Influence of Sulfur Dioxide Fumigant Dose on Residues and Control of Postharvest Decay of Grapes. J. L. Smilanick, Research Plant Pathologist, USDA-ARS, Horticultural Crops Research Laboratory, Fresno, CA 93727. J. M. Harvey, P. L. Hartsell, D. J. Henson, C. M. Harris, D. C. Fouse, and M. Assemi. Research Plant Pathologist (retired), Physical Science Technician, Agricultural Research Aide, Plant Physiologist, Agricultural Research Technician, and Research Apprentice, USDA-ARS, Horticultural Crops Research Laboratory, Fresno, CA 93727. Plant Dis. 74:418-421. Accepted for publication 9 November 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0418.

To determine if sulfite residues in grapes (Vitis vinifera) after sulfur dioxide (SO₂) fumigation were proportional to the product of the SO₂ dose and fumigation period, grapes were fumigated with 1,250 ppm SO₂ for 2 hr, 2,500 ppm for 1 hr, 5,000 ppm for 0.5 hr, or 10,000 ppm for 0.25 hr. The product of SO₂ dose and fumigation period in each test was 2,500 ppm/hr. Sulfite residues after fumigation were not significantly different among the first three time × dose combinations. Grape cultivars Flame Seedless and Thompson Seedless contained an average of 9.6 and 6.1 µg of sulfite per gram of fresh weight, respectively. Fumigation with 10,000 ppm for 0.25 hr left residues about 30% higher than the lower doses and bleached Flame Seedless grapes. To determine if SO₂ fumigant doses recommended for commercial use could be reduced to minimize sulfite residues and still control postharvest decay by Botrytis cinerea, three commercially packaged V. vinifera cultivars in storage at 0 C were fumigated 30 min weekly for 2 mo with 312, 625, 1,250, or 2,500 ppm SO₂. In three seasons of tests, decay control was not significantly different among the doses. Sulfite residues in grapes fumigated weekly with 312 or 625 ppm did not exceed the 10 µg of sulfite per gram of fresh weight residue tolerance, whereas those with higher fumigation doses occasionally did. Commonly used commercial doses (5,000–10,000 ppm for the first fumigation followed by 1,000–2,500 ppm weekly) were higher than needed for decay control. Before a low-dose fumigation recommendation can be made, however, the influence of other factors, such as temperature management, package design, load factor, SO₂ sorption, and venting, must also be examined.

Keyword(s): ion chromatography, pararosaniline, sulfur dioxide residues, table grapes, tetrachloromercurate.