EFFECTS OF LEAF BLAST ON GROWTH AND PRODUCTION OF A RICE CROP .2. ANALYSIS OF THE REDUCTION IN DRY-MATTER PRODUCTION, USING 2 MODELS WITH DIFFERENT COMPLEXITY

Effects of leaf blast on dry matter production of rice observed in a field experiment were analysed in terms of cumulative radiation interception by green foliage (PARCUM; MJm-2) and radiation use efficiency (RUE; g dry matter MJ-1). Leaf blast caused significant reductions in total dry matter production as a result of a decrease in PARCUM and a decrease in RUE. Reductions in radiation interception (RI; MJm-2d-1) and RUE were first observed shortly after the introduction of the disease. RI was significantly reduced till maturity. The reduction in RUE disappeared towards maturity, simultaneous with a gradual decline in disease severity. Analysis with a mechanistic model of crop growth showed that previously determined effects of leaf blast on leaf photosynthetic rate and respiration could explain the reduction in RUE during a large part of the growing season. During the first weeks after the onset of the disease the two effects were insufficient to fully explain the observed reduction in RUE. It was hypothesized that the unexplained part of the reduction in RUE in that period was a result of carbohydrate withdrawal by the pathogen for spore production.