producing Pseudomonas build up in the rhizosphere, making the soil suppressive.
- Cross-protection against citrus tristeza virus: in Brazil, mild strains were used from the 1960s to protect millions of sweet orange trees, and the method has also been used against papaya ringspot virus.
Suppressive soils
Some soils remain nearly free of a disease even when the pathogen and a susceptible crop are present. General suppression comes from the total microbial activity of a soil rich in organic matter. Specific suppression comes from particular antagonists, and can often be transferred by adding a small amount of suppressive soil to a conducive one, which shows its biological nature. Well-known examples include Fusarium wilt-suppressive soils in France and Central America, and take-all decline soils worldwide.
Organic amendments, composts, cover crops and rotations can raise suppressiveness, part of the reason healthy soils have less root disease.
How many spores reach each seed?
A Trichoderma product contains 2 × 108 colony-forming units (cfu) per gram, and the label rate is 4 g per kg of seed. A farmer treats wheat seed with a thousand-grain weight of 40 g.
- cfu applied per kg of seed = 4 × 2 × 108 = 8 × 108 cfu.
- Seeds per kg = 1,000 g ÷ 0.040 g = 25,000 seeds.
- cfu per seed = 8 × 108 ÷ 25,000 = 32,000 cfu per seed, assuming even coating.
For a larger-seeded crop such as chickpea (thousand-seed weight about 200 g, so 5,000 seeds per kg), the same dose gives about 160,000 cfu per seed. If the product has been stored badly and its count has fallen tenfold, each seed gets far less, which is one reason biocontrol results can be inconsistent.
Limits and good practice
- Inconsistency: performance varies with soil, temperature, moisture and the pathogen pressure, more than for most fungicides.
- Shelf life and quality: living products must be stored correctly and used before expiry; check counts and contaminants.
- Compatibility: many fungicide seed treatments kill biocontrol fungi; check before combining.
- Speed and spectrum: biocontrol usually prevents rather than cures, and each agent controls a limited range of diseases.
- Regulation: biocontrol agents are registered as biopesticides, with requirements varying by country.
Biocontrol works best as one part of integrated disease management, alongside resistant varieties, rotation, sanitation and, where needed, targeted fungicides.
Key points
- Biological control uses living antagonists to reduce pathogens or disease.
- Mechanisms: antibiosis, competition (including siderophores for iron), mycoparasitism, induced systemic resistance, hypovirulence and cross-protection.
- Main agents: Trichoderma, Pseudomonas fluorescens, Bacillus, Agrobacterium radiobacter K84, Coniothyrium minitans.
- Classic examples: K84 against crown gall, hypovirulence in chestnut blight, take-all decline, mild-strain protection against citrus tristeza.
- Suppressive soils can be general or specific; organic matter and rotations help.
- Biocontrol is less consistent than chemicals and works best in integrated programmes.
What the exams ask
- School-leaving and entrance papers: what biological control is, and examples of biocontrol agents such as Trichoderma and Pseudomonas.
- Degree and research-fellowship entrances: mechanisms of antagonism, ISR and SAR, suppressive soils, cross-protection, hypovirulence, and classic case studies.
- Professional examinations: formulation, application, quality control and registration of biopesticides, and their place in integrated disease management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can I mix Trichoderma with a chemical seed treatment?
Often not, because many fungicides kill it. Some products are compatible; otherwise apply the biocontrol agent to the soil or at a different time.
Why does biocontrol work in one field and not another?
Living agents need suitable conditions to establish and multiply. Soil type, temperature, moisture, organic matter, resident microbes and disease pressure all affect results.
Is biocontrol the same as using biofertilisers?
No, though some microbes do both. Biofertilisers supply nutrients, for example by fixing nitrogen; biocontrol agents suppress pathogens.
Sources
- Baker, K. F. and Cook, R. J. Biological Control of Plant Pathogens. W. H. Freeman, 1974.
- Kerr, A. "Biological control of crown gall through production of agrocin 84." Plant Disease 64, 25–30 (1980).
- Weller, D. M., Raaijmakers, J. M., McSpadden Gardener, B. B. and Thomashow, L. S. "Microbial populations responsible for specific soil suppressiveness to plant pathogens." Annual Review of Phytopathology 40, 309–348 (2002).
- Harman, G. E., Howell, C. R., Viterbo, A., Chet, I. and Lorito, M. "Trichoderma species: opportunistic, avirulent plant symbionts." Nature Reviews Microbiology 2, 43–56 (2004).
- Pieterse, C. M. J. et al. "Induced systemic resistance by beneficial microbes." Annual Review of Phytopathology 52, 347–375 (2014).
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