Pg. 4-9 Refinement of SO₂ sheet application in combination with fumigation

During the 2002/03 season, a preliminary study was conducted, aimed at evaluating the efficacy of sulphur fumigation sheets. The results indicated that the sheets have a beneficial effect when placed in the bottom of litchi cartons containing sulphur fumigated fruit. During the 2003/04 season, a comprehensive set of trials was conducted. Refinements included the lowering of the original SO₂ fumigation dosage, establishing the optimal placement of the sheets within the box and the assessing of paper liners for possible use in combination with the sheets.

From the results it is recommended that HLH Mauritius fruit be fumigated at full strength and that two sheets ( one on top and one at the bottom) be used, without a liner. This combination was found to be especially effective when shipping delays may require slightly longer storage periods, and did not increase the SO₂ residues in the aril either.

With McLean’s Red, fumigation at half strength in combination with one sheet was the only treatment to result in acceptable aril SO₂ residue levels. All other treatments (including full strength fumigation without any sheet) resulted in unacceptably high residue levels. Future research should focus on the relationship between maturity and the dosage rate at which the fruit are to be fumigated.

Summary:

• The study evaluates the efficacy of sulfur dioxide (SO₂) fumigation sheets for controlling fungal infections in litchi fruit during storage and shipping.
• Initial trials in the 2002/03 season showed that SO₂ sheets placed in the bottom of cartons reduce fungal infections without increasing SO₂ residues in the fruit aril.
• The 2003/04 season involved more comprehensive trials focusing on:
  • Reducing the original SO₂ dosage.
  • Determining optimal placement of the sheets (top, bottom, or both).
  • Assessing the use of paper liners in combination with sheets.
• Two varieties were tested: HLH Mauritius and McLeans Red litchis.
• Key findings for HLH Mauritius:
  • Full-strength fumigation with two SO₂ sheets (top and bottom) and no paper liner provided the best fungal control.
  • This combination did not increase SO₂ residues above acceptable limits.
  • Use of paper liners increased fungal growth, especially when used with sheets on both sides.
• For McLeans Red:
  • Full-strength fumigation caused excessive SO₂ residues above allowable limits, regardless of sheet use.
  • Half-strength fumigation with one sheet was needed to achieve acceptable residue levels, but fungal control was weaker.
  • The maturity of the fruit (McLeans Red being more mature) influenced efficacy and residue levels, highlighting the importance of maturity in treatment optimization.
• Storage conditions simulated typical export scenarios, with fruit stored at 1°C for 28 to 35 days followed by 7 days at 13°C shelf life.
• Residue analyses confirmed SO₂ levels in the fruit aril remained within safe limits under recommended treatments, mainly for HLH Mauritius.
• The study suggests fumigating HLH Mauritius fruit at full strength combined with two sheets and no liner is best for export handling when long storage or shipping delays occur.
• Further research is recommended particularly on fruit maturity effects on fumigation dosage and residue outcomes.
• Use of paper liners generally reduced the effectiveness of SO₂ sheets in controlling fungal growth and is not recommended.
• The article included detailed trial designs, storage durations, residue testing, and fungal growth assessments with statistical analysis of results.