Effect of Leaf Age and Nodal Position on Receptivity of Rice Leaves to Infection by Pyricularia grisea

The effect of leaf age and nodal position on leaf receptivity to rice blast, caused by Pyricularia grisea, was studied by inoculating potted Rosemont plants with blast pathogen race IC-17, isolate 92T107. at different growth stages. Regressioti equations were used to describe the effect of leaf age (degree days > IOC) and nodal position on lesion density, the relative frequency of lesion appearance, and the lesion area distribtition. The number of sporulating lesions per cmʹ of inoculated leaf area was highest for leaves inoculated early in their development and progressively lower for leaves inoculated later in their development. The average lesion density for less than 1- day-old leaves at nodal position 6 was about four times the density for 10-day-old leaves of the same nodal position. Lesion density was similar for leaves of less than 1-day-old from the 4th to the 7th nodal position, but dropped sharply from the 7th to the 11 th nodal position. The flag leaf was the least susceptible of all nodal positions. The average lesion density for young leaves of about 1-day-old at nodal position 6 was about 27 times the density for young flag leaves of the same age. The incubation period increased with leaf age. but was not obviously affected by leaf nodal position. Lesion area increased linearly with time, but was not affected by leaf age and nodal position. The results are of use for epidemiological investigations which rely on detailed quantitative leaf susceptibility data for accurate blast forecasts.