Minimum Constant Sulfur Dioxide Emission Rates to Control Gray Mold of Cold-Stored Table Grapes

Abstract

Sulfur dioxide generating pads are used worldwide for the control of gray mold, caused by Botrytis cinerea, during long-term cold storage and/or export shipment of table grapes. We propose using an emission rate (measured in µmol SO₂ per kilogram of fruit exposed per hour of exposure) to assess the amount of sulfur dioxide that a generating pad should emit during storage/shipment. Inoculated berries were weighed and placed inside gas-tight containers attached to a flow-through fumigation system and continuously exposed to 0.00, 0.25, 0.50, 1.00, 2.00, or 3.00 µL/L sulfur dioxide (inlet concentrations) at 0°C for 6 weeks. These low concentrations simulated continuous emission of sulfur dioxide from a hypothetical slow-release generating pad. Grids with spray-inoculated Redglobe berries and open petri dishes with a central syringe-inoculated berry in contact with surrounding healthy berries were used for evaluating gray mold incidence and gray mold nesting, respectively. None of these sulfur dioxide emission rates completely controlled berry decay, while nesting was effectively prevented by sulfur dioxide emission rates of 3.6 and 5.5 µmol/kg hr (inlet concentrations of 2.0 and 3.0 µL/L). Both gray mold incidence and nesting were higher among control fruit at 95 to 98% relative humidity than at 65 to 75%, but no significant differences were observed when an inlet sulfur dioxide concentration of 3.0 µL/L was applied. Sulfur dioxide was continuously sorbed by the grapes during exposure and did not noticeably injure any fruit in these tests.