Lack of correlation between incidence of rice grain discolouration and panicle brown spot

Grain discolouration of rice is a serious diseases intropical countries, caused both by fungal pathogens like Cochliobolus miyabeanus, Magnaporthe grisea and Sarocladium spp., and by pathogenic and saprophytic bacteria. In Italy, the disease was reported occasionally in the past, mainly as a consequence of severe epidemics of brown spot and blast. However, incidence of discoloured grains has recently increased, in spite of improved fungal disease control. Failure of fungicides to reduce incidence of grain discoloration suggests that pathogens other than fungi may cause the disease. We identified the most sensitive rice varieties grown in Italy and searched for factors increasing disease incidence. Sensitivity of rice varieties was assessed randomly, collecting samples all over the rice-cultivated area. Forty-six percent of the samples (N=39) were of cv Selenio, 18% of cv Loto and 8% of cv Balilla, which resulted the most sensitive varieties with an average discoloured grains incidence of 13.7%, 14.6% and 8.7%, respectively. Results of two-year field trials, carried out in Rosasco (Province of Pavia) on cv Selenio (subsp. Japonica), showed that the percentage of discoloured grains decreased by delaying the date of sowing. Disease incidence was not affected by increasing the level of nitrogen or by fungicide treatments and it was not correlated to panicle brown spot incidence and severity. We are currently studying the microflora associated with discoloured rice grains in order to identify the casual agent of the disease.