Persistence of elemental sulfur spray residue on grapes during ripening and vinification

Elemental sulfur (S⁰) is commonly used to control powdery mildew in vineyards, but S⁰ residues in musts have been correlated with increased hydrogen sulfide (H₂S) and sulfurous off-aroma formation during fermentation. As a consequence, S⁰ is often used sparingly late in the season, but defining appropriate preharvest intervals for S⁰ sprays has been challenging due to limited data on S⁰ persistence in vineyards and during prefermentation operations. Using a new quantification method, S⁰ residues were monitored in the vineyard over three years of field studies. Treatments varied in commercial formulation, application rate, and timing of the last application before harvest, all of which affected S⁰ concentrations on the fruit at harvest. Residue concentrations generally were lower for a wettable powder versus a micronized formulation applied at the same rate and timing and increased proportionally to the application rate when timing and formulation were constant. In all years, ceasing application ≥35 days prior to harvest resulted in S⁰ residues below the 10 μg/g concentration associated with increased H 2S production in several previous studies. S⁰ residues >1 μg/g correlated with increased H ₂S production in our current work and were observed on all fruit sprayed within 56 days of harvest. However, clarification decreased S ⁰ in must by >95% prior to fermentation in all treatments. Furthermore, fermentation on treated skins increased H₂S formation nearly 3-fold over fermentations without skin contact. Collectively, these results indicate that S⁰ residues are likely of low concern in white winemaking, whereas residue concentrations in red fermentations can exceed those associated with increased H₂S production when some S⁰ sprays are applied within eight weeks of harvest.