A wild host species would presumably exhibit a normal distribution of horizontal resistance. Each ecotype would possess the level of resistance necessary to balance the epidemiological competence of the parasite in that part of the ecosystem (see 6.4).
This assumption has important agricultural implications. Wild plants are consistently more resistant that cultivars, and yet we must assume that most wild plants have only the modal level of horizontal resistance. Given a normal distribution, this modal level is about half of the available resistance. The implication is that horizontal resistance can be domesticated, in the same way that both the yield, and the quality of product, of cultivars were domesticated far beyond their original levels in wild plants. If this assumption is correct, it is clear that there would be little difficulty in accumulating enough horizontal resistance to provide a virtually complete control of parasites in crops.
The evidence available for this assumption is mainly theoretical. Factual evidence is almost entirely lacking because there has been so little study of wild plant pathosystems, and so little study of horizontal resistance. Clearly, such studies are urgently required, and they would make excellent research projects for graduate students.
However, there is some factual evidence from agriculture. Before the industrial revolution, draught animals, such as horses or oxen, were used for ploughing and drawing carts. But all other work was done by hand. Furthermore, fertilisers were limited to farmyard manure, and there was never enough. These various factors meant that yields were generally rather low. The control of crop parasites was limited to rotation and, perhaps, the burning of crop residues. However, centuries, indeed, millennia, of selection by farmers had produced local landraces that must have had adequate resistance to all the locally important parasites. These crops were grown successfully and economically, without any crop protection chemicals. These cultivars are now known as 'heritage' or 'heirloom' seeds, and they are in demand among organic farmers. Had these heritage seeds been as susceptible as many modern cultivars, for which crop protection chemicals are essential, the world would have starved.
With the advent of modern agriculture, yields have been increased very considerably by mechanisation, by the use of artificial fertilisers and synthetic herbicides, insecticides, and fungicides, as well as by the development of modern high-yielding cultivars. Tragically, these increases have also led to a loss of horizontal resistance to crop parasites, because of inappropriate plant breeding in which there was negative selection pressure for horizontal resistance (see 5.5.4).
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