Just as the biodiversity of the planet is becoming more and more reduced, scientists and the general public are becoming more aware of its importance in "practical" terms. Microbial biodiversity is little known but even the most "humble" soil may contain strains of microorganisms with interesting antagonistic properties. In the past, the use of antagonists for biocontrol purposes was restricted to the culturability of the putative organisms detected. Nowadays, in the post-genomic era, our capabilitites of handling genetic material of organisms allow the detection of microorganisms or their metabolites in the environment. The classical approach for finding antagonists has been to search for them in places where their target did not cause damage or disease in spite of host presence. Clear examples of these places are suppressive soils.
The most studied sources for fungal antagonists of nematodes are soils naturally suppressive to plant-parasitic nematodes. The first example to be discovered was the decline of Heterodera avenae populations in cereal monocultures. Using previous data from nematode population dynamics and with a stepwise approach, Kerry (1987) and co-workers found fungal parasites of females (Nematophthora gynophila) and eggs (P. chlamydosporia) to be the biotic causes for nematode suppression. Similar situations have been found elsewhere in the world with soils suppressive to cyst nematodes. Root-knot nematodes have a much wider host range than cyst nematodes. Besides, many crop hosts of root knot nematodes are grown under intensive agricultural conditions with regular use of agrochemicals. Under these conditions soil antagonists are not favoured. Therefore, fewer examples of natural suppressive soils to root-knot nematodes have been found. Among these are tree orchards in the United States, where the egg-parasitic nematophagous fungus Dactylella oviparasitica was first isolated (Stirling and Mankau 1978). In a similar environment, ring nematodes (Criconemella spp.) were found to be suppressed by Hirsutella rhossiliensis, an endoparasitic nematophagous fungus (Jaffee and Zehr 1982). Both antagonists are examples of organisms with an important ecological role but difficult to use biotechnologically. Both fungi have a limited growth rate and sporulation. Soil suppresiveness to nematodes is a very complex phenomenon. In a study to find causes for nematode decline, Persmark et al. (1995) did not find differences in nematophagous fungi populations between putative suppressive and conducive soils in Central America.
Searching for nematophagous fungi in agroecosystem soils have yielded fungal antagonists with potential as BCAs of plant-parasitic nematodes. For instance, cyst nematodes were infected by the entomopathogenic species Lecanicillium lecanii (syn. Verticillium lecanii) in the United States (Meyer et al. 1990). In other instances, fungal antagonists have remained undescribed and their use halted for lack of sporulation, e.g., ARF-18, a sterile fungus infecting cyst nematodes in the United States (Kim and Riggs 1991). The search for nematophagous fungi in natural "undisturbed" ecosystems is another approach for finding antagonistic potential, e.g., the Antarctic (Gray 1982) or the tropical rain forest. Costa Rica still offers protected areas where such studies can be carried out. Our laboratory is presently involved in such a survey.
Another approach of using fungi for nematode control is to identify nematophagous metabolites. These substances can be identified from nonpreviously described nematophagous fungi, e.g., cuticular disruption of Caenorhabditis elegans by Byssochlamys nivea (Park et al. 2001). A search for nematicial molecules from Arthrobotrys and other fungi was carried out by Anke et al. (1995). In the future, genes (e.g., toxins, enzymes) can be a source of antagonistic potential against nematodes, which can be used for improvement of BCAs.
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