Biological control of crop diseases
Crop diseases are usually caused by microbes, and one of the oldest ideas in plant protection is to fight them with other microbes. Biological control asks whether the natural enemies of a pathogen can be recruited to keep it in check.
The idea
Wikipedia's article on biological pest control defines it as a method of controlling pests, whether animals such as insects and mites, weeds, or pathogens affecting animals or plants, by using other organisms. It relies on predation, parasitism, herbivory or other natural mechanisms, typically with an active human role in managing it, and it can be an important part of integrated pest management. The article describes three basic strategies: classical biological control, in which a natural enemy is introduced; augmentation, in which natural enemies are released in large numbers; and conservation, in which existing natural enemies are encouraged.
For plant diseases, the natural enemies in question are mostly microbes: fungi and bacteria that compete with, poison, parasitise or crowd out the organisms that cause disease. The article notes that microorganisms used this way are registered by regulators as biopesticides, a different category from insects and other larger control agents.
A troublesome fungus: rice sheath blight
A good example of the kind of pathogen that biocontrol research targets is Rhizoctonia solani. Wikipedia's article on Rhizoctonia solani describes it as a fungus with a wide host range and a worldwide distribution, which causes diseases such as root rot, damping off and wire stem. Among the many conditions the article lists are black scurf of potatoes, root rot of sugar beet and sheath blight of rice. The name, the article explains, is now applied to a complex of related species that await further research, and strains differ in the hosts they can infect and how aggressively they do so.
A pathogen like this, living in soil, attacking many crops and hard to eradicate, is a natural target for biological control. Its history is long: the article records that the German plant pathologist Julius Kühn described it on diseased potato tubers in 1858.
Two well-studied antagonists
Trichoderma
Wikipedia's article on Trichoderma describes it as a genus of fungi present in all soils, where they are the most prevalent culturable fungi, and notes that many species can form mutually beneficial relationships with plants. It records that several strains have been developed as biocontrol agents against fungal diseases of plants, through mechanisms that include antibiosis, parasitism, inducing resistance in the host plant, and competition. Put simply, a Trichoderma strain may produce substances that inhibit a pathogen, attack the pathogen directly, prompt the plant to defend itself, or simply take the space and food the pathogen needs.
Pseudomonas fluorescens
Among bacteria, Pseudomonas fluorescens is one of the most studied. Wikipedia's article on Pseudomonas fluorescens describes it as a common Gram-negative, rod-shaped bacterium and notes that some strains show biocontrol properties, protecting the roots of some plant species against parasitic fungi such as Fusarium and the water mould Pythium, as well as some nematodes and insect pests. The article adds that exactly how such strains promote plant growth is not entirely clear, which is a fair summary of much of this field.
Fusarium wilt
Fusarium wilt of tomato is another disease that often appears in biocontrol research. The fungus lives in soil and enters through the roots, and as it spreads through the plant's water-conducting tissue the leaves yellow and the plant wilts. Soil-borne diseases of this kind are hard to reach with sprays, which is part of why researchers look to root-dwelling microbes that might compete with the pathogen where it lives.
From dish to field
Research on a candidate antagonist usually begins in the laboratory with a dual culture: the pathogen and the candidate are grown on the same plate and watched to see whether one holds the other back. Promising strains then move to pot trials with plants and, much later, to the field. Each step is harder to pass than the last. A strain that inhibits a pathogen on agar may not survive in real soil, may not colonise the roots of the crop, or may work in one season and one field but not another.
This is the central difficulty of biological control. A chemical is the same wherever it is applied; a living agent has to establish itself in a complex community of other organisms, under weather and soil conditions no one controls. Good studies acknowledge this by testing across seasons and sites. When reading about biocontrol, it is worth noticing which stage of that ladder a result comes from.