Botrytis bunch rot

The seasons are getting increasingly unpredictable. If you're unlucky enough to have wet, humid weather leading up to harvest, these are the perfect conditions for an explosion of Botrytis bunch rot that is living under the radar in your vineyard.

Why should you care?

Botrytis bunch rot costs the Australian wine industry an estimated $70-75 million each year. As well as making machine harvesting and processing harder, the shrivelled and rotting fruit reduces yield and can taint wine, making downgrades or rejection likely. Botrytis bunch rot can take hold and spread within days. Only a relatively small amount of infected grapes can result in the rejection of an entire block.

Know your enemy

Understanding the disease and the risk factors in your vineyard will inform your decisions about how best to tackle it.

What is Botrytis Bunch Rot?

Botrytis bunch rot (also known as grey mould) is a disease caused by the fungus Botrytis cinerea. It causes a soft rot of grey mould on berries and can destroy whole bunches, typically near harvest.

Symptoms of Botrytis

Botrytis spores are present in most vineyards as the fungus lives on many plant species, not just grapevines. Spores survive over winter in old bunches, dead vine tissue or decaying plant matter on the vineyard floor. Early infections can enter through small wounds on developing berries from flowering onwards, then remain latent until ripening. As sugar levels rise and weather conditions become wet or humid, the fungus can become active and symptoms may appear.

Because of this hidden phase, the threat of Botrytis is often forgotten. Botrytis infections can stay invisible in bunches for months from flowering until grapes increase in sugar content (to approx. 9.5 Baumé) and the ‘right’ weather occurs. When these conditions line up, it can take off very quickly.

Is it Botrytis?

There are other moulds and rots that can look quite similar to Botrytis bunch rot, so it’s important to check.  A casual visual assessment may not be enough. Pull a few bunches apart and look closely for grey spores or ‘slip skin’ berries, where the skin slides over the flesh because it has started to rot under the surface. If in doubt, check with your agronomist.

How does the Botrytis fungus get into grapes?

The fungus can enter berries through natural openings or wounds in the skin. Thinner-skinned varieties are generally more susceptible. The scars left after flower caps fall off can provide an early entry point, but infections are often kept under control by naturally-occurring anti-fungal compounds in green berries. Although infections can occur at any time during berry development when moisture and damage are present, symptoms usually only become apparent as berries change colour and begin to ripen, particularly under wet or humid conditions.

Skin damage from powdery mildew, light brown apple moth, birds, hail, sunburn, machinery and berry splitting also create entry points for the fungi. 

Once inside bunches, Botrytis feeds on damaged material (e.g. bunch trash, damaged berries and juice) and can spread within and between bunches via insects, wind-borne spores, rain splash and direct bunch-to-bunch contact. Under favourable conditions, the disease can spread in a matter of hours.

Watch this AWRI video (from 4:28) with Dr Kathy Evans who explains how to identify Botrytis in the field.

Botrytis bunch rot at a glance

What it infects	Berries (especially in white varieties) and damaged vine material (e.g. leaves, shoots, tendrils, rachis and flower caps)
When it starts	From flowering until harvest, with latent infections appearing at ripening
Favourable conditions	Wet weather, high humidity, tight bunches, damaged or split berries, poor airflow
How it spreads	Wind, rain splash, insects (e.g. light brown apple moth)
What it looks and smells like	Grey or brown mould, musty smell
High-risk varieties	Semillon, Sauvignon Blanc, Pinot Noir, Riesling, Chardonnay

 

What you can do

Recommended treatments vary between regions and grape varieties, and for yield and quality goals. The likely cost of treatment relative to the season’s grape prices is also a factor to consider.

