What is downy mildew?
Downy mildew is caused by Plasmopara viticola, an organism now classified as an algae-like pathogen rather than a true fungus. Like most algae, it depends on free moisture for much of its life cycle, which is why wet weather and prolonged leaf wetness play such an important role in disease development.
The disease affects leaves, shoots, inflorescences, bunch stems and berries. Young green tissue is most susceptible, while mature tissue that has hardened or changed colour becomes increasingly resistant. This is why downy mildew is often described as a "green tissue disease".
Why it matters
The greatest risk from downy mildew is not usually the initial infection. Instead, the real threat comes from secondary spread within the canopy.
A small number of early-season infections can rapidly escalate into a vineyard-wide outbreak if warm, humid conditions persist. Individual oilspots can produce thousands of spores, enabling disease levels to increase dramatically within a short period.
For growers, this means:
- Early detection is critical.
- Missing primary infections can result in significant bunch infection later in the season.
- Disease pressure can increase rapidly during flowering and early berry development.
- Weather monitoring and spray timing are often more important than spray frequency alone.
Recognising the symptoms
On leaves
The classic symptom of downy mildew is a circular, yellow-green "oilspot" on the upper surface of young leaves.
As lesions age they may:
- become darker yellow
- develop a characteristic angular or "tapestry" appearance on older leaves
- produce white downy growth on the underside of the leaf under warm, humid conditions.
The white down consists of spores and indicates the disease is actively producing inoculum capable of spreading further through the vineyard.
On bunches
Young bunches are highly susceptible. Infected bunches often:
- turn brown
- shrivel and die quickly
- become covered with white down under favourable conditions.
Berries gradually become resistant as they develop. By pea size (E-L 31), berries are generally resistant to direct infection, although bunch stems and berry stalks remain susceptible.
Understanding the disease cycle
Downy mildew develops in two stages: primary infection and secondary infection.
Primary infection
Primary infection occurs when overwintering spores in the soil produce spores that are splashed onto grapevine foliage under suitable conditions.
Primary infections usually produce only a small number of oilspots, often just one to three oilspots per 50 metres of canopy row.
Although primary infections may appear insignificant, they establish the disease in the vineyard and create the source of inoculum for more damaging secondary spread.
Secondary infection
Secondary infection occurs when spores produced on oilspots spread throughout the canopy.
Spores move:
- leaf to leaf
- leaf to shoot
- leaf to bunch.
Because each oilspot can produce thousands of spores, disease levels can increase rapidly following warm, humid weather. Secondary infection is responsible for most crop damage and economic losses.
Conditions that favour infection
Downy mildew requires warmth, moisture and leaf wetness.
A useful guide for assessing primary infection risk is the "10:10:24" rule:
- at least 10 mm of rainfall
- temperatures of at least 10°C
- occurring within a 24-hour period.
These conditions indicate a high likelihood that primary infection could occur.
Secondary infections require:
- existing oilspots in the canopy
- warm temperatures
- very high humidity overnight
- leaf wetness the following morning.
Because these conditions can occur repeatedly through spring and early summer, monitoring weather events is an important part of disease management.
Practical management tips
Monitor weather, not just symptoms
Waiting until disease symptoms are visible can mean infection has already occurred.
Many successful growers use the following to identify high-risk periods before symptoms appear:
- weather forecasts
- disease alerts
- automatic weather stations
- vineyard monitoring programs.
Inspect the canopy thoroughly
Early infections are easy to miss and may go untreated unless vineyards are monitored carefully.
When monitoring:
- inspect inside the canopy as well as outer leaves
- check young bunches and bunch stems
- assess a representative number of vines across the block
- look for isolated oilspots after infection periods.
Older oilspots often indicate the disease has already begun spreading.
Focus on flowering and early berry development
The period around flowering and early berry development is one of the highest-risk stages for economic damage from downy mildew.
Young bunches are highly susceptible and favourable weather can allow disease levels to increase rapidly within the canopy. Rapid shoot growth can also leave newly emerged leaves and bunches unprotected only days after spraying, increasing the risk of infection.
While early leaf infections may appear minor, they can become a major source of inoculum during this period. Once established, the disease can spread quickly from leaves to bunches, where the greatest crop losses occur.
When conditions favour infection:
- maintain protection through flowering and early berry development
- monitor canopy growth and adjust spray timing or spray intervals to maintain coverage of new growth.
Prioritise spray coverage
Good disease control depends on more than product choice.
Effective programs require:
- the right product
- the right timing
- the right application technique.
Pay particular attention to:
- undersides of leaves
- developing bunches
- spray penetration into dense canopies.
Even the best fungicide program can fail if spray coverage is inadequate. Poor coverage is one of the most common reasons downy mildew develops despite spraying.
Manage canopy density
An open canopy helps reduce disease pressure by improving:
- airflow
- spray penetration
- drying of foliage after rainfall and dew events.
The following practices can support broader disease management efforts and improve fungicide performance:
- shoot trimming
- shoot positioning
- leaf removal around the fruit zone.
Confirming an active infection
If symptoms are unclear, a simple "bag test" can help confirm active downy mildew.
Place suspect leaves or bunch material into a lightly moistened plastic bag overnight in a dark, warm location. Active infections will often produce fresh white sporulation on the underside of oilspots by the following morning.
This can help distinguish active downy mildew from old lesions or other vineyard disorders.
Key takeaways for growers
- Downy mildew is highly weather dependent and can spread rapidly under favourable conditions.
- Early infections are easy to miss but can quickly become the source of vineyard-wide outbreaks.
- Young bunches are particularly vulnerable from flowering until berries approach pea size.
- Monitor weather and vineyards closely to identify infection risk and detect disease early.
- Effective control relies on early action, good spray timing, maintaining protection of new growth and achieving good spray coverage.
- Canopy management and vineyard monitoring are important tools alongside fungicide programs.