Tasvina R. Borah Scientist, Plant Pathology The soil is a living system that represents a finite resource vital to life on earth. It forms the thin unconsolidated mineral and organic matter on earth surface and develops slowly from various parent materials and is modified by time, climate, macro- and micro-organisms, vegetation and topography. It is a complex mixture of minerals, organic compounds and living organisms that interact continuously in response to natural and imposed biological, chemical and physical changes. Besides anchoring the roots, the soil is a dynamic system that serves as natural medium of growth for land plants and is inhabited by a wide range of microorganisms (SSSA, 9184). Rhizosphere is an environment that the plant itself helps to create and where pathogenic and beneficial microorganisms constitute a major influential force on plant growth and health. Microbial inhabitants consist of autotrophs, saprophytes, mutualists, parasites of plants and antagonists of other soil microbes. Growth of plants and the soil microorganisms may be influenced by both direct stimulation and indirectly through a variety of interactions. Plant pathogens are a component of these systems with their impact dependent on many interacting factors, including plant species population dynamics and community composition. Plant pathogens are affected by these interactions and also act reciprocally by modifying their nature. The soil fungi: Soil fungi are a diverse group of eukaryotic organisms that encompass economically important root-infecting plant pathogens, beneficial mycorrhiza, root endophytes, nematode-trapping fungi and mycoparasites. As a consequence of their saprotrophic activities, soil fungi are the principal degraders of biomass in terrestrial ecosystems and are responsible for much of the organic re-cycling in the environment. As a group of organisms, they are arguably the most difficult to study since their natural habitat, soil, is chemically and biologically complex and remains largely uncharacterized. Close associations of fungi with other soil-borne micro-organisms such as bacteria, actinomycetes and oomycetes presents a significant challenge for detection and quantification of individual genera or species and their often copious production of asexual conidia causes significant restrictions in the accurate quantification of population dynamics. Soil borne fungi and their economic importance: In most agricultural ecosystems, soilborne plant pathogens can be a major limitation in the production of marketable yields. They are also more recalcitrant to management and control compared to pathogens that attack the above-ground portions of the plant. Soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the infection court where the pathogen encounters the plant and establishes a parasitic relationship. This is also where the complex rhizosphere community, both microflora and microfauna, can interact with the pathogen and influence the outcome of pathogen infection. There are four main groups of plant pathogens, but only two of them are major players in the soil: fungi (true fungi and oomycetes) and nematodes. Only a few groups of bacteria are considered to be soilborne, probably because non-spore forming bacteria cannot survive well in soil for long periods. Environmental conditions in the soil are generally not favorable for fungal growth, due to high or low temperatures (frozen ground) or extremely dry conditions. Pathogens survive in the soil as resistant propagules, such as chlamydospores, sclerotia, thick-walled conidia or hyphae, or survive in plant roots and crop residues. Rhizoctonia solani, Sclerotinia sclerotiorum, Sclerotium rolfsii etc. are the sclerotia forming fungi. When conditions are favorable and when a seed or root approaches the dormant propagule, the fungus is stimulated to germinate by root or seed exudates and chemotactically grows toward the plant and cause infections. Soilborne pathogens can cause seed decay, damping-off (both pre- and post-emergence), and can also move into the base of the stem, causing crown rot and wilt. In perennial trees, fungi can move into the collar of the tree, girdling the tree, or inoculum can splash onto the fruit, causing decay and rot. Losses from soilborne pathogens are even more difficult to estimate, because of the difficulty of diagnosis. Some estimate that soilborne pathogens cause 50% of the crop loss in the US. Spatial aspects of soil fungi and root diseases: Soilborne pathogens not only spread through time but also through space. Some soilborne pathogens in infected crop debris or soil can be spread by wind that blows during harvesting or cultivation. Some soilborne pathogens, such as Sclerotinia sclerotiorum or Rhizoctonia solani, produce aerial sexual spores that are ejected into the air and spread by wind. Pathogens can move above ground with irrigation water or rain run-off, which can carry soil particles into adjacent fields. Influence of agricultural practices on soil fungi: Management of the biotic and abiotic properties of a soil is an important approach to promote the activities of beneficial microorganisms in the rhizosphere and thus limiting the densities and activities of soilborne pathogens to a tolerable level. Adaptation of cultural practices has been proposed as a means to decrease the soil inoculum potential or increase the level of suppressiveness to diseases. Indeed, disease suppressiveness has been obtained through crop rotation, intercropping, residue destruction, organic amendments, tillage management practices and a combination of those regimes.
