Nitrogen form and silicon nutrition effects on resistance to blast disease of rice

The effect of N form and Si nutrition on rice (Oryza sativa L.) susceptibility to blast disease (caused by Pyricularia oryzae Cav.) was assessed in the greenhouse with nutrient solution culture. The N form supplied to the susceptible cultivar IR50 affected the relative infection efficiency (RIE) of P. oryzae measured as lesions/cm² leaf. Plants given NO₃ ^- were more susceptible than plants receiving NH₄ ⁺-N. This result may partially explain why plants grown in nonflooded soil, where NO₃ ^- is the main source of inorganic N, are more susceptible to blast than plants grown in flooded soils, where NH₄ ⁺ is the main inorganic N source. Nitrate-N and Mn concentration were higher in leaf blades of plants grown with NO₃ ^-. Total-N, Si, and Fe concentration were not affected by N form. The addition of Si significantly increased IR50 resistance to blast. With 2.2 mol m^-3 Si in solution, RIE values were lower by more than 90% than the control with no Si added in solution. The effect of Si accumulation in leaves at various positions was further studied in cultivars having differing levels of resistance (IR50, IR36, and IAC165). Silicon addition significantly reduced RIE in the three cultivars. Silicon concentration in the topmost leaves (the only leaves showing typical blast lesions) was not significantly different among the three cultivars when 2.2 mol m^-3 Si was used. Silicon was an important component in the mechanism of resistance to blast and it was effective regardless of the original level of resistance of the cultivar used. Contribution from the Agronomy Unit, Agronomy-Physiology-Agroecology Division, International Rice Research Institute (IRRI), P.O. Box 933, 1099 Manila, Philippines, and Colegio de Postgraduados, Mexico. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. degree.