Читать книгу Plant Pathology and Plant Pathogens - John A. Lucas - Страница 23

The relationship between the amount of disease and loss of income is complex due to the many possible interactions between symptoms of disease and the final determinants of crop yield and quality. Figure 1.8 defines a number of yield levels for a hypothetical crop. The theoretical maximum yield is a value based on predictions from crop physiology; under field conditions, this yield level is not a practical possibility and hence there is some unavoidable loss. Attainable yield indicates the maximum level to be expected under optimum conditions in the field. With optimum inputs of fertilizer, water, and pesticides, this is the best yield the farmer can realistically hope for. The difference between this value and the actual yield (also described as farmer yield) obtained from the crop can be defined as an avoidable loss. In practice, attainable yield is not a realistic goal for most crops for simple economic reasons. To increase yield to this level requires so many inputs that the cost is greater than the eventual return at the end of the season. Instead, we can define a slightly lower threshold, the economic yield, which represents the break‐even point at which the input cost is balanced by the extra productivity of the crop. Any shortfall below this level is an avoidable loss which justifies the expense of a control measure.

Figure 1.8 Relationship between yield levels and crop loss, indicating economic benefits of control.

Source: Zadoks and Schien (1979).

For some crops, especially high‐value fruits, vines, vegetables or ornamental plants, the quality of the product is as important as the yield. Under these circumstances, very little disease is tolerated, as any damage or blemish may have a disproportionate effect on crop value. Not surprisingly, the most intensive disease and pest control regimes available are used for such crops.