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 in tensities, assessed visually using a disease scale (scores 0 to 5) or by an estimation of the fraction of leaf area with necrosis using an i mage analyser. The photosynthetic rate of foliar discs was measured us ing a leaf disc electrode; for plants it was assessed by CO2 exchange rate measurements in a closed chamber. A reduction in the net photosyn thetic rate of diseased plants was correlated with an increase in dise ase score. A 53-56% reduction was found in foliar discs with a mean di sease score of 3. On whole plants, mean scores of 3.5 and 4.0 were ass ociated with reductions of 36% and 98% respectively. The disease also induced a decrease in the photosynthetic efficiency of the non-necroti c leaf area. The photosynthetic rate was zero when 30-40% of foliar ar ea was necrotic on foliar discs and on whole plants. A function was de rived from the foliar disc data that described the dependence of photo synthetic rate on the fraction of leaf area without necrosis. The mode l is discussed with reference to data obtained from entire plants and other pathosystems.