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During the period 1986–1988 field studies were conducted on the epidemiology of the tar spot disease complex (TDC) of maize (Zea mays) caused by Phyllachora maydis, Monographella maydis and Coniothyrium phyllachorae. Under field conditions we found that P. maydis symptoms always appeared first, followed by symptoms of either M. maydis or C. phyllachorae. M. maydis causes leaf necrosis and has the most devastating effect. The primary symptoms covered about 12% of the leaf area below the ear leaf, whereas the total necrotic leaf area amounted to 30–60%, here considered as a secondary effect. Maximum TDC severity occurred during the winter season of 1988, which was characterized by a temperature range of 17–22°C, a mean RH >75%, and > 7h of leaf wetness per night. The highest numbers of windborne ascospores of P. maydis were trapped at an RH > 85% and at temperatures of 17 to 23°C in the winter of 1987 and 1988, although large numbers were also caught at temperatures of >23°C and RH <70%. Spore release was strongly influenced by light conditions and followed a similar diurnal curve throughout three seasons, reaching a maximum at 17.00–21.00 hours. The spread off. maydis within the field was very homogeneous. The incubation period of P. maydis was 12 to 15 days, and most of the ascospores were released within 3 weeks after formation of the ascostromata. M. maydis inoculum in plant debris was reduced by 90% within 3 to 4 months.

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SUMMARY 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.