summaryWheat (Triticum aestivum L. cv. Longbow) was grown hydroponically with either 0, 0.33 or 3.3 mol Pi m−3 and, after 12 d, was infected with powdery mildew (Erysiphe graminis D.C. ex Merat f.sp. tritici). Fructan content of infected first leaves was increased by mildew infection but infection increased starch content only at 0 Pi. Photosynthetic 14CO2 fixation was reduced by infection, especially at 0 Pi. Accumulation of 14C in fructan rose with reduced Pi supply but was unaffected by mildew; accumulation of 14C in starch was lower in infected leaves. Pi deficiency increased the content of sugars in the apoplast, as did mildew infection, the greatest content being in infected leaves of low Pi status. Acid invertase activity was increased by infection but was not affected by Pi status. Thus P‐deficiency alone does not mimic the effects of mildew but Pi status and mildew interact in their effect on leaf carbohydrate metabolism.