1Biology Department, IENS, Lancaster University, LA1 4YQ, UK; 2IACR-Long Ashton Research Station, University of Bristol, Bristol BS18 9AF, UK

The ultraviolet (UV) component of sunlight has long been known to influence plant pathogenic fungi. Firstly, sporogenesis in a wide range of fungi may be stimulated by UV, especially by longer wavelength UVA (315-400 nm), but also by shorter, more energetic UVB wavelengths (290-315 nm). Sunlight, and by implication its UV component, may also reduce the survival of spores during dispersal or during the early stages of infection, although the photobiology of such responses is mostly poorly defined, and sometimes based on studies using environmentally irrelevant UVC radiation (200-290 nm). Interest in the biological effects of solar UVB has been stimulated since ground-level irradiances of this waveband will increase if stratospheric ozone is significantly depleted by the release of halogenated hydrocarbons such as CFCS. Indeed, increases in UVB have been recorded over the Northern temperate latitudes during episodes of substantial ozone depletion. We report here the results of a series of experiments in which we studied the effects of increased UVB radiation on leaf blotch of wheat resulting from infection by the fungus Septoria tritici (perfect stage Mycosphaerella graminicola).

Materials and methods
The responses of a range of S. tritici isolates were studied in vitro, using a xenon-arc lamp and narrow bandpass filters to provide treatments precisely defined in terms of dose and spectral composition. Studies in planta were conducted in controlled environment (CE) chambers which provided environmentally realistic UVB treatments with near field doses of UVA and photosynthetically active radiation (PAR: 400-700 nm). In planta responses were confirmed in the field including a season-long study with natural field inoculum of S. tritici, which used state-of-the-art 'modulated' UVB supplements, in which UVB added from lamps was constantly adjusted in proportion to changes in incident sunlight.

Results and conclusions
UVB radiation inhibited conidial germination and germ tube growth in some S. tritici isolates, but there was marked intra-specific variation in response, not only between isolates from diverse geographical origins but also between different isolates from within a limited area of south-west England. In sensitive isolates, the spectral responses of S. tritici quantified in vitro showed that shorter wavelengths within the UVB waveband were the most inhibitory to germination and germ tube growth. UVB effects on S. tritici infection of wheat were investigated after inoculation of potted plants in controlled environments and outdoors. In CE studies, infection by an S. tritici isolate whose early development was inhibited by UV radiation was significantly reduced by increased doses of UVB post-inoculation, but not by pre-inoculation treatments. An S. tritici genotype known to be UVB-tolerant in vitro was unaffected by either pre- or post-inoculation UVB treatments in planta. We concluded that UVB inhibits infection by S. tritici due to the direct effects of radiation on the pathogen, rather than through changes in the host plant.

The prediction that increased UVB would decrease the severity of leaf blotch of wheat caused by S. tritici infection was tested in the field during two growing seasons. The field data confirmed that leaf blotch could be reduced by increased UVB, but this response was rather variable and appeared to depend on interactions with other environmental factors which influence the early development of the pathogen. It was concluded that any decrease in leaf blotch which might result from increases in UVB of the magnitude currently expected to result from depletion of stratospheric ozone, was unlikely to be agronomically significant. However, the data also confirmed the possible role of variation in ambient UVB on the epidemiology of S. tritici.