Читать книгу Plant Pathology and Plant Pathogens - John A. Lucas - Страница 21

It is often assumed that disease outbreaks are less frequent and less severe in wild plant populations than in crops. This is because there are several important differences between natural and agricultural plant communities (Table 1.3). Wild species are more diverse, both genetically and in the age structure of the population. Hence individual plants will differ in their relative susceptibility to infection. Natural populations also tend to be spatially dispersed as part of a mixed plant community, thereby reducing opportunities for the spread of disease. A recent survey of grassland plots differing in the number of plant species present found that the number of groups of fungal pathogens increased with diversity of the plant community, but that the severity of infection on individual plants decreased. This supports the idea that disease is commonplace in natural plant populations but that mixed communities are less prone to severe disease outbreaks. Often, nutrients are added to crops as fertilizers, and in some cases this can lead to increased susceptibility to infection. Finally, it is likely that plants in natural communities have co‐evolved with their pathogens over long periods of time, leading to some kind of host–pathogen equilibrium. Analysis of wild populations of the genetic model plant Arabidopsis thaliana found very high levels of polymorphism in genes determining recognition of pathogens, supporting the idea that reciprocal selection between host and pathogen over time has maintained a natural diversity for disease resistance. Modern, intensive agricultural systems and the deployment of crops in new areas have disturbed this balance.

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.