Two Kinds of Host Damage

The frequency of parasite damage is the proportion of the host population that is parasitised. The injury from parasites is the amount of damage inflicted on each host individual, normally expressed as an average. The total parasite damage is the product of frequency and injury.

In a wild plant pathosystem, the total parasite damage is controlled very effectively. This control ensures that the parasite does not impair either the ecological or the evolutionary ability of the host to compete. This control is fundamental to the entire concept of parasitism, which is neither competition nor cooperation between host and parasite. This pathosystem control is the result of self-organisation, which produces biological order.

In the crop pathosystem, this balance has been damaged. Indeed, in many crop pathosystems, it has been lost entirely. This is demonstrated by the fact that, without crop protection chemicals, the crop loss from parasitism can often be total. The principle message of this book, and the main function of self-organised crop improvement, is to restore the balance in our crop pathosystems, without damaging their essential agricultural functions.

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