Crop and Wild Plant Pathosystems Compared

There are two basic categories of plant pathosystem. The wild pathosystem is a fully autonomous subsystem of a natural ecosystem, in which people have not interfered. It is a balanced and dynamically stable system. This is axiomatic because, were it not balanced and dynamically stable, it could not have survived ecological and evolutionary competition until the present. The primary components of the wild pathosystem are the host, the parasite, and the environment. The wild plant pathosystem is a complex adaptive system. It is self-organising, stable, and resilient, with many homeostatic mechanisms based on negative feedback.

The crop pathosystem differs in that it has the fourth component of human control. People have changed the host species genetically, first by domestication and, more recently, by plant breeding and genetic engineering. We have also altered the host population by cultivating it as a single stand of only one species. And we usually cultivate these single stands as genetically uniform clones, pure lines, or hybrid varieties. We have altered the environment by numerous agricultural practices such as clearing, ploughing, weeding, irrigation, and fertilising. And we have altered the parasite population by exposing it to some very abnormal selection pressures that would never occur in a wild pathosystem.

Consequently, the crop pathosystem is markedly different from the wild pathosystem, and it is often an unstable system, as we know to our recurring dismay. Even more serious, many of our crops could not survive without the protection of fungicides and insecticides. This is in spite of many decades of breeding these crops for resistance to their parasites. Indeed, the use of crop protection chemicals has been increasing dramatically in recent years, and this is a measure of our failure to breed our crops for resistance to their parasites.

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