EFFECTS OF LEAF BLAST ON PHOTOSYNTHESIS OF RICE .2. CANOPY PHOTOSYNTHESIS

The effect of Pyricularia oryzae, the causal organism of leaf blast in rice, on photosynthetic rate of a rice crop was determined with mobil e equipment in the field. Canopy CO2 exchange rate (CER) was significa ntly reduced in plots inoculated with P. oryzae. The experimentally ob tained data were used to evaluate the performance of a model for the e ffects of leaf blast on canopy photosynthesis. The model comprised pho tosynthesis and respiration routines of a mechanistic crop growth mode l, extended with submodels for effects of leaf blast on both processes . Canopy photosynthesis and the effect of leaf blast on CER were accur ately simulated with the model. Analysis showed that the reduction in canopy photosynthesis was mainly due to an adverse effect of lesions o n leaf photosynthetic rate and to shading by dead leaf area resulting from disease induced senescence. A sensitivity analysis demonstrated t he importance of the vertical distribution of the disease in the canop y. This implies that disease monitoring for crop loss assessment shoul d consider vertical disease distribution.