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Physiology and Biochemistry

Inhibitory Activity of Sulfur Dioxide on the Germination of Spores of Botrytis cinerea. J. L. Smilanick, U.S. Department of Agriculture, Agricultural Research Service, Horticultural Crops Research Laboratory, 2021 South Peach Avenue, Fresno, CA 93727; P. I. Hartsell, D. Henson, D. C. Fouse, M. Assemi, and C. M. Harris. U.S. Department of Agriculture, Agricultural Research Service, Horticultural Crops Research Laboratory, 2021 South Peach Avenue, Fresno, CA 93727. Phytopathology 80:217-220. Accepted for publication 28 August 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-217.

Spores of gray mold (Botrytis cinerea) were exposed to sulfur dioxide for 4 hr in dilute, buffered grape juice at pH 3.0, 3.5, and 4.0, acidities approximating those of mature table grapes. Concentrations of 2.8, 9.6, or 33.8 ?g/ml total SO₂, respectively, were required to kill 99% of the spores. The concentration of sulfur (IV) species (SO₂ and HSO₃^?) at each acidity was calculated using the Henderson-Hasselbalch equation from the total sulfur dioxide concentration that killed 50% (LD₅₀) of the spores. ?Molecular? sulfur dioxide (SO₂) was present at 0.10?0.11 ?g/ml, whereas bisulfite (HSO₃^?) ranged from 1.6 to 17.1 ?g/ml at LD₅₀. This confirmed that the ?molecular? form of SO₂ was the primary toxic species. Spores were exposed to SO₂ in dilute, buffered grape juice at pH 4 for 4 hr at 0, 10, 15, 20, 25, and 32 C. Concentrations of 886, 343, 300, 57.8, 49.3, and 25.8 ?g/ml total SO₂ were required, respectively, to kill 99% of the spores. The concentration and distribution of total SO₂ residues within fumigated grapes were determined by ion chromatography. After a 30-min fumigation with 5,000 or 10,000 ppm SO₂, the skin and pulp contained 7.6?16.5 and 0.7?0.9 ?g/g SO₂, respectively. Residues on or near the surface of fumigated grapes approached the toxic dosage above 20 C, whereas residues in the pulp were insufficient to kill the spores in dose-mortality tests.