The effect of heat on the growth and recovery of Aspergillus spp. from the mycoflora of onion seeds

Temperature optima for growth of Aspergillus niger and A. flavus on agar lay between 30°C and 35°C; the optimum for A. fumigatus was 40°C. A. flavus grew less rapidly in culture than the other two species. These fungi were recovered when a single sample of onion seeds, produced in Sudan, was plated out onto agar and incubated over a range of temperatures from 15°C to 45°C. In line with the growth optima of the fungus, the recovery of A. niger was greatest between 25°C and 35°C; recovery of A. flavus was greatest between 30°C and 35°C and recovery of A. fumigatus greatest between 40°C and 45°C. Hot‐water treatment for durations of up to 60 min at 50°C failed to reduce the incidence of recovery of seedborne A. niger and A. flavus from seeds incubated at 30°C on agar; A. fumigatus was not recovered from seeds treated in this way. However, when seeds were hot‐water treated at 60°C and incubated on agar at 30°C, A. niger was virtually eliminated by a treatment duration of 15 min or more; the incidence of recovery of A. fumigatus was significantly increased compared with the 50°C treatment and there was no change in the incidence of A. flavus. Hot‐water treatment at 60°C for more than 30 min significantly reduced seed germination.