Effects of sulfur dioxide on the development of powdery mildew of cucumber

Environment is a major factor that does influence host parasite relationships. Air pollution caused by SO2 may directly alter the environment around the plant and pathogen. It is hypothesised that plants may respond differently to foliar pathogens in air polluted environments. To test this hypothesis, effects of intermittent exposures of SO2 at 143, 286 and 571 mu g m(-3) were investigated on the development of powdery mildew of cucumber (Cucumis sativa) caused by Sphaerotheca fuliginea, using pre-, post- and concomitant-inoculation exposures in closed-top chambers. Sulfur dioxide (except 143 mu g m(-3)) and the fungus acting alone caused chlorosis and/or necrosis, and mildew colonies on leaves, respectively and both reduced the plant growth and yield of cucumber. Fungus colonization was relatively greater on the plants exposed to 143 mu g SO2 m(-3), but at the higher concentrations, the colonies were greatly suppressed. Gas injury on fungus-infected plants was also less in the other treatments. Conidia of S. fuliginea collected from exposed plants varied in size. Conidial germination was considerably greater at 143 mu g SO2 m(-3). This concentration also promoted germination of the conidia exposed on glass slides. Higher concentrations (286 and 571 mu g m(-3)), however, suppressed the germination of conidia from exposed plants or exposed on glass slides. The number of fibrosin bodies declined at all the concentrations. Synergistic effects of 143 mu g SO2 m(-3) and S. fuliginea were recorded on plant growth and yield of cucumber. Sulfur dioxide at 571 mu g m(-3) and powdery mildew infection had an antagonistic effect on plant growth. (C) 1998 Elsevier Science B.V. All rights reserved.