Root diseases

Abstract

Underground parts of plants live in a much more complex biological medium than shoots in that soil normally contains large and fluctuating numbers of fungi, bacteria, actinomycetes, Protozoa, nematodes, insects and other organisms which interact and are affected by its chemical and physical conditions. Natural soil in which plants grow is in a dynamic state, and such agricultural operations as ploughing, addition of fertilizers and manure, application of crop protection chemicals, burning of crop residues, irrigation and indeed any procedure which alters its chemical composition, structure, moisture content or pH are likely to disturb to varying extents its microbiological equilibrium. As Park (1963) points out, the fairly simple host—pathogen (H—P) interaction which commonly obtains with air borne pathogens is rarely encountered in root diseases — a triple interaction between host, pathogen and soil microbial population (S) being the rule. Each of these three components is affected by such environmental factors as temperature and moisture. Although a straight H—P interaction is probably an over-simplification for air borne pathogens, in that the activity of pathogens on shoot surfaces is influenced by nearby epiphytic microorganisms, this effect is much greater in the case of root pathogens surrounded as they are by the numerous other inhabitants of the soil and root surface.