Walter A. H.
van der Werf
First author: DLO Research Institute for Agrobiology and Soil Fertility, P.O. Box 14, 6700 AA Wageningen, the Netherlands; second and third authors: Department of Theoretical Production Ecology, Wageningen Agricultural University, P.O. Box 430, 6700 AK, the Netherlands
Go to article:
Accepted for publication 24 May 1999.
Virtual lesion extension was proposed as a measure to summarize the effects of foliar diseases with single spreading lesions on CO2-exchange parameters at the whole-leaf level. Visible lesion plus virtual lesion extension constitute a virtual lesion, in which CO2 exchange was postulated to be nil. Virtual lesion extension can be derived for each photosynthesis parameter from gas-exchange measurements. Using a leaf-shape function, one-dimensional lesion length was translated into two-dimensional lesion area, and a relationship between visible and virtual severity can be established. The model was applied to measurements of leaf CO2 exchange in rice leaves infected with Xanthomonas campestris pv. oryzae, the causal organism of rice bacterial blight. The model resulted in a virtual lesion extension of 1.1 cm for the gross CO2-exchange rate at light saturation, -3.9 cm for dark respiration rate, and 0 for initial light use efficiency. Reduced light interception due to a visible lesion caused reductions in net CO2 assimilation, and small virtual lesion extensions only marginally reduced net CO2 assimilation further. The additional reduction was smaller in case of longer leaves. Measurement of net photosynthesis rate along a transect from the base to the tip of infected leaves indicated that the location on the leaf blade where net photosynthesis decreased from normal to nil was centered around the lesion tip.
© 1999 The American Phytopathological Society