Leaf Age Related Partial Resistance toPyricularia oryzaein Tropical Lowland Rice Cultivars as Measured by the Number of Sporulating Lesions

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