RATIO BETWEEN VIRTUAL AND VISUAL LESION SIZE AS A MEASURE TO DESCRIBE REDUCTION IN LEAF PHOTOSYNTHESIS OF RICE DUE TO LEAF BLAST

The effect of Pyricularia oryzae, the causal organism of blast in rice, on net photosynthetic rate of rice leaves was measured in the field and in a greenhouse experiment. Leaf blast reduced photosynthesis not only through a reduction in green leaf area, but also through an effect on photosynthesis of the remaining green leaf tissue. A function was derived to relate the net photosynthetic rate of diseased leaf area (P(x)) to the photosynthesis of comparable healthy leaf area (P(O)) and disease severity (x): P(x) = P(O) (1 - x)beta. This function is based on the assumption that the visual lesion is part of a virtual lesion in which photosynthesis is negligible. The parameter beta-expresses the ratio between virtual and visual lesion size and characterizes the effect of the pathogen on leaf photosynthesis for the entire range of measured disease severities. A value of beta between 3 and 4 gave a good description of the effect of leaf blast on net photosynthetic rate of rice leaves, indicating a much stronger effect of leaf blast on leaf photosynthesis than expected on the basis of visible lesion size.