Increased UVB radiation, arising from stratospheric ozone depletion may influence crops and native vegetation by altering a range of plant-microbe interactions, including disease and decomposition. Such effects of UVB may result from direct effects on micro-organisms or indirect effects, mediated via changes in the host plant. The responses of plant diseases to increased UVB reflect both direct and indirect effects. However, our studies of Septoria tritici infecting wheat highlight the role of direct UVB effects on the fungus in this system. In controlled environment experiments infection was significantly decreased by elevated UVB after inoculation, but UVB prior to inoculation had no effect. Field experiments using a modulated UVB supplement also showed significant reductions in natural S. tritici of a wheat crop, but such reductions were transient. appeared to be dependent on weather conditions. and resulted in no change in crop yield. The responses of microbial decomposition to increased UVB also reflect both direct and indirect effects. For example, we have shown highly marked interspecific variation in the response of decomposer fungi to direct UVB exposure. and changes in the fungal decomposer community following decomposition under increased UVB conditions. lndirect UVB effects on decomposition also occur, and are evident in altered patterns of microbial respiration. Such responses may reflect changes in host nitrogen concentration, and may have their greatest ecological effects via changes in nutrient cycling.