Impact of <Emphasis Type="Italic">Erysiphe alphitoides</Emphasis> on transpiration and photosynthesis in <Emphasis Type="Italic">Quercus robur</Emphasis> leaves

Oak powdery mildew, (Erysiphe alphitoides) causes one of the most common diseases of oaks. We assessed the impact of this pathogen on photosynthesis and water relations of infected leaves using greenhouse-grown oak seedlings. Transpiration of seedlings infected by oak powdery mildew was also investigated. Altogether, E. alphitoides had a low impact on host gas exchange whether at the leaf or whole plant scale. Maximal stomatal conductance of infected leaves was reduced by 20–30% compared to healthy controls. Severely infected seedlings did not experience any detectable change of whole plant transpiration. The reduction in net CO₂ assimilation, A_n, was less than proportional to the fraction of leaf area infected. Powdery mildew reduced both the maximal light-driven electron flux (J_max) and the apparent maximal carboxylation velocity (Vc_max) although Vc_max was slightly more impacted than J_max. No compensation for the infection occurred in healthy leaves of partly infected seedlings as the reduced photosynthesis in the infected leaves was not paralleled by increased A_n levels in the healthy leaves of the seedlings. However, E. alphitoides had a strong impact on the leaf life-span of infected leaves. It is concluded that the moderate effect of E. alphitoides on oak might be related to the small impact on net CO₂ assimilation rates and on tree transpiration; nevertheless, the severe reduction in leaf life-span of heavily infected leaves may lead to decreased carbon uptake over the growth season.