Sensitivity of Mucor piriformis to Natamycin and Efficacy of Natamycin Alone and with Salt and Heat Treatments Against Mucor Rot of Stored Mandarin Fruit.

Mucor rot caused by Mucor piriformis is an emerging postharvest disease of mandarin fruit in California. Natamycin is a newly registered biofungicide for postharvest use on citrus and some other fruits. In the study, baseline sensitivity to natamycin in 50 isolates of M. piriformis was determined in vitro. The mean EC50 (effective concentration to inhibit sporangiospore germination by 50%) and MIC (minimum inhibitory concentration to inhibit mycelial growth by 100%) values were 0.59 µg/ml and less than 1.0 µg/ml, respectively. Natamycin at the label rate of 920 µg/ml alone or in combination with 3% potassium sorbate (PS) or 3% sodium carbonate (SC) applied at 20 or 50°C was evaluated for control of Mucor rot on inoculated 'Tango' mandarin fruit. Natamycin alone reduced Mucor rot incidence on stored mandarin fruit from 100% among nontreated control fruit to approximately 30%, a reduction of more than 70% compared to the nontreated control, while 3% PS and 3% SC had no to little control. When applied at 50°C, natamycin and 3% PS reduced Mucor rot incidence by 65.0 and 31.2%, respectively; while natamycin in combination with 3% PS reduced disease incidence by 92.5% compared to the nontreated control after 2 weeks of storage at 5°C. This combined treatment remained effective even when the application of the treatment was delayed for 6 and 12 h after inoculation. However, the effectiveness of the treatments declined when storage was extended to 3 or 4 weeks. Natamycin can be an effective tool to control Mucor rot on mandarin fruit, and minimizing the period of extended storage could help maintain the control efficacy of natamycin.