Grain discoloration (GD) is ranked second among the most economically important rice diseases. The influence of nitrogen levels, planting date and air spora concentration on the disease severity was studied in the field. GD severity in the rice cultivar BRS Bonança was assessed on two planting dates (30/11/2006 and 21/12/2006), using five nitrogen levels (0, 30, 60, 120 and 240 kg of N.ha-1) and a randomized split plot block design with three replicates. The effect of N level on GD severity was not significant. The correlation between GD severity and empty spikelets was positive and significant. The fungus population in the air (air spora) was quantified using the volumetric trap Rotorod Sampler from panicle emergence to maturation. Grain discoloration linearly increased with time (r = 0.98; P < 0.01), but the same did not occur with the total number of fungi, which varied from 0.23 to 2.97 spores/liter of air/minute. The fungi present in the air in decreasing order of spore concentration were Nigrospora sp., Pyricularia oryzae, Pithomyces sp., Alternaria sp., Cercospora sp., Fusarium sp., Curvularia sp. and Bipolaris sp. These fungi, Phoma sp. and others were also detected in the seed health testing. The correlation between the quantity of spores of P. oryzae and other fungi was linear and positive (r=0.80, P < 0.01). The number of spores increased with the increase in relative humidity and exponentially decreased with the increase in maximum temperature.
Oryza sativa; volumetric spore trap; diseases of final growth stages; epidemiology
