Many organic farmers believe that well-nourished plants have more resistance to crop parasites than poorly nourished plants. However, this is a misconception. Plant nutrition is a physiological phenomenon, and plant resistance to parasites is a genetic phenomenon. These two phenomena are independent of each other. While well-nourished plants will grow better, and will probably both taste better, and provide better human nourishment, they are not necessarily more resistant to their parasites, and physiological nourishment has little effect on genetic resistance.
Organic farming can reduce the incidence of pests and diseases by restoring natural controls that were lost with the advent of conventional farming. These include biological controls (see 6.7), an improved soil microbiological activity, greater biodiversity, and a reduced host population density. But a reduced incidence of parasitism is not necessarily due to an increased resistance to parasitism.
It must be appreciated that organic farmers are usually able to escape serious outbreaks of pests and diseases only because their farmer neighbours are keeping parasite populations down by using crop protection chemicals. This results in a decreased incidence of parasitism due to epidemiological isolation (see 6.10.5). If many conventional farmers were to change to organic farming, the overall parasite populations would increase dramatically, and the losses from crop parasites would become prohibitive. Crop parasites set an absolute ceiling to the amount of organic farming that is possible with existing cultivars. Consequently, further increases in the amount of organic farming will eventually require major increases in the levels of resistance to parasites. In their turn, these increases in resistance require plant breeding for horizontal resistance.
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