Combined Effect of CO₂ and Temperature on Wheat Powdery Mildew Development

The effect of simulated climate changes by applying different temperatures and CO₂ levels was investigated in the Blumeria graminis f. sp. tritici/wheat pathosystem. Healthy and inoculated plants were exposed in single phytotrons to six CO₂+temperature combinations: (1) 450 ppm CO₂/18-22°C (ambient CO₂ and low temperature), (2) 850 ppm CO₂/18-22°C (elevated CO₂ and low temperature), (3) 450 ppm CO₂/22-26°C (ambient CO₂ and medium temperature), (4) 850 ppm CO₂/22-26°C (elevated CO₂ and medium temperature), (5) 450 ppm CO₂/26-30°C (ambient CO₂ and high temperature), and (6) 850 ppm CO₂/26-30°C (elevated CO₂ and high temperature). Powdery mildew disease index, fungal DNA quantity, plant death incidence, plant expression of pathogenesis-related (PR) genes, plant growth parameters, carbohydrate and chlorophyll content were evaluated. Both CO₂ and temperature, and their interaction significantly influenced powdery mildew development. The most advantageous conditions for the progress of powdery mildew on wheat were low temperature and ambient CO₂. High temperatures inhibited pathogen growth independent of CO₂ conditions, and no typical powdery mildew symptoms were observed. Elevated CO₂ did not stimulate powdery mildew development, but was detrimental for plant vitality. Similar abundance of three PR transcripts was found, and the level of their expression was different between six phytotron conditions. Real time PCR quantification of Bgt was in line with the disease index results, but this technique succeeded to detect the pathogen also in asymptomatic plants. Overall, future global warming scenarios may limit the development of powdery mildew on wheat in Mediterranean area, unless the pathogen will adapt to higher temperatures.