Wheat yields in 13th Century Europe have been estimated at 385 kg ha-1 (1 & 2), more than half a millennium later, by 1939 they had been increased to little more than 2 t ha-1. Subsequently, in the period 1952-1986 scientific and technologically based innovation applied to farming increased yields by an average value of 2.6 % pa. It is predicted that wheat yields will rise to 10.48-13.69 t ha-1 by 2015, with a current theoretical biological ceiling of 19.2 t ha-1 (3). These rising yields have been accompanied by a 10-fold increase in the amount of nitrogen applied to wheat in 1943/1945-1994 (2). They represent one example of what has been achieved by a combination of genotype (plant breeding) and environmental modification increasing the nutrients available to the plant in step with its physiological demands and fending-off consequential invasions by pests and pathogens attendant on rapid growth and high yields. Burgeoning human populations, reaching an estimated 10 billion by 2050, demand that similar yield increases are continued and accelerated. The environmental and climatic consequences arising from a policy of raising yield solely by increasing inputs are becoming apparent as dangerously unsustainable. Dramatic changes in the manner by which crops are husbanded are required with the aim of achieving increased yield but with minimal damage to the World’s ecosystems (4). Concomitant with the need to continue and accelerate yield increases in order to fill empty stomachs and enhance lifestyles far greater control of pests and pathogens using intelligent, integrated and environmentally benign methods based on sound scientific knowledge of the intimate relationships of beneficial microbes, roots, soil and pest and pathogen biology is required for the health of our Planet’s environment and biodiversity. So far, our knowledge of soil microbiology is at best scant and fragmentary and at worst non-existent. Yet as shown in this Book the agronomic opportunities for enhanced productivity offered by the soil and its inhabitants are huge.
Soils contain very substantial numbers of phytopathogenic microbes capable of devastating crops worldwide. These include representatives of the bacteria, fungi, mycoplasma, protista and viruses. Some pathogens have very limited and highly specific host ranges while others are generalists, to a lesser or greater degree, causing diseases across many host taxa. Similarly, their geographical ranges may be restricted or alternatively spread around the entire globe. The intensity of pathogenesis varies also from those which devastate crops by ultimately killing their hosts in the processes of colonisation and reproduction to those which are only mildly aggressive and possibly almost commensalists. The forms of disease syndrome generated in crop hosts vary from simple root invasion and rotting through to altered root growth e.g. root galling and clubbing, to collar rotting and damping-off into vascular wilting and colonisation throughout all the aerial organs of the plant. Some pathogens invade early in the host life-cycle and may then enter a quiescent or dormant phase from which they re-emerge devastating maturing flowers and fruit or destroying products stored after harvesting from the crop. Indeed some pathogens are only apparent from the toxins which they elicit in stored products.
The extensive range of pathogenic lifestyles available to soil-borne microbes increases the problems facing plant pathologists striving to unravel their biology. These difficulties are compounded by the sheer physical obscurity resulting from dwelling either wholly or at least partially in soil. Attempting to ascribe taxonomic identity and to understand the life-cycle biology of organisms that conduct all or part of their life-cycles and subsequently are enclosed in the bodies of their host plants is a complex and at times confusing task. Adding to these problems are barriers in understanding the physiology and metabolism of host roots which are the primary targets for invasion by soil-borne pathogens. Roots possess a modular structure as described by Hodge (5) permitting responses to their soil environment and adaptation to changes, some of which are a result of the presence of pathogenic microbes. In achieving adaptation the roots differentiate between adopting partnership modes with benign microbes which enhance the potential efficiency for resource capture (see other Chapters in this volume) and adopting defensive modes as a reaction to the presence of pathogens. The root-cap region appears to be the main environmental sensing and response control centre. Recent research indicates apparent root-to-root interactions and the capacity for recognising ‘self’ and non-self’ roots (6). Root exudates possibly form one means for positive communication with benign microbes and negative responses to pathogens (7).
Developing husbandries which are both economically and environmentally sustainable demands a thorough understanding of the impact and interactions between host roots, benign microbes and pathogenic organisms. Pathogens are capable of wreaking either rapid crop destruction or causing the long-term degeneration and decline of hosts possibly without their presence being easily obvious. This Chapter discusses the abilities of pathogenic microbes to cause diseases as moderated by the soil environment and influenced by husbandry practices. Soil-borne microbial pathogens are well evolved and efficiently fitted for their ecological niches, for example as described by Dixon (8) for Plasmodiophora brassicae, the causal agent of Clubroot Disease in the Brassicaceae. Ecologically soil-borne microbes exploit their edaphic and in planta environments with high degrees of effectiveness. Their traits of pathogenic success are indicative of organisms with long established and well tested lifestyles. Herein is one of the major mysteries associated with these crop pathogens. The pathogenic microbe is frequently found only sporadically within natural ecosystems or may be apparently entirely absent. They appear to have evolved their current lifestyles in parallel with their hosts as these were domesticated by man.
Devastating disease epidemics appear to result from or be associated with extensive and intensive mono-cropping of vulnerable host genotypes. In this context the pathogens cause “iatrogenic diseases of cultivation”. The consequence of this status for sustainable crop production demands environmentally balanced disease control achieved by using husbandry systems which are compatible with host growth and productivity and the maintenance of benign microbe populations which benefit the host. This means integrating husbandry, biological, chemical, genetical and legislative techniques into logical and coherent strategies. Acceptance of this approach reflects changing agronomic attitudes driven at least partially by the withdrawal of toxic chemical agents such as the sterilant methyl bromide (9). Considerable re-appraisal of the importance of cultural controls and the needs for crop protection strategies using integrated systems aiming to stimulate host growth and reproduction, encourage soil health and repress pathogens result from this approach
