Polyethylene (PE) films having various Blue/ultraviolet (B/UV) ratios (20.4–76.8) of the transmitted light have been investigated with regard to their effects on the epidemiology of grey mould in artificially-inoculated greenhouse-grown tomato plants and onin vitrosporulation ofBotrytis cinereaPers. ex Fr. The highest number of infection sites was found on plants grown under the sheet having the lowest ratio of B/UV transmittance (20.4) in both control or artificially-inoculated and fungicide-treated (Rovral) greenhouses. It was also found that the rate of development of the epidemic in greenhouses covered with an improved sheet having a higher ratio of B/UV transmittance (32.7) was remarkably slower as compared to those covered by sheets with a low B/UV ratio. When experimental sheets having even higher B/UV ratio (up to 76.8) were tested, a clear negative association was observed between B/UV ratio and number of spores produced in culture. Such films are suggested as a cladding material for both conventional and organic greenhouse crops where the grey mould disease is a major economic problem.