• Before flowering. Reduce old trash and overwintered spores by removing old bunches and tendrils from shoots at pruning. Monitor for insect pests, in particular light brown apple moth, to determine if action is necessary.
• Flowering. Reduce berry damage from pests and diseases. Apply Botrytis sprays to the bunch zone to protect wounds from infection as per recommended preventative strategies.
• Bunch closure. Remove leaves and shoots around bunches to improve airflow and reduce humidity. Manage irrigation to avoid berry damage and splitting. When warm and wet weather is forecast, use Botrytis fungicide sprays to protect the bunches.
• Ripening. Watch the weather closely and monitor high-risk blocks for early symptoms, especially after warm, humid or wet periods. Check inside bunches. When warm and wet weather is forecast, use approved Botrytis sprays to protect bunches.
• Close to harvest. Adjust harvest timing and picking order so at-risk blocks are picked first to reduce spread and minimise quality loss.

What is the risk of Botrytis bunch rot in your vineyard this season?

A number of Tasmanian growers are benefiting from adopting a systematic approach to identifying their Botrytis risk and managing it. Whilst they are doing it under field trials with the support of the Tasmania Institute of Agriculture (University of Tasmania), there are some great resources that are available for you to use (see below).

Botrytis checklist	To help identify and rank the factors in your block that will influence the likely severity of Botrytis and management choices.
Botrytis management	Reducing spore load, infection and spread.
Botrytis: questions and answers	Identifying the disease and practical management.
A systematic trial approach	Step by step instructions on how to approach Botrytis systematically, rather than shooting from the hip.

How to detect botrytis early

Quick, decisive action is essential. In the right weather conditions growers have seen bunches go from fine one day and mouldy the next. Sometimes fruit won’t be quite ready to pick but leaving it any longer could cost the whole crop. Sound, quick decision making requires facts and understanding.

Constant vigilance is critical

The key weather factors that favour Botrytis are:

• Wetness. Fungal spores need water to germinate. This can be from rain, fog, mist or dew. Even high humidity could be enough to cause condensation inside vine flowers or grape bunches that will set it off.  Conversely, a drying wind that gets into and between bunches will dry surfaces and reduce fungus germination.
• Temperature. Once germinating, temperatures of 18-21^ºC (day or night) will result in the fastest spread of infection. The longer the wetness, the less important the temperature.  

Weather forecasting

The more accurately you can predict the weather at your vineyard location, the better. When conditions favour Botrytis, acting early gives you the best chance to stop it before it takes hold. 

What are other growers using to better predict and monitor vineyard conditions?

• Weather forecasting apps. More than a simple BOM weather forecast, apps can pull data from local weather stations and use past weather conditions to produce forecasts tailored for your property. Some also highlight when conditions are ideal for Botrytis development and suitable for spraying chemicals.

• Disease watch services. Ask your regional association if it has Crop Watch, Grow Care or regional weather stations funded under Wine Australia’s Regional Program. These services provide local vineyard information and more relevant data in greater detail than the BOM.
• Botrytis decision support tools. The Botrytis Decision Support Model, developed by New Zealand Plant and Food with New Zealand growers, can help predict Botrytis risk in your vineyard. By replacing New Zealand weather data with your own local data, the model becomes relevant to your conditions.

Spotting Botrytis bunch rot and critical levels

Walk your blocks regularly, especially high risk or previously infected ones and after substantial rainfall post-veraison. Every couple of days is reasonable in high-risk periods. Look for skin damage, slip skin and early symptoms. Pull apart bunches and smell inside where Botrytis can hide. Target bunches in clumps, around posts and in the shadiest parts of the vine.

Non-chemical control

The most effective Botrytis bunch rot control is completed well ahead of harvest. 

Growers using the University of Tasmania’s systematic ‘predict-monitor-act’ approach are seeing better results in high-risk seasons.  

Vineyard design

• Avoid planting highly susceptible varieties in Botrytis prone areas (low lying, poor drainage, next to lakes and large dams).
• Plant rows so air can flow down rows and dry bunches after rain.
• Choose a trellis type and pruning techniques that open up the bunch zone and let air move through to dry bunches.
• Choose mid-row crops that don’t restrict airflow down rows. 

Canopy and bunch control

Dense canopies and high crop loads hold moisture and slow drying, which creates ideal conditions for Botrytis. Manage canopy density and crop load to meet yield and quality goals while also reducing Botrytis risk. In high-risk seasons, reducing crop load and opening the canopy around bunches can help slow Botrytis spread. When reducing crop load, target tight bunches in clumps and around posts for removal. 

