Dr. Carien Coetzee
29 September 2021
Elemental sulphur (S0) is an effective, durable and economical fungicide for the management powdery mildew in vineyards. The advantage of S0 over alternatives include its low cost, good efficacy, low risk of resistance development and its acceptability within various production systems.
But sulphur residues present on the grapes at harvest can have adverse effects and concentrations exceeding 10 µg/g in musts are associated with increased hydrogen sulphide (H2S) formation during fermentation1–3. Growers and winemakers are therefore cautious when applying elemental sulphur closer to the harvest date for fear of increased residual elemental sulphur being transferred into the juice.
Three years of field studies investigated the persistence of S0 in the vineyard as well as the concentrations during pre fermentative winemaking stages. The findings of the study were published in an article titled
Elemental S0 spray was applied at different times and dosages. Different commercial formulations were also tested. The transfer of S0 to the must after harvest and crushing as well as the influence of vinification factors such as whole bunch pressing, length of skin contact and must clarification on the proportion of S0 transferred into the must were also investigated and will be discussed in Part 2 of this blog series.
Materials and Methods
Vineyard trials were conducted on Chardonnay and Riesling vines. S0 was applied using a sprayer operating at 2070 kPa and delivering 935 L/ha through seven hollow cone nozzles.
Two commercial S0 formulations were tested:
1) a micronized formulation
2) a wettable powder formulation
The method of S0 residue quantification is described in the original publication4.
Vineyard trial 1
During trial 1, a single micronized formulation was applied either 68, 40 or 12 days before harvest at a rate of either 2.7 or 5.4 kg/ha (6 treatments). This was compared to a control where no S0 was applied (1 control).
Trial 1: Results
- The control vines did not have any measurable residues
- At harvest, there was no practical difference in S0 residue concentration between the application rate of 2.7 vs 5.4 kg/ha S0 and both dosages delivered approximately the same concentration (which varied according to the timing of application).
- The timing of application was shown to be the major factor affecting the residue concentration:
- Applying S0 12 days before harvest resulted in residues of approximately 40-60 µg/g.
- Applying S0 40 days before harvest resulted in residues of less than 5 µg/g.
- Applying S0 68 days before harvest resulted in residues of even less than the 40 days application and was not statistically different from the control sample where no S0 was applied.
Vineyard trial 2
In trial 2 the first micronized formulation was applied at vériason whereafter sprays were applied at approximately two-week intervals and continuing until either 50, 35, 22 or 8 days before harvest. The treatment which ceased 50 days before fermentation was applied at a rate of 2.7 kg/ha S0, whereas the latter three timing regimes received applications of either wettable sulphur at 2.7 or 5.4 kg/ha S0 or micronized sulphur at 5.4 kg/ha S0 (10 treatments). This was compared to a control where no S0 was applied (1 control).
Trial 2: Results
- In trial 2, both the application (formulation-concentration) and the timing of application impacted the final residue concentration on the grapes at harvest.
- When applied until eight days before harvest the residue concentration exceeded 10 µg/g (all three rates of application). The 5.4 kg/ha micronized sulphur delivered the highest concentration of residual S0, while the wettable sulphur formulations resulted in lower residual S0. The application of 2.7 kg/ha S0 in wettable powder formulation was only about one third the concentration of those following applications of 5.4 kg/ha in a micronized form.
- When the sprays ceased at 22 days before harvest residues from the wettable powder at the lower rate resulted in an average concentration of around 6 µg/g. Applications of either formulation at the higher rate resulted in levels exceeding 10 µg/g.
- At 35 days before harvest, all three S0 treatments (formulation and dosage) resulted in S0 residues below 5 µg/g.
- At 50 days before harvest, the micronized formulation at 2.7 kg/ha S0 resulted in a residue concentration of less than 0.5 µg/g.
Vineyard trial 3
In trial 3 the vines were sprayed at a rate of 4.5 kg/ha S0 in either micronized or wettable powder formulation beginning on 67 days before harvest and continuing at approximately two-week intervals until 54, 38, 25 or 12 days before harvest. An additional treatment was included that received micronized sulphur at 4.5 kg/ha in the first application and 2.2 kg/ha in the final two applications, 54 and 38 days before harvest.
Trial 3: Results
- Residues were near or well above 10 µg/g when either formulation was applied until either 25 or 12 days before harvest.
- At 12 days before harvest, the micronized sulphur resulted in a higher residual S0 concentration when compared to the wettable sulphur. At 25 days before harvest, the opposite trend was observed with the wettable sulphur resulting in a higher residual S0 concentration when compared to micronized sulphur.
- There was no significant difference in residue concentration between the 38 and 54 days before harvest
S0 application rate and specific formulation affected the residual S0 concentration on the grapes at harvest. Residues consistently exceeded the 10 µg/g threshold when S0 was applied within 25 days of harvest and in all the trials, only those treatments ceasing more than 50 days before harvest were below 1µg/g. The S0 residues did not exceed 4.6 µg/g when application ceased by 35 to 38 days before harvest and were typically below 3 µg/g.
In general, residue concentrations were lower for wettable powder formulations versus a micronized formulation applied at the same time and dosage. The S0 residue concentration increased proportionally to the application rate when the timing and the formulation were constant. In all the trials, ceasing the application 35 days before the harvest date resulted in S0 residues below the 10 µg/g concentration associated with increased H2S production.
Part 2 of this two-part series will briefly outline the fate of these residues during pre fermentative grape and must processing operations4.
(1) Acree, T. E.; Sonoff, E. P.; Splittstoesser, D. F. Effect of Yeast Strain and Type of Sulfur Compound on Hydrogen Sulfide Production. American Journal of Enology and Viticulture 1972, 23 (1), 6 LP – 9.
(2) Rankine, B. C. Nature, Origin and Prevention of Hydrogen Sulphide Aroma in Wines. Journal of the Science of Food and Agriculture 1963, 14 (2), 79–91. https://doi.org/10.1002/jsfa.2740140204.
(3) Schutz, M.; Kunkee, R. E. Formation of Hydroge Sulfide from Elemental Sulfur During Fermentation by Wine Yeast. American Journal of Enology and Viticulture 1977, 28 (3), 137–144.
(4) Kwasniewski, M. T.; Sacks, G. L.; Wilcox, W. F. Persistence of Elemental Sulfur Spray Residue on Grapes during Ripening and Vinification. American Journal of Enology and Viticulture 2014, 65 (4), 453–462. https://doi.org/10.5344/ajev.2014.14027.
Click image to download the article