Aim: To explore whether ultraviolet (UV) light treatment within a closed circulating and filtered water drainage system can kill plant pathogenic species. Methods and Results: Ultraviolet experiments at 254 nm were conducted to determine the inactivation coefficients for seven plant pathogenic species. At 200 mJ cm-2, the individual species log reductions obtained for six Ascomycete fungi and a cereal virus were as follows: Leptosphaeria maculans (9 center dot 9-log), Leptosphaeria biglobosa (7 center dot 1-log), Barley stripe mosaic virus (BSMV) (4 center dot 1-log), Mycosphaerella graminicola (2 center dot 9-log), Fusarium culmorum (1 center dot 2-log), Fusarium graminearum (0 center dot 6-log) and Magnaporthe oryzae (0 center dot 3-log). Dilution experiments showed that BSMV was rendered noninfectious when diluted to > 1/512. Follow-up large-scale experiments using up to 400 l of microbiologically contaminated waste water revealed that the filtration of drainage water followed by UV treatment could successfully be used to inactivate several plant pathogens. Conclusions: By combining sedimentation, filtration and UV irradiation within a closed system, plant pathogens can be successfully removed from collected drainage water. Significance and Impact of the Study: Ultraviolet irradiation is a relatively low cost, energy efficient and labour nonintensive method to decontaminate water arising from a suite of higher biological containment level laboratories and plant growth rooms where genetically modified and/or quarantine fungal and viral plant pathogenic organisms are being used for research purposes.