Warning about disease pressure

25 November 2022

Following an eBulletin on 27 October 2022, which focused on downy mildew, this is a reminder that weather conditions in some regions this season are also likely to be highly conducive to the development of powdery mildew and botrytis bunch rot. These diseases have a habit of ‘flying under the radar’ and then emerging just before Christmas or harvest at levels that are very difficult to control. Growers are encouraged to take steps now to avoid disease problems closer to harvest.

Powdery mildew
Powdery mildew in grapevines is caused by the fungus Erysiphe necator. The distinctive grey-white powdery growth affects green tissue, including leaves, shoots and fruit, and has negative impacts on both grape yield and quality. All grapevines are susceptible to powdery mildew, but the risk is greatest in vineyards that were affected by an outbreak in the previous season. Some varieties are particularly susceptible, namely Chardonnay, Semillon and Verdelho. Powdery mildew can develop and progress independently of free moisture. The conditions that favour the development and spread of the disease are mild weather (20 – 30°C), low light levels and relative humidity greater than 40%.

Controlling powdery mildew
The ideal spray program for powdery mildew involves regular applications of protectant chemicals early in the season, effectively protecting all growth from the disease. Stopping the disease from getting established early in the season is critical in reducing the disease load later in the season. Unfortunately, this season has not allowed for early-season sprays to be applied in many vineyards, or the sprays that have been applied have been hit with periods of heavy rain which may have diluted them. These circumstances have created a high level of risk for powdery outbreaks. Where this is the case, action to prevent an epidemic of powdery disease is paramount.

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. This can be confirmed using wettable paper or a short run with a kaolin clay tank mix. If there are gaps in coverage, it is advisable to adjust one setting on your tractor or spray unit at a time and re-test until good coverage is achieved.

An active powdery mildew infection can be water repellent, so it is important to check that enough spray is reaching all targets. Growers should consider using an adjuvant to improve coverage on what is a hard to wet surface. Some sulfur products such as Thiovit Jet and Microthiol Disperss have an adjuvant built into the formulation, which improves spread on the plant surfaces. Many other sulfur products may require some assistance with spread, so check with your chemical on-seller if an adjuvant is required.

Some single site chemicals are known to have curative activity against powdery mildew but their use against active infection is not recommended due to the increased risk of resistance development. The CropLife Australia recommendation for single site fungicides is that they should be applied before colony formation (preventatively) for powdery mildew control.

Elemental sulfur is in short supply so it’s important to plan how you will use the chemical stock you can access. It is recommended to allocate sulfur to those blocks where a powdery curative is required and use other agrochemicals where powdery mildew has been contained and prevention is still an option. Monitoring is important and instructions on how to recognise powdery mildew are provided in the resources linked below. 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.

Operating a spray unit in the same direction for every spray runs the risk of missing the same hard-to-reach parts of the canopy each time, potentially creating a powdery mildew hotspot. Spraying in a different pattern so that the opposite direction is travelled up the row has the potential to improve overall coverage.

Canopy management practices are an essential part of managing disease risk, especially in challenging seasons. Manipulation of canopy conditions to allow light, air and agrochemical sprays to reach the ‘inside’ foliage helps create a more challenging environment for fungal diseases, including powdery mildew, downy mildew and botrytis bunch rot.

Botrytis bunch rot
Botrytis bunch rot is caused by the pathogen Botrytis cinerea, which is a common environmental fungus that enters grapevine tissue through wounds (including scarring from powdery mildew infection and natural openings) and takes hold once the right conditions are met. Effective Botrytis management requires an integrated approach using both chemical and cultural control measures.

Flowering (E-L 25) and pre-bunch closure (E-L 31) are recognised as the critical growth stages for Botrytis sprays; however, rain and/or high humidity at or around harvest can cause devastating losses in a short period of time. Getting good coverage of the bunch zone is critical with any Botrytis spray, so take the time to check that you are hitting the intended target. Consider the potential entry points for Botrytis infection from initiation of flowering onwards, factoring how growing conditions could affect the risk of Botrytis infection. Also consider previous disease load in your vineyard and the susceptibility of your variety mix when designing control strategies.

New Zealand research investigating various cultural practices to reduce Botrytis risk in high pressure years found that leaf plucking to maintain 70-90% bunch exposure from late flowering to pre-harvest was the most effective treatment (Andrew and Lupton 2013). However, leaf plucking should be applied with care, since excessive leaf removal may result in sunburn, especially if applied later in summer in hot regions. Removing the trash from inside and around developing bunches post-flowering, using a harvester or pulsed air leaf plucker, can also reduce Botrytis risk by taking away a source of Botrytis inoculum and altering the bunch environment for improved airflow and bunch drying.

Andrew, R., Lupton, T. 2013. Understanding Botrytis in New Zealand vineyards, Auckland, New Zealand: New Zealand Winegrowers: 215 p.

Additional resources
· The AWRI’s resources on powdery mildew
· The AWRI’s resources on Botrytis
· Ask the AWRI publication – Spray application
· Wine Australia’s resources on powdery mildew
· Wine Australia’s resources on Botrytis

AWRI webinars
· Getting the most out of ‘biological’ sprays for Botrytis control (October 2021)
· Late season Botrytis: the disease and options to control it (February 2021)
· Fungicide resistance in Australian viticulture (January 2022)

Liz Riley (Vitibit), Kerry DeGaris (Treasury Wine Estates), Warren Birchmore (Accolade Wines), Philip Deverell (Pernod Ricard Winemakers), Scott Paton (Nutrien Ag Solutions) and Scott Mathew (Syngenta Australia) are thanked for their contributions to this eBulletin.

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