The effect of downy and powdery mildew on grapevine (Vitis vinifera L.) leaf gas exchange

The impact of powdery (Uncinula necator) and downy mildew (Plasmopara viticola) on grapevine leaf gas exchange was analysed. Gas exchange measurements (assimilation A, transpiration E, stomatal conductance g(s), intercellular concentration of CO(2)C(i)) were made on three different leaf materials: (i) healthy tissue of diseased leaves, (ii) infected tissue of diseased leaves, (iii) healthy tissue of healthy leaves (control treatment). Using the same source of leaf tissue, photosynthetic pigment concentration (chlorophyll a, b) and fluorescence levels (minimal fluorescence F(0), maximal fluorescence F(m) and the optimal quantum yield [F(m) - F(0)]/F(m)) were determined to explain the mechanism of action of the two diseases on leaf assimilation. The results indicated that powdery and downy mildew reduced the assimilation rates, not only through a reduction in green leaf area (visual lesions), but also through an influence on gas exchange of the remaining green leaf tissues, determining a 'virtual lesion'. The ratios between virtual and visual lesions were higher in powdery mildewed leaves than in the downy mildewed leaves. The photosynthetic fluorescence level (F(v)/F(m)) was affected by neither of the two pathogens. The reduction in intercellular concentration of CO(2) and photosynthetic pigment may explain the lower assimilation rates in the healthy tissues of powdery and downy mildewed leaves respectively.