Fall 1994 (v6n4)
	The role of soil microfauna in plant-disease suppression. Elroy A. Curl CRC Critical Reviews in Plant Sciences 7(3):175-196. 1988 The role soil microfauna play in the cycling of nutrients in ecosystems has been recognized for many years (see previous article, Verhoef and Brussaard, 1990). This review article (123 references) broadens our view of the soil community by examining the impact soil microfauna have on plant diseases. The concept of suppressive soils was described by Baker and Cook (1 9 74) in their first book on the biological control of plant pathogens. This treatise described suppressiveness as the ability of a soil to prevent, truncate or delay the development of a disease even though favorable environmental conditions exist. Further research has attempted to explain the actual mechanisms of disease suppression. These studies have looked at the influences of various microbial antagonists, soil fungistasis (inhibition of fungal growth), and physical and chemical properties of soils. Investigations into the role of soil fauna in this process are relatively new. From the start, this article argues that the effect of soil fauna on plant growth must be examined in the context of the whole microbiotic community. The author, therefore, puts considerable effort into describing the members of this community, both flora and fauna, that dwell in the rhizosphere. A brief section highlights the soil microflora: bacteria, actinomycetes, and the fungi. More detail is provided on the microfauna. This diverse group includes the protozoa, nematodes, and some arthropods, primarily mites and springtails (collembola). Organic Matter Decomposition In keeping with its holistic emphasis, the article examines the relationship between flora and fauna in organic matter decomposition and nutrient cycling before looking narrowly at plant disease. The author reminds us that as nematodes and protozoans feed upon microbial populations, they consequently will affect organic matter decomposition. It is likely that such feeding ultimately liberates nutrients immobilized in microbial cells or reduces competition between microorganisms so that mineralization is actually accelerated. " These activities not only influence the general nutrition, health, and vigor of higher plants (which relates to disease susceptibility), but also determine t e competitive behavior of root-infecting fungi and their microbial antagonists." These indirect and direct impacts are important ones for further study. The role of microarthropods in mineralization is also discussed. Mites and collembola are known to fragment organic matter as they feed on soil microflora. This fragmentation to finer particles creates new surface areas for microbial colonization and consequently speeds the decomposition and mineralization processes. Along with mineralization effects, this section of the article describes several other interesting features of the microbiotic community, including increased nitrogen fixation by nitrogen-fixing bacteria (Azotobacter spp.) where protozoa are present, and the behavior of bacterialand fungal-feeding nematodes. Rhizosphere Effects The rhizosphere is the soil zone surrounding and influenced by plant roots. To complete the picture of the microbiotic community, the author examines the specific characteristics of the rhizosphere that affect microflora and fauna. The primary influences in this zone are root exudates and the chemical and physical features of the root-soil interface. Root exudates and sloughed root cells in the rhizosphere provide a rich carbon source for microbes. As the microbial population increases, so does the activity of microbe-grazing fauna, leading eventually to the release of inorganic nitrogen and phosphorus. Root exudates can also have a strong influence on soil pathogens. "The nutrients in exudates release dormant spores and sclerotia from the grip of soil fungistasis, a natural phenomenon imposed by chemical inhibitors and microbial competition for nutrients. Propagules fteed from fungistasis germinate and establish infection if they are not first consumed by mycophagous [fungus-eating] animals. Root exudates are also important in the activation of quiescent nematode stages and in the movement of plant parasitic forms to infection sites." Specific Soil Fauna-Plant Disease Relationships This analysis of the soil fauna and the author's description of the niche the fauna have in the rhizosphere are sufficient to indicate that soil fauna do regulate pathogen populations to some degree. Investigations since 1975 have given some detail to the actual mechanisms involved and highlight some interesting relationships. These are summarized in table 1. Conclusions The studies reviewed in this paper point to the fact that the soil microfauna play an important role in suppression of plant pathogens and represent a significant biological control potential. However, there has not been adequate field testing to verify if this biocontrol potential can be exploited in cropping systems. In addition, any evaluation of a candidate organism must include the possibility of adverse effects upon plants, animals, or the environment. A problem with the fungal-feeding amoebae, for example, maybe their effect on other beneficial organisms such as the mycorrhizal fungi. "In the final analysis, efforts to manipulate and exploit the friendly fauna populations for crop benefit must be compatible with microbial symbionts, and other plant-growth promoting rhizosphere organisms, and with fungi and bacteria that are being promoted for biological control of diseases." This is clearly an area with great opportunities for further research. For more information write to: E. Curl, Dept of Plant Pathology, Auburn University, Auburn, AL 36849. (DEC.318) Contributed by David Chaney Table 1. Summary of biological control activities of soil micro-and meso- fauna. Biocontrol Agent Action Amoebae At least 5 different species Conidiall perforations and/or hyphal feeding on numerous species of fungi including Cochliobolus sativus, Thielaviopsis basicola, Fusarium oxysporum F. sp. melonis, F. roseum, F. solani, Verticillium dahliae, Gaeumannomyces graminis, and Phytophthora cinnamomi. Also may feed on bacteria, flagellates, blue green algae, diatoms, and nematodes. Nematodes Numerous species from 7 genera Feed on various species of Rhizoctonia, Aternaria, Pyrenochaeta, Botrytis, Fusarium, and Agaricus. Microarthropods Mites Fungal feeders, but probably play a minor role in disease suppression. Collembola Two species found to feed extensively on several plant pathogenic fungi including Rhizoctonia solani, and Fusarium oxysporum. Show marked preference for particular types of fungi. [ Back | Search | Feedback ]