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Leaf Age Related Partial Resistance to Pyricularia oryzae in Tropical Lowland Rice Cultivars as Measured by the Number of Sporulating Lesions. E. C. Roumen, Associate expert, Netherlands Directorate General for International Cooperation, P.O. Box 20061, 2500 EB The Hague, Netherlands; J. M. Bonman(2), and J. E. Parlevliet(3). (2)Plant pathologist, International Rice Research Institute (IRRI), P.O. Box 933, 1099 Manila, Philippines; (3)Professor of plant breeding, Wageningen Agricultural University, P.O. Box 386, 6700 AJ Wageningen, The Netherlands. Phytopathology 82:1414-1417. Accepted for publication 16 June 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-1414.

To study the effects of leaf age on resistance, plants of four rice cultivars were inoculated with a virulent isolate of Pyricularia oryzae. Susceptibility of leaves declined rapidly with increasing leaf age. In older leaves, fewer sporulating lesions developed per unit of leaf area, and eventually leaves became completely resistant. The period during which newly formed leaves were susceptible, as well as the initial level of susceptibility of new leaves, differed greatly between cultivars. Cultivars with high levels of partial resistance to leaf blast showed typical susceptible lesions, but the resistance in leaves rapidly increased with age, and the initial level of susceptibility of new leaves was low.

Additional keywords: Magnaporthe grisea, rice blast disease.