Why this matters for grape growers
Non-botrytis bunch rots can reduce yield, crop value and wine quality. Severe infections may make a crop uneconomic to harvest.
Rot-affected fruit may produce mouldy or musty off-flavours. Some bunch rot fungi may also contribute to colour loss and oxidative spoilage in wine.
The effect on wine depends on the type and severity of the rot and the intended wine style. Research is limited and there is no single tolerance threshold for all non-botrytis bunch rots. In one study, wine made from fruit with fewer than 3 per cent of bunches infected by ripe rot had noticeable off-flavours and colour loss.
Understanding the risk factors
Most non-botrytis bunch rots are caused by fungi that produce large numbers of spores. These spores can spread through:
- Wind and dust
- Rain splash
- Insects carrying spores on their bodies
- Infected vineyard debris
- Mummified berries
- Dead wood, canes, bark and spurs
- Soil and decaying organic matter
Many of these rot-causing pathogens survive between seasons in the vineyard. Mummified berries, infected wood and vineyard debris can act as ongoing sources of infection. Some pathogens can also survive on other plant hosts surrounding the vineyard.
The risk of infection generally increases as fruit approaches maturity, particularly when berries are damaged through:
- Rain-induced splitting
- Hail
- Insect feeding
- Bird damage
- Sunburn
- Mechanical injury
Once berry skins are compromised, many rot-causing pathogens can rapidly colonise the fruit.
Common non-botrytis bunch rots in Australian vineyards
Several diseases may occur in the same vineyard or even on the same bunch.
Alternaria rot
Alternaria is commonly present on healthy grape berries but usually becomes problematic when berries split or are damaged. Infected berries develop grey to brown-black fungal growth through cracks in the skin. Wet bunches and prolonged humidity increase the risk.
Aspergillus rot (black sooty mould)
More common in warm and hot regions, Aspergillus produces dark brown or black spores that give berries a sooty appearance. Some strains can produce ochratoxin A, a mycotoxin of concern to human health. It is frequently associated with damaged fruit and sometimes occurs alongside sour rot.
Bitter rot (Greeneria rot)
Bitter rot is characterised by concentric rings of black fungal growth on berries. It is most common in vineyards experiencing warm, wet conditions near harvest and has been regularly observed in eastern Australian growing regions.
Botryosphaeria rot
Commonly associated with grapevine trunk diseases, Botryosphaeria fungi can also infect berries under favourable conditions. Older vineyards are generally more susceptible, with infections often occurring where bunches come into contact with infected wood.
Cladosporium rot
Cladosporium typically causes soft, dark circular lesions on berries and produces distinctive olive-green fungal growth under humid conditions. It is most often associated with late-harvest fruit following rainfall and generally affects individual berries rather than entire bunches.
Penicillium rot (blue mould)
This disease is identified by its blue-green spores and is frequently associated with berry splitting after rainfall. It tends to be more common in cool-climate vineyards and is often found alongside sour rot.
Rhizopus rot
Affected berries develop a soft brown rot with juice leakage and cobweb-like black fungal growth. The disease spreads readily through damaged berries within a bunch and is commonly associated with sour rot.
Phomopsis rot
While Phomopsis is more commonly associated with cane and leaf disease, severe infections can also lead to bunch rot where warm, wet weather combines with berry damage and infected vegetative material.
Ripe rot (Colletotrichum rot)
Ripe rot causes berries to lose firmness before developing distinctive orange to salmon-pink spore masses. It is commonly associated with warm, wet conditions near harvest and can be particularly problematic in subtropical regions.
Sour rot
Sour rot is a complex disease involving yeasts, bacteria and fungi. It is easily recognised by the smell of vinegar and the presence of watery berries with thin skins. Vinegar flies and other insects play an important role by both damaging berries and spreading disease-causing microorganisms.
Practical management for vineyard managers
There is no single solution for managing non-botrytis bunch rots. Successful management relies on an integrated approach throughout the season.
Select varieties suited to your climate
Variety choice can significantly influence disease risk.
Consider:
- Earlier-ripening varieties in regions prone to late-summer rainfall
- Varieties with looser bunch architecture
- Varieties less prone to berry splitting
- Site suitability when planning new developments
In regions where harvest frequently coincides with wet weather, earlier harvest windows can reduce exposure to disease pressure.
Optimise canopy management
Good canopy management remains one of the most effective disease prevention tools.
Aim to:
- Improve airflow through the canopy
- Reduce prolonged humidity around bunches
- Improve spray penetration
- Support faster drying after rain events
However, balance is important. Overexposed fruit can suffer sunburn and heat stress, which may increase susceptibility to some rots such as ripe rot. Retaining some fruit protection on the western side of rows may help reduce heat-related damage in hot regions.
Avoid excessive vineyard moisture
Reducing periods of extended wetness can limit disease development.
Practical strategies include:
- Maintaining effective drainage
- Avoiding waterlogging
- Using drip irrigation where appropriate
- Managing weeds to improve airflow
- Removing excess shoots and suckers
- Considering row orientation and prevailing winds
These practices help bunches dry more quickly following rainfall, dew or irrigation events.
Reduce inoculum sources
Many bunch rot pathogens survive between seasons.
Reducing inoculum sources may include:
- Removing diseased bunches where practical
- Removing infected wood during pruning
- Managing vineyard debris
- Monitoring trunk disease symptoms
These practices help reduce the amount of disease inoculum available to infect fruit in future seasons.
Use fungicides strategically
Fungicides may form part of a management program, but they are only one tool for managing non-botrytis bunch rots.
Keep in mind that:
- Products used for botrytis bunch rot may not control non-botrytis bunch rots
- Fungicide options can be limited close to harvest because of withholding periods and residue requirements
- Fungicides are generally most effective when combined with practices that reduce berry damage, humidity and inoculum sources
- High disease pressure in warm, wet seasons can still lead to disease outbreaks despite fungicide use
Integrated management remains essential.
Climate and seasonal risk
The bunch rot profile in a vineyard changes from season to season.
General patterns include:
| Seasonal conditions | Rots more commonly observed |
|---|---|
| Warm and wet | Bitter rot, ripe rot |
| Warm and dry | Aspergillus rot |
| Cool and wet | Penicillium rot |
These seasonal associations are general patterns only and multiple rot organisms may occur together in the same vineyard, bunch or berry. Warmer and wetter summers are likely to increase the risk of non-botrytis bunch rot outbreaks.
Key takeaway
Non-botrytis bunch rots can reduce grape yield and affect wine quality. The greatest opportunities to reduce risk come from proactive vineyard management, including selecting suitable varieties, managing the canopy to improve airflow, reducing prolonged moisture around bunches, limiting sources of infection and making timely harvest decisions. Early identification of disease risk, minimising berry damage and reducing prolonged bunch wetness are often more effective than relying solely on fungicides.