Relevance of plant root pathogens to soil biological fertility

Abstract

In this chapter we use the term soil biological fertility to describe the ability of soil biota to perform various (1) plant essential functions to support the growing plant with its nutritional and other biological requirements, and (2) ecosystem functions that maintain the quality of soil resource. A number of soil functions essential for plant growth and crop productivity are regulated by different groups of biota. These include (i) mineralisation and uptake of major nutrients (e.g. N, P and S) and trace elements (e.g. Zn), (ii) beneficial, pathogenic and associative interactions affecting root and shoot growth, (iii) degradation of chemicals harmful for plant growth (e.g. herbicides from a previous cropping season), and (iv) formation of soil structural components that provide optimal aeration and water-filled pore space for plant growth. A unique balance between the three components of a soil system, i.e. physical, chemical and biological, is necessary for long-term sustainability of crop production, soil health and other essential ecosystem functions. Soil biota regulate processes that impact on the physical and chemical properties of soil and conversely the physical and chemical attributes of soil greatly influence the populations and activities of soil biota. The optimum functioning of the biological components of soil requires both a suitable habitat (pH, habitable pore space, oxygen concentration etc.) and optimum environmental conditions (temperature, moisture level etc.). For example, the activities of different groups of soil biota have important roles in various components of soil structure i.e. the burrowing activities of macrofauna influencing soil pore structure (Lee and Foster 1991) and binding and entanglement of soil particles by microflora (including pathogenic fungi) in the aggregate formation and stabilisation (Gupta and Germida 1988, Tisdall 1991, Tisdall et al. 1997). Conversely stable aggregates are an important component of soil structure for maintaining aeration and porosity for favourable microbial growth including that of plant pathogenic fungi. These activities may affect the physical and chemical properties, which are known to determine suppressiveness of soils to plant root diseases (Hoper and Alabouvette 1996). Management practices involving surface retention of crop residues are recommended for improving soil organic matter, soil structure and biota populations. However, they can result in providing food source for the survival of some pathogenic microorganisms especially during off-season. In addition, the surface retention of residues has increased the potential for the movement of residues around the farm and across farms and might have increased the carryover of soilborne root pathogens (Neate 1994, Allen and Lonergan 1998). Allen (2000) summarised the trends in diseases in Australian cotton based on 17 years of survey data on disease incidence and severity. Results show that there has been a steady decline in the mean seedling mortality during the 10-year period ending in 1998, i.e. from 50% mean seeding mortality in 1987/88 to <30% seedling mortality in 1997/98. The incidence of diseases such as bacterial blight (Xanthomonas campestris) and verticillium wilt (Verticillium dahliae) has declined whereas diseases such as black root rot (Thielaviopsis basicola) and fusarium blight (Fusarium oxysporum f.sp. vasinfectum) have become a severe threat to the sustainability of Australian cotton industry. Presence of the inoculum of a pathogen does not necessarily result in the outbreak of the disease and the severity of the disease is ultimately determined by the environmental conditions. Irrigated wheat often succumbs to pathogens different to rainfed crops. In the Pacific northwest (Cook and Baker 1983) fusarium crown rot dominated rainfed crop while take-all (caused by Gaeumannomyces graminis var. tritici) was the major problem in irrigated wheat. Fusarium dominates in soils that are relatively dry and with relatively low microbial (mainly bacterial) activity, while the take-all fungus although not as saprophytically competent as Fusarium spp., is active in moist soils. The extent of threat from major pathogens varies with crops and regions of Australia (Murray and Brennan 2001, unpublished). For example, Fusarium crown rot of wheat, especially that caused by F. pseudograminearum, is predominantly a problem in the north and central cropping regions of Australia, while the crops in the south central wheat growing region are more severely affected by pathogens such as Rhizoctonia solani AG8 and take-all fungus (Gaeumannomyces graminis var. tritici). © 2007 Springer.