A wet season update – managing fungal diseases between veraison and harvest

11 January 2024

During the Christmas and New Year period, extensive rainfall, and hail in some regions, affected several grapegrowing regions in Australia. These conditions were conducive to the development of fungal diseases such as downy mildew, powdery mildew and Botrytis in grapevines. Tools and resources available to grapegrowers and winemakers to manage disease assessments are detailed within this Ask the AWRI article on assessing grapes for disease.

Looking ahead, the forecast for rainfall from January to March suggests a likelihood of above-median rainfall in the south-east, while the north and west of Australia may experience below-median rainfall. Immediate action is crucial to mitigate the risk of significant crop losses from fungal disease. This eBulletin outlines strategies to reduce both the risk and severity of fungal infections during periods of heightened disease pressure.

General tips for best-practice disease control
In high-vigour blocks, consider shoot thinning, trimming, leaf removal and bunch thinning to enhance canopy openness, allowing better light and air penetration. This practice not only accelerates canopy drying post-rainfall but also facilitates improved spray penetration and chemical deposition.

Vineyard floor management, such as keeping mid-row cover crops low to support airflow and canopy drying, is another strategy to put in place. In addition, if a neighbouring vineyard has opted to ‘rest’ their block, a strategic approach could involve treating a few boundary rows to mitigate disease pressure stemming from those vines.

Chemical options for powdery mildew, downy mildew and Botrytis control are listed in the ‘Dog book’. These recommendations are for fruit destined for export wine. Agrochemicals applied to fruit destined for the domestic wine market have shorter withholding periods and these are found on the label. Ensure you consult with your grape purchaser or the AWRI if you are considering chemical options that do not follow the ‘Dog book’ recommendations for export wine.

Effective spray coverage is critical for successful disease control. Test your spray coverage using water sensitive papers or a kaolin-based clay sunscreen. Once your equipment is set up well, check that you’re applying the correct chemical rate to avoid underdosing. Operating a spray unit in the same direction for every spray application runs the risk of missing the same hard-to-reach parts of the canopy each time, potentially creating a disease hotspot. Spraying in a different pattern so that the opposite direction is travelled up the row has potential to improve overall coverage.

Ask your supplier about available products and their features, particularly regarding rainfastness (the ability of a product to remain on the target after rain or heavy dew). Different products perform differently during prolonged wet periods. Also, check the compatibility of these products with adjuvants designed to improve rainfastness. The relationship between spray coverage and retention is a function of agrochemical mixes and rates, spray application volumes, canopy type and the target canopy tissues (leaves, wood or bunches of fruit) (Gaskin et al. 2002). Choosing the right product for the current conditions is crucial, as it might differ from what’s suitable during drier years.

In situations where resources and time are constrained, prioritisation becomes crucial. Identify your most profitable blocks and concentrate on securing a healthy crop from those areas. Monitoring is important and instructions on how to recognise powdery mildew, downy mildew and Botrytis are provided in the resources linked below.

Downy mildew
Applying preventative sprays prior to an infection event is crucial to controlling downy mildew in wet years. Fungicides are in high demand during wet seasons, so keeping a chemical supply available is wise. Most registered products for downy mildew act as protectants. Avoid using these on an active infection to prevent resistance, which could lead to losing the efficacy of the product in the future. If you have had a downy mildew infection event and are using a metalaxyl (group 4) for curative control, then a second group 4 spray on a 7–10-day interval is required.

While the AWRI website provides information on phosphorous acid use, it is crucial to communicate with your winery or grape purchaser before applying this product. Some export markets maintain low tolerance for phosphorus acid residues, necessitating careful consideration before its use.

Additional downy mildew resources

Powdery mildew
The optimal strategy for combatting powdery mildew involves consistent application of protective chemicals early in the season to protect all growth from this disease. Preventing its establishment in the initial stages is pivotal for minimising the disease load later on. Unfortunately, this season posed challenges, hindering the application of early sprays in numerous vineyards or diluting those already applied due to heavy rain periods. These conditions have heightened the risk of powdery outbreaks. In such scenarios, taking preventive action to avoid a widespread powdery disease epidemic becomes absolutely essential.

Elemental sulfur is a multi-site fungicide which is not susceptible to resistance and has some curative activity. To gain control of an existing infection using sulfur, good spray coverage is essential. An active powdery mildew infection may repel water, so ensuring adequate spray coverage is crucial. Consider using an adjuvant to enhance coverage on challenging-to-wet surfaces. Some sulfur products include built-in adjuvants, enhancing their spread on plant surfaces. For other sulfur products, consult your chemical supplier to determine if an adjuvant is needed to improve spread.

Some production zones across Australia are experiencing earlier ripening conditions compared to previous seasons. Ensure applications of sulfur adhere to the pre-harvest cut-off timeframes if used for late-season powdery mildew management.

Some single-site chemicals exhibit curative activity against powdery mildew but using them on active infections is not advised due to the heightened risk of resistance development. CropLife Australia recommends applying single-site fungicides preventatively before colony formation for effective powdery mildew control.

Additional powdery mildew resources

Botrytis bunch rot
Late-season Botrytis expression can create significant challenges for growers formulating strategies for variable site locations and varietal mixes. This requires a sound understanding of the pathogen and how it interacts with various late-season fungicide modes of actions. Many biological fungicides provide optimal protection when used prior to visual expression of disease symptoms. Disease control may be less than optimal when products are employed reactively for curative purposes. All spray options for late-season Botrytis management require good coverage of bunches and berries. Strategic canopy and fruit zone management not only enhance spray droplet coverage but also promote better airflow around the bunch zone.

When Botrytis is already evident in tightly clustered bunches, causing juice leakage from the berries, spraying might not be effective. In such cases, hand removal of affected bunches or selective harvesting may be an option. Controlling for downy and powdery will also help manage Botrytis.

Research from New Zealand suggests that maintaining 70-90% bunch exposure by selectively removing leaves from late flowering to pre-harvest significantly reduces Botrytis risk (Andrew and Lupton 2013). However, caution is necessary with leaf plucking to avoid sunburn, especially in hot regions, if excessive leaf removal occurs later in summer.

Botrytis can cause problems during winemaking as it produces an oxidative enzyme called laccase, which, in the presence of oxygen, can cause oxidative spoilage and rapid browning of must. Laccase activity is the main marker for Botrytis infection and can be detected using enzymatic test kits so it is advised to have these ordered or in stock. Botrytis can produce polysaccharides that cause clarification problems in juice and filtration issues in wine. It can also impart mouldy flavours, and together with bacteria, increase the chance of elevated levels of acetic and lactic acid. The AWRI fact sheet Managing Botrytis-infected fruit, must and wine details winemaking strategies for processing both white and red grapes affected by Botrytis.

Additional Botrytis resources

Don’t forget to look after yourself and your mates
When conditions are challenging, it can increase stress and add to existing pressures. Check on your mates and if you are in a difficult spot, it is important to talk about it. There is help available in your local community and from these organisations: Beyond BlueLifelineHeadspaceR U OK?Black Dog Institute.

Additional resources on fungal diseases and dealing with wet conditions

This eBulletin is supported by Wine Australia with levies from Australia’s grapegrowers and winemakers and matching funding from the Australian Government. Liz Riley (Vitibit), Warren Birchmore, Philip Deverell (Pernod Ricard Winemakers) and Scott Paton (Nutrien Ag Solutions) are thanked for their contributions.

For more information or assistance, please contact the AWRI helpdesk on 08 8313 6600 or helpdesk@awri.com.au.