is usually based on likely profitability, rather than resistance to pests or pathogens. Unless a financial return is guaranteed, control measures may be ignored or reduced in scale. As a consequence, the significance of disease, as perceived by the grower, will depend to a large extent on the market value of the crop. Inputs of chemicals or other actions designed to reduce disease are only justified when the likely impact on yield or quality will outweigh the cost of the measure. Even a possible bonus, such as restricted carry‐over of the pathogen to the following season, may not provide sufficient incentive for any financial outlay.
Other parties with an interest in crop diseases are government advisory or extension pathologists, consultants, and representatives of the agrochemical industry. The relative resistance of new crop varieties to pathogens and the efficacy of commercial formulations of pesticides are assessed by advisory scientists under field conditions; recommendations for use may be based on these field trials. Independent consultants offer growers an overall package of advice for crop management, part of which concerns disease. Agrochemical companies provide information on the performance of their crop protection products, whilst government agencies regulating use of chemicals in the field issue guidelines and can impose restrictions on pesticide application. Nowadays, such advice includes strategies to reduce the risk of resistance developing to different pesticide classes. In recent years, more stringent legislation on the registration and use of agrochemicals, especially in Europe, has led to withdrawal of many crop protection products, and also affected the availability of new pesticides. Hence decisions on disease and pest control are influenced by many factors and often involve compromises driven by economic considerations or the regulatory system.
Crop Yield and Quality
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.
The Impact of Disease
The impact of plant disease depends on agricultural, biological, socioeconomic, and historical factors. In developed agricultural systems, impacts are usually measured in terms of reduced crop yield and quality, and overall effects on farm profitability. In less developed systems, the consequences of disease can be much more serious, affecting food security, regional and national economies, and social stability. Invasive pests and pathogens can also destroy native trees and alter natural ecosystems, as well as impacting on biodiversity (Table 1.4).
Some examples of these impacts and the pathogens responsible are given in Table 1.5. Prior to understanding of the germ theory of disease, and discovery of chemicals and other means of controlling epidemics, disease outbreaks could devastate crops and consequently cause famine and social disruption. It is believed that a plant disease, most likely a virus, contributed to the collapse of the ancient Mayan civilization in the ninth century. But the most notorious case is the potato late blight outbreak of 1845–1846, caused by the oomycete pathogen Phytophthora infestans, that spread rapidly across Europe and had particularly disastrous consequences in Ireland, where many communities were dependent on potatoes as their sole source of food. Around 1 million people died of starvation and countless others were displaced, many emigrating to the USA. Fortunately, with diversification of food sources, and improved crop protection, this scenario is now much less likely to be repeated in developed countries, but in subsistence agriculture is still a constant threat.
Table 1.4 Some impacts of plant disease
| Developed agriculture |
| Reduced crop yield |
| Reduced crop quality |
| Compromised product safety, e.g., mycotoxin contamination |
| Reduced profitability |
| Developing agriculture |
| Food security – malnutrition and famine |
| Impact on communities or national economies |
| Social instability |
| The natural environment |
| Loss of key species or natural communities |
| Damage to landscapes and leisure amenities |
Table 1.5 Some examples of the impacts of specific plant diseases
| Type of impact | Disease | Causal agent | Country/region affected |
| Famine | Late blight of potato | Phytophthora infestans | Europe 1845–1846 |
| Brown spot of rice | Helminthosporium oryzae | India 1942–1943 | |
| Failure of maize crop | Maize mosaic virus? | Guatemala, ninth‐century Mayan civilization | |
| Cassava mosaic disease | Cassava mosaic Gemini viruses | East Africa 1980s to present | |
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