EFFECTS OF LEAF BLAST ON GROWTH AND PRODUCTION OF A RICE CROP .1. DETERMINING THE MECHANISM OF YIELD REDUCTION

Rice crops grown under irrigated conditions were inoculated with Pyric ularia oryzae during early growth stages to study the effect of leaf b last on yield formation. The inoculations led to severe epidemics of l eaf blast around maximum tillering, characterized by the presence of t ypical blast lesions and an accelerated senescence of heavily infested leaf tissue. Leaf blast led to a prolonged tillering and a delay in f lowering and maturity. Crop growth rate and leaf area formation declin ed sharply during establishment of the disease and continued to be red uced till maturity. This resulted in a marked reduction of total dry m atter production and grain yield. Dry matter distribution was not affe cted. Leaf blast reduced spikelet number, 1000 grain weight, and the f raction filled grains. From this last observation it was concluded tha t the reduction in grain yield was exclusively source determined. Nitr ogen uptake of the inoculated crops before flowering was reduced compa red to the N uptake of the control crop, but shoot N content of the in oculated crops at flowering was higher. Uptake of nitrogen after flowe ring was negligible in both healthy and inoculated crops. Redistributi on from vegetative tissue was therefore the main source of N for grain growth. The higher N content of the shoot organs in the inoculated cr ops during ripening led to the conclusion that the reduced N uptake wa s not responsible for the yield reduction observed. Consequently, the reduction in grain yield was solely determined by a reduced carbohydra te supply.