The potential of diseases to cause diversity in mixed-species plant communities and to influence the development of those communities is reviewed. Fungal diseases in natural populations are as severe and damaging as on crops. Epidemics occur on endemic host plants and in mixed-species communities with the same characteristics as in crops. Natural communities differ in the proportion of species that are hosts of fungal pathogens, and only a small proportion of diseases re-occur every year in the same community. Disease proneness in communities differs between sites and between years. Also the parasitic mycoflora differs between sites in the same vegetation type, variation being partly due to the fluctuating composition of host species. The presence of a host species at a particular site does not ensure the occurrence of a particular disease.
Climate and weather greatly influence the distribution and density of plants. These, in turn, and together with direct effects, determine the prevalence and intensity of diseases which vary markedly. As fungi are naturally amongst the pioneer decomposers of plant material, most hosts are affected after flowering and/or fruit set. There is no compelling evidence that high disease intensities are correlated with high plant densities in natural communities. Diversity in a community is not an automatic safeguard against high disease incidence or severity. Also, disturbance of balanced communities by man is not a prerequisite for damaging epidemics. The impact of disease on the proportions of host species comprising a community depends on the consistent occurrence of sufficiently high disease intensities, particularly during those plant growth stages most prone to negative effects from disease. The impact of exotic diseases can be very serious during the first few years after their introduction, but populations can recover once the exotic pathogen has become endemic. In endemic plant populations the effect of diseases, despite occasional severe outbreaks, is limited or erratic. Disease reduces the reproduction and survival of plants, but practically nothing is known about the competitiveness of a diseased plant species in the next season.
In conclusion, the effect of plant diseases on adult plants and, thereby, on the history of a community should not be overrated. More important for the diversity of communities, particularly during immigration and re-colonization by plant species, could be attacks of fungi on juvenile plants since the few available in situ studies suggest that fungal pathogens may cause high mortality. Very little evidence is, however, yet available. Benefits for comparative epidemiology of studies in natural plant communities are briefly discussed.