A field experiment was set up in Mogi Mirim, SP, in 2003/2004 to study the effect of N fertilizer on the incidence of rice blast, caused by Pyricularia grisea in two rice cultivars. The experimental design was a split-plot in randomized blocks with 4 replications. Rates of N sidedressed 40 days after seed germination (0, 50, 100, and 150 kg ha-1 N) were placed in the plots and rice cultivars (IAC 201 and IAC 202) in the subplots. Fertilization common to all treatments was applied in the seed furrow comprising 16, 56, and 32 kg ha-1 of N, P2O5, and K2O, respectively. An intense incidence of blast was observed. Rice plants were evaluated for severity of disease in panicles (using a grading system) and in seeds (by the blotter test), sterile spiklets, grain density and yield, and percentage of broken grain. The incidence of rice blast in seeds as well as the panicle blast disease severity were significantly higher in plants fertilized with N and were also higher for IAC 202 than for IAC 201. However, IAC 201 could not be considered resistant/ tolerant do P. grisea. A highly significant linear correlation coefficient was observed between incidence of P. grisea in seeds and disease severity in panicle. Grain yield was negatively correlated with blast incidence and severity, and with the percentage of sterile spiklets. Therefore, sidedressed N fertilization increased seed damage caused by P. grisea, the percentage of sterile spiklets, and the percentage of broken grains, thereby causing a decrease in rice yield (22% lower at the higher N rate) and grain density. The results of the present study suggest that the use of fungicides is necessary when rice cultivars susceptible to blast are fertilized with nitrogen.
Oryza sativa; nitrogen; blast; grains yield; Pyricularia grisea
