MINIMUM GASEOUS SULFUR-DIOXIDE CONCENTRATIONS AND EXPOSURE PERIODS TO CONTROL BOTRYTIS-CINEREA

Sulphur dioxide (SO2) Concentrations required to kill the spores and mycelium of Botrytis cinerea and control post-harvest decay of fresh grapes were determined in chamber studies. Effective concentrations and exposure times were simultaneously expressed in a single dosage term, parts per million-hours (p.p.m.-h). This unit is the product of the concentration Of SO2 in parts per million multiplied by the duration of the fumigation in hours. SO2 dosages that killed 99% of the spores (ED99) were 78.3 +/- 22.3 p.p.m.-h at 0-degrees-C and 20.3 +/- 2.0 p.p.m.-h at 20-degrees-C. These values were determined with continuous concentrations of 25, 50, 100 and 200 p.p.m. SO2 at high humidity. The mycelium on the surface of infected berries was injured by dosages of 10 p.p.m.-h and was no longer viable after fumigation with dosages of 50-100 p.p.m.-h at 0-degrees-C. Infection of inoculated berries at 0-degrees-C compared with the controls was 68,47,2 and 0% after dosages of 20, 50, 100 and 200 p.p.m.-h, respectively. Reduction in the percentage infection after spore inoculation of berries required a larger dosage (approximately 100 p.p.m.-h) than that which controlled mycelial growth (approximately 50 p.p.m.-h). For initial fumigation of grapes after harvest, where control of spores on the surface of grapes is required, 100 p.p.m.-h should be sufficient; for subsequent storage fumigations where control of mycelial growth from latent infections is required, 50 p.p.m.-h should be sufficient. Colorimetric dosimeters have been developed to quantify exposures of humans to SO2 rapidly and inexpensively for safety purposes. They consist of a sealed glass tube containing a colour-indicating substrate that reacts with SO2 to produce a coloured product. One end of the tube is opened before use, and the length of the colour change indicates the accumulated dosage in units of p.p.m.-h. They should be placed inside grape packages before fumigation and examined promptly after fumigation. Their accuracy was sufficient for determination of adequacy of SO2 dosages to control decay.