Plant disease is caused by abiotic and biotic factors. While other chapters deal with the way fungi improve plant health directly, this chapter will ask the question “Can fungi be used to reduce the effects of biotic causes of plant disease?” As most biotic causes are fungi, we will concentrate on fungus to fungus interactions. The topic of fungus to fungus interactions has been reviewed in depth on many occasions from many different points of view. This chapter will not attempt to summarise this work. Rather we will dip into some of the stories.
The interactions between fungi can be manipulated to reduce the damage caused by one fungus on the plant. Essentially, these interactions include:
The direct mechanisms are not mutually exclusive and all may be evident within one interaction at the same or different times.
Mycoparasitism is where one fungus derives its nutrition from another without any benefit in return. The interaction can be where the parasite is biotrophic or necrotrophic. Biotrophs tend to have highly specialised relations with specific hosts. The parasite is extremely difficult to manipulate, and as a consequence few have ever been used in biocontrol.
Necrotrophs tend to kill the cell or tissue from which they feed. They tend to have a broad host range, and their activity commonly includes excretion of antibiotics. For instance, the necrotroph Trichoderma has the capacity to parasitise a great number of soil fungi, including pathogens Rhizoctonia, Scerotinia, Fusarium and Verticillium, and the Straminopiles Pythium and Phytophthora. Trichoderma grows from the host where it is an endophyte, towards hyphae of other fungi where it binds following branching. Trichoderma releases hydrolytic enzymes that digest the walls of the host prior to penetration. Trichoderma is also thought to release a range of toxins that reduce any response from the host to invasion.
For biocontrol to be successful, the parasite must be placed in an environment favourable for its competitive success. Most soils are oligotrophic. LINK Thus, the introduced fungus is unlikely to be favoured against isolates that are adapted to the local conditions. By judicious increases of organic amendments at inoculation, biocontrol can be attained followed by a decline in the population size of the parasite as the amendment degrades. In addition, biocontrol agents that colonise epidermal or cortical cells of the root have access to abundant nutrients, placing them at an advantage over the plant pathogen.
Antibiosis can be defined as the inhibition of one organism by metabolites of another. Metabolites that kill another organism tend to be called toxins. Thus, dilute solutions of toxins may inhibit and high concentrations of antibiotics may kill. LINK
Most fungi produce inhibitory metabolites. For instance, Gliocladium produces a diketopiperazine that kills Pythium probably because of coagulation of proteins in the cytoplasm. Use of the fungus on seed appears to reduce seedling damping-off as much as a common seed treatment. Volatile pyrones produced by Trichoderma appear to reduce damping off caused by Rhizoctonia. The potential to exploit these metabolites in biocontrol appears feasible.
The role of antibiotics in biocontrol is still unclear. Some spectacular experiments, when repeated in the field, have resulted in enormously variable outcomes indicating that other mechanisms may be important.
Competition is the active requirement for resources in excess of those immediately available to two or more organisms. The resource most commonly considered to be important for fungi is organic energy. Thus much competition exists for access to energy for growth and maintenance. However, competition between fungi is extremely difficult to quantify. The mycelium is constantly changing. LINK Resources within the hyphae are constantly being shifted between different parts of the thallus, with consequent changes in what is being assessed over very short periods of time.
Competition for nutrients can take place between survival propagules, germinating units and mycelium in any arrangement. The result of competition is stasis of the less successful competitor. Inoculum density does not decline and the potential to initiate disease remains. Thus the application of competition by itself to biocontrol appears limited.
Some examples of fungi used against fungal pathogens with some success:
|Scerotinia, Fusarium, Verticilium||Puccinia|
Trichoderma viride, Trichoderma harzianum, Trichoderma polysporum and Trichoderma hamatum (all Ascomycota) have all been formulated for commercial use against soil fungi. These fungi are common inhabitants of heated soil where the almost sterile medium is conducive to their explosive colonisation following germination of ascospores of the fungi. Over time other fungi colonise the soil, competitively excluding the species of Trichoderma. Thus their obvious presence in soil may be short-lived. They colonise the soil, interact with the pathogen but then generally decline. For instance, Trichoderma can be used in potting media to reduce seedling diseases.
For the fungus to be succesful in field soil, it must establsih an endophytic association with the roots of the crop plant. Given the benefit of a constant source of energy, the biocontrol agent may explore beyond the internal plant tissues interacting with pathogens. Success in field soils is less common than the theory indicates. Many fungi are commercially available but we still have much to understand before relaible control of pathogens can be expected.
While laboratory and glasshouse experiments indicate fungi have a role in biocontrol of plant pathogenic fungi, the results of field application of a single fungus have been discouraging. While it appears that some agents may have a place in integrated approaches to reducing plant pathology, we do not appear to understand the basis of interactions between fungi. Until this understanding is improved, use of fungi in biocontrol is likely to remain a dream.
New theoretical developments argue that release of multiple organisms will have greater potential to regulate populations of ALL microbes, both pathogens and saprotrophs. The real issue is whether the diversity effect is due to taxonomic diversity or functional attributes of the diverse inoculants. If this impact is due to functional diversity, incorporation of organic amendments inocuated with fungi selected to suppress deleterious microbes and other pests may be a sustainable mechanism to reduce reduce reliance on chemical treatment of pests and pathogens. Treatment would be a regular management process, rather than a reaction to a problem.
Baker K.F. & Cook R.J. (1974) Biological Control of Plant Pathogens. Freeman, San Francisco. This fascinating book was the stimulus to explore alternative ways of dealing with biological problems. They following this book with a much broader look at Biological Control in: Baker K.F. & Cook R.J. (1983) Nature and Practice of Biological Control. APS. The two books have since been emulated by others, but without the immediacy these books bring to the subject.
Butt TM, Jackson CW & Magan N 2001 Fungi as Biocontrol Agents. CABI.
Jeger MJ & Spence NJ 2001 Biotic Interactions in Plant-Pathogen Interactions. CABI.
Robson GD, van West P & Gadd GM (eds) 2007. Exploitation of Fungi. CUP