Effects of Ascochyta blight ( Mycosphaerella pinodes ) on the photosynthesizing leaf area and the photosynthetic efficiency of the green leaf area of dried‐pea ( Pisum sativum )

The effects of Ascochyta blight (caused by Mycosphaerella pinodes ) on the net photosynthetic rate of glasshouse‐grown dried pea were studied on foliar discs and whole plants in pots showing different disease intensities, assessed visually using a disease scale (scores 0 to 5) or by an estimation of the fraction of leaf area with necrosis using an image analyser. The photosynthetic rate of foliar discs was measured using a leaf disc electrode; for plants it was assessed by CO 2 exchange rate measurements in a closed chamber. A reduction in the net photosynthetic rate of diseased plants was correlated with an increase in disease score. A 53–56% reduction was found in foliar discs with a mean disease score of 3. On whole plants, mean scores of 3.5 and 4.0 were associated with reductions of 36% and 98% respectively. The disease also induced a decrease in the photosynthetic efficiency of the non‐necrotic leaf area. The photosynthetic rate was zero when 30–40% of foliar area was necrotic on foliar discs and on whole plants. A function was derived from the foliar disc data that described the dependence of photosynthetic rate on the fraction of leaf area without necrosis. The model is discussed with reference to data obtained from entire plants and other pathosystems.