Iatrogenic Plant Diseases

Abstract

Crop production is essentially an exercise in the management of artificial plant ecosystems. The objective is to achieve, subject only to commercial constraints, the maximum yield of valuable produce per unit area of land. Optimum conditions for growth and development of the crop should be provided. These include freedom from competing plant species (weeds) and from pests and diseases. Control of weeds, pests, and diseases-crop protec­ tion-is thus an important element in crop production systems and One which frequently depends On the use of agrochemicals-herbicides, growth regulators, insecticides, fungicides, etc. These are, of necessity, compounds with high biological activity which, despite careful selection, tend to have some effects on nontarget organisms or physiological processes. Undesirable side effects are common and these include the induction of new diseases or, more usually, the exacerbation of diseases already present. Diseases which result from or are increased in severity by the use of a specific crop protection chemical may be referred to as iatrogenic, a term derived from human medicine, but, as indicated by Horsfall (46), equally applicable in plant pathology. Only brief reviews of iatrogenic disease in plants have hitherto been published (38, 47). Examples of such diseases are commOn but information frequently is limited to the observation of enhanced disease with a particular treatment. In some instances, however, sufficiently detailed studies have been undertaken to reveal the probable mechanism or mechanisms in­ volved. This review is restricted to these examples. They may be divided into three broad groups (38) according to whether disease is induced or exacerbated through action of agrochemicals On (a) the host plant, (b) the pathogen, or (c) the ecosystem in which host and pathogen coexist.