Fungi utilise many different sources of carbon, some living and some dead. The observation that nematodes can be colonised by fungi, led to the possibility that the interaction might be used to control pest nematodes of plants and animals. Closer examination of the interaction has led to the belief that only partial control of nematodes is possible, and that in a practical sense, populations of pests may be suppressed not eliminated. In the case of plant and animal parasitic nematodes, fungi may be used to regulate nematode populations in conjunction with other processes.
Nematodes found in plants, animals and soil use various sources of organic energy. Plant parasites feed from plant cells. Sedentary feeders select and then establish a single feeding sites.
Other plant-parasitic nematodes are migratory, moving from site to site on the root, rarely feeding from a single cell for a long time. Migratory plant parasites can be further categorised as internal and external feeders, meaning that they migrate within or outside the root.
Animal parasites of agriculturally important ruminants tend to have a life cycle whereby colonisation of the host is followed by a soil phase. Larvae are consumed by grazing animals along with grass. The larvae establish a feeding location within the digestive system of the host. Adults lay eggs within the intestines, and the eggs fall to the ground with the faeces. Eggs subsequently hatch in the faeces and larvae move to blades of grass, where they are consumed.
Fungi that colonise nematodes fall into several different categories, each of which requires a different approach if it is to be used successfully to control pest nematodes. The fungi range from highly specialised nematode parasites, to generalists with no specific host. Some fungi may colonise organic matter, insects and incidentally also colonise eggs, larvae or adult nematodes. Other fungi may trap larvae or adults and colonise their body, using the body as a source of nutritients.
Endoparasitic fungi are generally highly specific to single species or genera of nematodes, and are difficult to culture in the absence of their host. Hirsutella, Meria, Nematophthora and Nematoctonus have all been suggested as ideal biocontrol agents because of the specificity for economically important nematodes. However, the difficulty of culturing them and lack of experimental support for their activity has limited any commercial development.
Nematode trapping fungi utilise either sticky pads or constricting rings to immobilise nematodes. The fungus then digests the internal organs of the nematode and form reproductive cells. The main fungi in this group are Arthrobotrys and Duddingtonia. These fungi are found in soil where they live on readily-available organic carbon, in the absence of nematodes. They are generalist feeders, being able to trap and digest many nematodes, not just those that infest plants. Thus, the fungi are rare, unable to exist at sufficiently high densities to control nematode populations, and parasitic on beneficial as well as pest species of nematode. Because of these difficulties, exploration of the use of trapping fungi in the field has largely ceased, though see below for a case study of animal pests and Duddingtonia.
Parasitic fungi have been isolated from either eggs, larvae or adults nematodes. Several of these have been found to preferentially parasitise the nematode and thereby reduce the size of the nematode population. Research efforts in some crops are now concentrating on using fungi found locally to control local pests. The general approach is to go to locations where nematodes have reached high densities. Parasitised individuals are extracted from soil, the fungi cultured and then tested as parasites of the pest nematode. The theoretical basis for this approach is that high densities of nematode will enable fungal parasites to increase population size dramatically. Thus by selecting fungi adapted to the host (nematode) and environment, inundative inoculation will control the pest nematode.
Biocontrol in problem fields has been achieved in a surprising number of cases. The fungal species used include Paecilomyces lilacinus. The fungus appears to have a wide range of potential hosts, both insect and nematode and plants, and yet isolates have a degree of specificity for each host. The most interesting examples parallel success with control of problematic insects and fungi. Endophytic fungi colonise roots of plants where they access host energy. Hyphae grow from the root where they may contact the parasitic nematodes. The nematode is colonised and damage to the plant limited. Again, sustaining populations of both nematode and parasitic fungus is paramount.
Sheep and cattle have traditionally been drenched to control parasitic nematodes. However, nematodes are now developing resistance to the common active ingredients in drenches. Drenches using different active compounds are not being developed because the economical returns are low for the investment. Thus alternative approaches are being sought.
In one approach, nematode trapping fungi are placed into nutrient-rich faeces. As nematodes hatch, they become colonised by the fungus. The fungus Duddingtonia has been inoculated to the dung via a drench to the animal. Sclerotia of the fungus pass through the intestine, hatch in moist dung, and effectively reduce larval densities IN SOME CASES. While further research is needed before commercial adaptation, this is one example where understanding of the biology of fungi, lateral thinking and some judicious research, has led to the development of a new approach to a serious problem.
Biocontrol of nematodes by fungi appeared to be possible when nematode trapping fungi were first discovered. However, it has taken some considerable time before their potential could be realised. Successful control relies on having sufficiently high densities of fungus where high densities of the pest are maintained. The requirement for high densities of agent to control the pest applies across all biocontrol efforts.
Butt TM, Jackson CW & Magan N 2001 Fungi as Biocontrol Agents. CABI.
Mittal N, Saxeena G, & Mukerji KG. (1999) Biological Control of root-knot nematode-destroying fungi. In: Singh J, & Aneja KR. Eds. From Ethnomycology to Fungal Biotechnology. Kluwer, pp 163 – 171.
Rye grass is grown widely in Australia and New Zealand, where it is a common pasture component. The use of rye grass in pastures, especially the annual Lolium rigidum, is of some concern because of its association with the bacterium Rathayibacter (Clavibacter) toxicus. The bacterium causes Annual Rye Grass Toxicity (ARGT), a condition where toxins from the bacterium cause serious toxicosis in animals grazing infested rye grass. The fungus Dilophospora alopecuri is a minor pathogen of grasses, including rye grass. The presence of Dilophospora alopecuri is correlated with an absence of ARGT.
Anguina funesta is a nematode that feeds from the stems, leaves or flowers of some grasses. Larvae hatch from the galls as seed germinates next season. Larvae are attracted to the seedlings. In the case of ARGT, larvae attach themselves to ryegrass seedlings, moving up the stem in a water film as it elongates. The larvae then form galls in place of the seed. The galls, in the absence of other factors, fall to the ground where the eggs mature and then hatch, completing the life cycle.
Anguina funesta carries the bacterium Clavibacter toxicus and the fungus Dilophospora alopecuri on its cuticle. As the nematode penetrates the plant, it can take with it either the fungus or the bacterium. Conidia of the fungus attach to the cuticle of the nematode by means of extracellular appendages. The mechanisms of competition between bacterium and fungus are unclear.
McKay A. C. & Ophel K.M. (1993)Toxigenic Clavibacter / Anguina associations infecting grass seedheads. Annual Review of Phytopathology 31: 153 – 169.
Riley I.T. (1994) Dilophospora alopecuri and decline in annual ryegrass toxicity in WA. Australian Journal of Agricultural Research 45: 841 – 850.
Riley et al (1998) Allozyme analysis of Australian isolates of Dilophospora alopecuri . Mycological Research 102: 301 – 307.
Robson GD, van West P & Gadd GM (eds) 2007. Exploitation of Fungi. CUP