In natural ecosystems, disease is one of the many factors which regulate populations (Figure 1.1) and hence determine the spectrum of species which are successful in any habitat. Pathogens affect the reproduction and longevity of plants and hence act as agents of natural selection. Figure 1.7 shows the effect of rust infection on the survival and reproductive capacity of the annual weed groundsel; infected plants produced fewer mature flowers, and hence seeds, and died earlier than uninfected plants. Repeated over several generations, such disease would therefore have a significant effect on population size. There is a practical spin‐off from this observation as some pathogenic fungi may be useful as agents to control weeds, known as mycoherbicides. Experiments in which foliar fungal pathogens were allowed to infect or were excluded from a grassland community by fungicide treatment showed that natural levels of disease reduced the overall biomass of the community, but actually increased biodiversity by reducing the abundance of the most dominant grass species. In particular, disease will tend to limit the spread of species to less favorable geographic regions or habitats, as the impact of pathogens will be greater on plants growing under suboptimal conditions.
Table 1.3 Natural and agricultural plant communities compared
| Natural | Agricultural | |
| Genotypes | Diverse | Uniform |
| Age structure | Mixed | Uniform |
| Distribution | Dispersed | Crowded |
| Nutrient status | Often low | Usually high |
| Co‐evolution span | Long | Short |
Figure 1.7 Effect of infection by the rust fungus Puccinia lagenophorae on survival and reproduction of the annual weed groundsel (Senecio vulgaris). (a) Percentage of the original population surviving in control and rust‐infected populations. (b) Number of mature inflorescences formed by control and rust‐infected plants.
Source: After Paul and Ayres (1986a,b).
Disease may also accelerate change within established plant communities. For example, forest trees may be killed, thereby opening the canopy and allowing regeneration to proceed. The global epidemic of Dutch elm disease (Table 1.5) completely altered the structure and species composition of woodlands where elm was one of the dominant trees. An even more dramatic example of a disease outbreak in natural vegetation occurred in Australia where the root‐rot pathogen Phytophthora cinnamomi has killed large areas of native forest (Table 1.5). The unusual feature of this epidemic was that many different species of trees and shrubs were affected by the same pathogen.
One explanation for the severity of such invasive diseases is that the pathogen responsible was almost undoubtedly introduced from another continent, and hence spread through a host population not previously exposed to infection. The term new‐encounter disease has been proposed to describe such an epidemic arising from contact between a previously separate host and pathogen.
Disease in Agriculture, Horticulture, and Forestry
In agricultural ecosystems, disease is one of the factors influencing crop yield and quality. Farmers, foresters, and horticulturalists are, by and large, interested only in those changes in crop performance which influence cash return per hectare. The ideal situation, in which pathogens are avoided, excluded or eliminated, is therefore a theoretical rather than practical goal. On the farm, other priorities may prevail; choice of crop or cultivar 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.