Effect of elevated concentrations of CO2 on infection of barley by Erysiphe graminis

Although there was no difference in the percentage of powdery mildew conidia that germinated on the second leaf of barley plants grown in either 350 or 700 ppm CO2, the percentage of conidia that progressed to produce colonies was lower in plants grown in 700 than in 350 ppm CO2. The lower percentage of conidia producing hyphae in 700 ppm CO2 was due to a higher proportion of the spores being arrested at the appressorial stage. The reduction in penetration of spores in 700 ppm CO2 was due neither to 700 ppm CO2 per se, nor to ontogenetic changes in the host tissue. Removing the epicuticular waxes from the surface of the leaf had no effect on the development of conidia on the surface of leaves in 350 or 700 ppm CO2, showing that increased epicuticular waxes were not causing the increased resistance to primary penetration of powdery mildew in 700 ppm CO2. We relate reduced rates of primary penetration in barley grown in 700 ppm CO2 to higher rates of net photosynthesis allowing increased mobilisation of resources into resistance including the production of papillae and accumulation of silicon at the sites of appressorial penetration. Established colonies of powdery mildew grew faster in 700 ppm CO2 than in 350 ppm CO2, coincident with accumulation of host carbohydrate in the source leaf.