Effect of chemicals, nitrogen, time of sowing and panicle brown spot epidemics on rice grain discolouration in Italy

Rice grain discolouration has a complex etiology and cannot be diagnosed prior to harvest. In Italy, the disease was thought to be associated with fungal infections, however several studies contradicted this hypothesis. We report the distribution of grain discolouration, the most susceptible Italian rice varieties, and the effects of epidemics of brown spot, induced by Cochliobolus miyabeanus, and abiotic factors, such as incidence of grains with open glumes, chemical treatments, nitrogen fertilization, and time of sowing, on disease incidence over three years. Among 39 heavily diseased rice lots, the early cvs Selenio and Loto were the most susceptible. We did not find any significant correlation between incidence of grain discolouration and incidence of brown spot. In contrast, incidence of discoloured grains was weakly correlated with incidence of grains with open glumes (r = 0.521 P = 0.001), while incidence of brown spot was better correlated with incidence of grains with sealed glumes (r = 0.570, P < 0.001) than to incidence of grains with open glumes (r = 0.355, P = 0.026). On susceptible cv. Selenio, percent of grains with open glumes varied among years, but increased significantly from milk dough stage to fully ripe maturation of kernels (P < 0.001). The highest percent of grains with open glumes was found for late-sown rice. However, late sowing reduced significantly (P ≤ 0.001) the incidence of grain discolouration, whereas chemical and fungicide treatments had no significant effect. We speculate that grain discolouration may have a bacterial rather than a fungal aetiology.