Environmental and Experimental Botany 42 (1999) 163 – 171 Effects of intermittent ozone exposures on powdery mildew of cucumber Mujeebur Rahman Khan a, *, M. Wajid Khan b a Department of Plant Protection, Institute of Agriculture, Aligarh Muslim Uniersity, Aligarh 202 002, India b Department of Botany, Aligarh Muslim Uniersity, Aligarh 202 002, India Received 20 October 1998; received in revised form 1 June 1999; accepted 5 June 1999 Abstract Exposure of plants to O 3 may influence foliar diseases caused by fungi. It is hypothesized that concentrations of O 3 at or below economic threshold may enhance severity of the fungal disease, whereas higher levels may decrease the severity. To test this hypothesis, effects of intermittent exposures at 50, 100 and 200 ppb O 3 and powdery mildew infection by Sphaerotheca fuliginea, using pre-, post- and concomitant-inoculation exposures were investigated on cucumber in closed-top exposure chambers. For impact assessment, plant growth, flowering, fruit-setting, foliar ozone injury, fungal colonization, conidia size and their germination were considered. Ozone (except 50 ppb) caused necrotic lesions on leaves and reduced the plant growth and fruit-setting of cucumber. Plants inoculated with S. fuliginea conidia developed powdery colonies in the intercostal region of leaves. Ozone injury was relatively moderate on fungus-inoculated plants. Powdery mildew development was, however, severe on the plants exposed to 50 ppb O 3 , but at higher concentrations there was significant decline in fungus colonization. Conidia examined from such plants (exposed to 100 or 200 ppb) were smaller in size and contained fewer fibrosin bodies and showed poor germination. Ozone exposures at 50 ppb, however, stimulated the conidial germination. In an in – vitro experiment, the conidia directly exposed to ozone on glass slides showed more or less similar response to the gas. Ozone at 50 ppb and S. fuliginea interacted synergistically and caused significantly greater decrease in the number of fruits/plant. At 200 ppb O 3 , the mutual effects were antagonistic. Fungus infection provided partial protection to plants against 200 ppb ozone, and mildew infection was also mild. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Cucumber; Cucumis satia ; Fungus disease; Ozone; Plant growth; Sphaerotheca fuliginea www.elsevier.com/locate/envexpbot 1. Introduction Ozone is considered the most economically im- portant air pollutant and it causes decline of many forest trees (Woodbury et al., 1994; Lau- rence et al., 1996) and reduces growth and yield of crop plants in America (Pell and Pearson, 1984; Kress et al., 1986; Temple, 1990) and Europe (Ashmore, 1984; Grandjean and Fuhrer, 1989; Adaros et al., 1991). Ambient concentration of O 3 in USA (NESCAUM, 1993) and Europe (Volz and Kley, 1988; Bytnerowicz et al., 1993) may * Corresponding author. S0098-8472/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII:S0098-8472(99)00029-5