Effect of elevated levels of CO₂ on powdery mildew development in five cucurbit species

The environment is the key factor that influences the host-parasite relationship. Elevated CO₂ levels resulting from various anthropogenic sources may directly affect the surroundings around pathogens and plants. It is hypothesized that plants may respond differently to pathogens in the environment containing an elevated concentration of CO₂. To test the hypothesis an experiment was conducted to examine the effects of intermittent exposures of elevated levels of CO₂ viz., 400, 500 and 600 ppm (5 hr/day on alternate days) on the development of Sphaerotheca fuliginea causing powdery mildew disease on five cucurbits species using open-top chambers. The elevated levels of CO₂ acted as a growth promoter and significantly enhanced the plant growth of all five cucurbit species. Inoculation with the fungus incited specific mildew symptoms on the leaves and decreased the plant growth and biomass production of the cucurbits tested except bitter gourd. The intermittent exposures with elevated levels of CO₂ aggravated the disease development. As a result, severe mildew developed on all five cucurbits, including bitter gourd, which expressed tolerance to the pathogen. Fungus colonization in terms of the number of conidia/cm² leaf surface was significantly greater on the plants exposed to 500 or 600 ppm CO₂. The stomata and trichome density and stomatal pore width were increased in the leaves of CO₂ exposed plants. The CO₂ exposures also accelerated the photosynthesis rate, but transpiration, stomatal conductance, salicylic acid and total phenols were decreased; fungus inoculation caused the effects just reverse of CO₂. Interaction between S. fuliginea and CO₂ was found synergistic at 500 ppm, whereas with rest of the concentrations it was near to additive.