Effect of the combined treatment with gamma irradiation and sodium dichloroisocyanurate on postharvest Rhizopus soft rot of sweet potato

Rhizopus soft rot, caused by Rhizopus stolonifer, is a serious postharvest disease of sweet potatoes. Generally, the pathogen infects through wounds during storage and shipment after harvesting. To prevent losses, sweet potato packing houses have been using the fungicide dicloran, but many retail markets no longer allow dicloran residues. Recently, gamma-irradiation as a phytosanitary treatment was used to control plant pests and pathogens. In this study, we evaluated the combined effect of gamma irradiation and sodium dichloroisocyanurate (NaDCC) for control of Rhizopus soft rot. For an investigation of the effect of gamma irradiation and NaDCC treatment on conidia viability, we employed a dilution plate method. The results showed that gamma irradiation and NaDCC completely inhibited the growth of R. stolonifer at a 2.8 kGy dose and 20 ppm, respectively. The growth inhibition rate of R. stolonifer compared to the untreated control was 37.2% for the irradiation dose of 0.8 kGy, 61.5% for NaDCC at 10 ppm, and 73.6% for the combined treatment of 0.8 kGy irradiation dose and 10 ppm NaDCC. Based on the colour and weight loss measurement of the sweet potato slices, gamma irradiation treatment did not affect the physical quality. In an in vivo assay, the combination treatment of 0.6 kGy gamma irradiation and 10 ppm NaDCC inhibited the fungal development. These results suggest that NaDCC treatment in combination with gamma irradiation is able to control Rhizopus soft rot disease of sweet potato or other fruits after harvesting.