Inactivation of Rhizopus stolonifer Sporangiospores by Single and Combined Treatments of Heating, Chilling, and Gamma-Irradiation

Germinating sporangiospores of Rhizopus stolonifer (Ehrenb. ex Fr.) Lind. were more susceptible than nongerminating sporangiospores to inactivation (loss of colony forming ability) by single treatments of heating, chilling, or gamma-irradiation. Susceptibility to heat injury increased with increasing pretreatment incubation time up to 6 hours maximally allowed for germination. Sensitivity to radiation, in contrast, was greatest in spores incubated for 3 hours. The temperatures (39-46°C) and irradiation doses (50-200 krad) used in most treatments were below the maxima tolerated by fruits commonly attacked by this fungus after harvest. Although the nongerminating spores were markedly resistant to single applications of heat or radiation within those limits, a strong inactivation effect (<1% survival) was obtained when radiation + heat (125 krad + 46°C/5 minutes) were applied in sequence. The interaction was less pronounced with treatment in reverse sequence. With germinating spores (6-hour incubation), an even greater synergistic effect (<0.1% survival) resulted from heating (39°C/5 minutes) followed by irradiation (125 krad). Thus, maximum synergism was obtained in nongerminating spores with a radiation-heat sequence and in germinating spores with a heat-radiation sequence. With both nongerminating and germinating spores, heat and radiation had a stronger interaction than heating and chilling.