In the present study onion plants at different ages viz., 15, 30, 45, 60 and 75 days were inoculated separately with conidial suspension of Alternaria porri. With the increase in the host age there was a significant increase in disease development. Highest per cent disease index (54.43) was recorded on plants inoculated at 60 days followed by plants inoculated at 75 days (51.75). Plants inoculated at 45 days (36.25) and 30 days (28.83) showed less per cent disease index relative to 60 days and 75 days old plants. Plants inoculated at 15 days showed the least per cent disease index (21.08). With the increase in the inoculum concentration, there was a significant increase in disease development of A. porri. Highest per cent disease index (57.03) was recorded on plants inoculated with inoculum of 108 spores/ml concentration followed by
plants inoculated with inoculum of 106 spores/ml concentration (52.00) and 104 spores/ml concentration (43.75). Plants inoculated with 102 spores/ml concentration (29.50) showed less per cent disease index