ORIGINAL ARTICLE Ecological impact of solar ultraviolet-B (UV-B: 320–290 nm) radiation on Corynebacterium aquaticum and Xanthomonas sp. colonization on tea phyllosphere in relation to blister blight disease incidence in the field T.S. Gunasekera and N.D. Paul Division of Biological Sciences, Lancaster University, Lancaster, UK Introduction Micro-organisms on the surfaces of leaves are exposed to extreme environmental conditions, such as desiccation, high temperature and solar ultraviolet radiation. Solar ultraviolet radiation (UVR), particularly shorter wave- length UV-B (290–320 nm), is generally damaging to phyllosphere micro-organisms (Ayres et al. 1996; Gunase- kera et al. 1997a; Sundin and Jacobs 1999; Sundin and Murillo 1999; Jacobs and Sundin 2001). Microbial strat- egies for UV tolerance can include DNA repair mecha- nisms that confer increased UVR survival (Sundin and Murillo 1999; Gunasekera and Sundin 2006), protection of vital cell targets by screening UVR via pigmentation (Jacobs et al. 2005), cross protection via aggregation (Monier and Lindow 2004), or avoidance of UVR by the colonization of protected sites. However, bacteria exposed directly to UV-B are subject to DNA damage, including the formation of photoproducts, such as cyclobutane pyrimidine dimmers (Pfeifer 1997). It has been shown that solar UVR can result in rapid selection of phyllo- sphere micro-organisms, with the relative abundance of UVR-tolerant individuals increasing rapidly under high UV conditions (Jacobs and Sundin 2001; Kadivar and Stapleton 2003). Relatively few studies have directly examined the effects of UVR on phyllosphere organisms, either by modifying solar UVR in the field (Gunasekera et al. 1997b; Jacobs Keywords Camellia sinensis, Corynebacterium aquaticum, Exobasidium vexans, phyllosphere, rainfall, UV-B radiation, Xanthomonas sp. Correspondence Thusitha S. Gunasekera, Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056, USA. E-mail: gunasets@muohio.edu 2006/0801: received 3 June 2006, revised 15 November 2006 and accepted 16 November 2006 doi:10.1111/j.1472-765X.2006.02102.x Abstract Aims: To assess the effects of solar UV-B radiation on phyllosphere bacteria of tea leaves in relation to blister blight disease in the field. Methods and Results: The effects of UV-B radiation on the phyllosphere microbiology of tea (Camellia sinensis) were studied in contrasting wet and dry seasons at a tropical site. Wavelength-selective filters were used to separate the effects of UV-B from those of other factors. Bacterial populations were quanti- fied in relation to the incidence of blister blight disease. Attenuation of UV-B increased the survival of Xanthomonas sp. when populations were not water limited, and increased the incidence of blister blight, but had no effect on Corynebacterium aquaticum. Conclusions: The effects of solar UV-B on phyllosphere bacteria were substan- tial but depended on both species and interactions with other environmental variables. Xanthomonas sp. was more sensitive to UV-B than C. aquaticum, but this did not result in differences in population density under high radiation conditions (dry season), but only in the wet season when other factors were not limiting. Significance and Impact of the Study: The role of UV-B on leaf surface micro- biology in the tropics is marked but depends on other conditions, and the con- trasting UV-B responses of different organisms can be masked by other limiting factors. Letters in Applied Microbiology ISSN 0266-8254 ª 2007 The Authors Journal compilation ª 2007 The Society for Applied Microbiology, Letters in Applied Microbiology 44 (2007) 513–519 513