Central parts of Western Australia are currently experiencing the start of heatwave conditions. These extreme temperatures will extend over southern Western Australia through Friday 10 January and extend across South Australia through Saturday and Sunday. From Sunday 12 January a period of heatwave conditions will be in place across much of southern Australia, with temperatures increasing for most areas through the following week, and peaking on Friday 17 January. There is a possibility that extreme temperatures could remain in place for many areas past that date. Growers should be aware of the tools available to predict heatwaves and the steps that can be taken to minimise vine damage.
Extreme heat after veraison (as grapes soften) may cause berry shrivel or sunburn and grapevine function can be severely impaired. Irrigation management is critical before, during and after a heat event:
- Before the heat arrives, apply irrigation to enable leaf cooling that occurs when leaves transpire. Postpone any canopy manipulation (leaf removal or canopy lifting) that may increase bunch/berry exposure.
- During the heatwave, maintain soil moisture and if using overhead irrigation, apply at night to avoid foliage burn. Also, wind can cause the canopy to roll over and expose the fruit. Look to implement management strategies which can reduce the likelihood of this occurring.
- After the heatwave, irrigate to replace lost soil moisture and decrease soil temperature. Monitor for pests and disease that may exploit damaged berries.
To monitor expected conditions through the event, Bureau of Meteorology (BOM) 7 day forecasts of temperatures can be seen via the Met Eye tool http://www.bom.gov.au/australia/meteye/
A pilot project heatwave warning service was released by the BOM on 8 January http://www.bom.gov.au/australia/heatwave/. This indicates areas likely to experience prolonged high maximum and minimum temperatures over the following three-day period from each start date, covering up to a week ahead. Feedback from this pilot service will be valuable in establishing the usefulness of this product for the grape and wine industry and can be submitted via this link http://www.bom.gov.au/other/feedback/
More detailed information on managing grapevines during heatwaves can be found in the GWRDC Innovators Network factsheet.
Due to recent outbreaks of powdery mildew across warmer areas of SA, NSW and Victoria, the AWRI help-desk team has received several queries regarding the effectiveness of elemental or crystalline sulfur on Powdery Mildew infections, and the possible impact residues might have on fermentation.
Elemental sulfur is classed as a non-systemic protectant fungicide that is best used prior to powdery mildew infection. It will not eradicate an existing powdery mildew infection but it can inhibit mycelia growth and spore germination. This reduces disease development and infection; however good coverage is critical.
For the control of Powdery Mildew, the AWRI recommends a thirty day withholding period (WHP) for elemental sulfur. Applications within the WHP increase the risk of residues entering the must. This may lead to fermentation issues as detailed below. The WHP is set to allow enough time for elemental sulfur to oxidise to sulfur dioxide (SO2) and degrade to levels that reduce the potential for fermentation problems. Research indicates that as the length of time between the application of sulfur in the vineyard and harvest increases, the impact on fermentation declines.
Contact your winery or grape purchaser before using sulfur within 30 days of harvest.
Elemental sulfur residues in must can be converted under the reducing conditions of fermentation to hydrogen sulfide (H2S), which imparts the objectionable odour of rotten-eggs. Hydrogen sulfide may then undergo reactions with other wine components to form mercaptans, which can have detrimental effects on wine quality and which may be difficult to remove.
The main factors that control H2S production from elemental sulfur are:
- pH and temperature (more H2S is produced at lower pH and higher temperature);
- reduction potential (more H2S is produced at higher reduction potential – i.e. more H2S is formed when there is very little, or no oxygen, available); and
- ethanol concentration (less H2S is produced at lower ethanol concentrations).
Given higher H2S production is associated with residual sulfur, it is best to use a yeast which is classified as a low H2S producer.
In white wine production, the problems resulting from the presence of dusting sulfur in the juice can be reduced by careful settling and racking of the juice before fermentation, as the sulfur should settle out with the lees.
It is advisable to ensure adequate YAN (yeast assimilable nitrogen) levels in juice to facilitate good yeast performance and manufacturer recommendations should be carefully followed when rehydrating active dried wine yeast. In addition, the yeast may benefit from rehydration with inactivated yeast preparations, which contain a range of micronutrients including amino acids, lipids and minerals. Higher than usual yeast inoculum rates could also be beneficial, in order to compensate for the possible negative effects residual elemental sulfur might have upon the yeast.
Once the ferment is actively fermenting, aeration in combination with a DAP addition will not only decrease the reduction potential (decreasing the amount of H2S produced), but will boost yeast performance. Aeration should be performed between one third and half-way through fermentation (i.e. when one-third to one-half of the sugar has been consumed). A steady fermentation rate should be maintained by minimising temperature fluctuations, and high fermentation temperatures should be avoided.
It is best to treat wines exhibiting H2S aromas due to residual sulfur with copper just prior to the completion of fermentation (approximately 1 Baumé) in order to avoid stressing the yeast, as it is likely a larger than usual amount of copper will be required. The likelihood of residual copper problems is also reduced if the copper is added at this time, as the yeast will bind up any excess copper that might be present. For more information, see FAQ.
Should you require further assistance regarding heatwaves or elemental sulfur use, please contact the AWRI help-desk on 08 8313 6600 or by email.