• Remove leaves and shoots to open the canopy
• Control excessive vigour with appropriate management of water and nutrients
• Reduce irrigation near harvest to avoid berry splitting

Learn more about canopy management and water monitoring and irrigation technology being used effectively by growers.

Pest and disease control

The Botrytis fungus needs an entry point to infect a berry. Skin damage from light brown apple moth (LBAM), powdery mildew, sunburn and other pests and diseases or mechanical damage creates these entry points. Protecting berries from damage reduces Botrytis risk.

Further information is available on the AWRI website.

Vine clean up by shaking

Dried up bunches and tendrils from last season and rubbish from flowering in this season’s bunches provide sites where Botrytis can feed and germinate. Removing old bunches and tendrils at pruning can slightly reduce the risk of Botrytis bunch rot in the coming season. Some growers have also had success with using either a machine harvester or blower to remove rubbish from flowering that has become stuck in bunches, resulting in less bunch rot. These methods are best used alongside other control techniques.

A machine harvester can be used after berry set (between peppercorn and pea size) to shake the rubbish out of developing bunches. If used along with fungicides, it has been shown to reduce bunch rot at harvest by around 50% (learn more). Whilst not beneficial in years when the disease is not rampant, it’s important to note that mechanical shaking at this stage of development does NOT have a negative impact on wine quality (learn more).

Mechanical shaking for rot reduction (pages 16-19) NSW DPI Grapevine management Guide 2025-26 – insights from trials and demonstrations in NSW vineyards.

Video showing vine shaking method and results.

Fruit management

If Botrytis hits hard and late, and conditions are ideal for rapid spread, quick and decisive action is essential. Depending on fruit maturity, labour availability and skills, winery processing capacity and flexibility, it’s worth considering:

• thinning out bunches and removing infected fruit to improve airflow and reduce bunch-to-bunch contact
• harvesting early to salvage the crop before Botrytis spreads even more
• prioritising high value blocks and harvesting them first
• separating blocks, or parts of blocks, with different levels of infection for separate harvesting and processing
• hand-picking in two passes (skilled rot removalist team first, followed by general pickers).

In severe cases, the only viable option may be to abandon the fruit and review what worked and didn’t work for future management.

Chemical control

Protective fungicides can be sprayed at key stages. ‘Preventative spraying is more effective than spraying after infection has occurred and is recommended for preventing development of fungicide resistance’ (AWRI Vitinotes).

Critical stages:

• 80% capfall
• pre-bunch closure

If Botrytis risk is high, fungicides can also be applied at veraison or pre-harvest (so long as disease impact is still low and winery withholding period restrictions allow it).

Pre-harvest spraying

Late season spraying potentially buys more time, but Botrytis bunch rot will develop if spores are still present and the weather is right. In recent years, several new fungicides have been registered for late season application, but they must be applied at the right time and have good coverage to be effective.

As bunches fill up near harvest, spray penetration through canopies and around berries drops significantly. At this stage, the cost of product and labour may outweigh the benefit of spraying, so consider the economics carefully.

Always discuss pre-harvest spraying with your winery first. Refer to the AWRI/Wine Australia Agrochemicals Booklet (Dog Book) for details of type and dosage of chemicals permitted for various export markets.

PMS / KMS (potassium metabisulphite) works, sometimes

• A dose of 3 g/L PMS has been shown to slow Botrytis germination and development so long as it is applied within 48 hours after infection. 
• Increasing the dose or spraying later than 48 hours after infection show little benefit.
• Read the full research paper: Improving fungicide control of Botrytis bunch rot

Avoiding chemical resistance

Avoiding fungicide resistance in Botrytis bunch rot, Agrochemical update, The Australian Wine Research Institute

Worth a look

AWRI webinar recording – Late season Botrytis: the disease and options to control it.

Further reading and advice

• University of Tasmania Botrytis resources
• AWRI Botrytis resources
• Late-season Botrytis and key resources (AWRI)