Effects of Soybean Leaf and Plant Age on Susceptibility to Initiation of Infection by Phakopsora pachyrhizi

To help resolve the question of relationship of soybean leaf and plant age to susceptibility to infection by Phakopsora pachyrhizi, we inoculated 50-day-old plants of cv. Williams 82 with a suspension at 3 × 10³ urediniospores/ml, placed in growth chambers under a range of temperature conditions, and later examined for number of lesions. In addition, plants ranging in age from 20 days to 79 days were inoculated, placed in a greenhouse, and examined by leaf for number of lesions. Results showed that Williams 82 did not vary in susceptibility among leaves at different positions regardless of temperature, or among leaves at different positions (up to 11 trifoliolates) on individual plants ranging in age from 20 to 79 days at inoculation. However, on a per-plant basis, there was a gradual decrease in numbers of lesions as plants increased in age at inoculation. For example, whereas 37 lesions/cm² leaf area were produced on 20-day-old plants, only 10 were produced on 79-day-old plants. Although the study does not rule out the possibility that there might be cultivar dependent differences in age response, it does strengthen the possibility that older plants are less susceptible, and that the increase in disease observed in the field during fall months may be due to an increase in favorability of the environment for disease or increase in inoculum pressure.