ABSTRACT
Soilborne plant pathogens (SBPPs) are organisms that penetate the roots or other underground plant parts, invade the vascular systems and other aboveground parts of the plant, and finally lead to the disease syndrome. Even in heavily infested soils, SBPPs constitute a very small fraction of the total population of soil organisms (Curl, 1986). For example, inoculum density (ID) of Verticillium in naturally infested field soils may be 0.2 microsclerotia per gram soil, or even less (Ben-Yephet and Szmulewich, 1985). The density of soil inoculum of V. dahliae ranged from 1.9 to 4.9 propagules per gram in 1 year and from 0.2 to 17.7 for the second year (Nicot and Rouse, 1987). Populations of Macrophomina phaseolina in soil were in the range of 2-16 sclerotia per gram soil in one study (Mihail and Alcorn, 1987) and in the range of 0.64-7.7 sclerotia per gram in another study (Van der Gaaf, 1993). In spite of the extremely low frequency, the impact of SBPPs on the growth of plants can be enormous. In certain cases they lead to complete destruction of crop plants and to total loss of yield. As a result of recurrent cropping of susceptible plants, there is a buildup of SBPP inoculum in agricultural production systems, resulting in a subsequent increase in disease incidence. In natural habitats, the ID of a SBPPs is much lower, their populations are hardly detectable, and disease outbreaks are rare. Thus, the outbreak of diseases caused by SBPPs, namely, a harmful interaction of roots with microorganisms, is an unusual magnification of a natural phenomenon which is enhanced by agricultural practices. In contrast, interactions of roots with beneficial microorganisms such as nitrogen-fixing bacteria or mycorrhizae are much more frequent in nature (see Chapters 50 by Kottke, 49 by Sieber, 47 by Vance, and 48 by Kapulnik and Okon in this volume). Infested soils may become unsuitable for cultivation of certain crops. This occurrence is typical for monoculture agricultural systems (Schippers et al., 1987). It should be noted that, in contrast to foliar pathogens, reports on occurrence of SBPPs in undisturbed soils or on plants growing in the wild are rare.
