Downy mildew risk can change quickly because infection depends on temperature, moisture and the availability of viable Downy inoculum.

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Viti Pulse uses two different biological models to represent the two main ways Downy mildew can develop:

• Primary infection — new infections originating from overwintered inoculum.

• Secondary infection — new infections produced from Downy mildew that is already established in the vineyard.

These two pathways are kept separate because they represent different biological processes.

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Primary infection

Primary infection is modelled using the Rossi et al. (2008) mechanistic model.

The model considers whether weather conditions are suitable for an initial infection, including:

• temperature

• rainfall and moisture

• the conditions required for infection to occur.

Viti Pulse reports the result as a primary infection probability (%).

A high probability means the weather has provided conditions favourable for a primary infection. It does not mean that infection is certain.

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Secondary infection

Once Downy mildew is established, the disease can produce new infectious material and begin further infection cycles.

Viti Pulse models this process using its adaptation of the Brischetto et al. (2021) weather-driven secondary infection model.

The model follows the biological sequence:

Existing lesions → sporulation → viable inoculum → infection → new disease

This means secondary risk can remain high even when there is no new primary infection occurring.

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Existing inoculum matters

This is an important difference between Downy mildew and a simple weather-risk model.

Rain or humidity alone does not automatically produce a high Downy risk.

For secondary infection to occur, there must be viable inoculum available.

If there is no viable Downy mildew inoculum, suitable weather by itself cannot generate a secondary infection in the model.

Conversely, if viable inoculum is already present, a relatively short period of suitable weather can create significant secondary infection pressure.

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Primary and secondary risk are separate

Viti Pulse does not simply add the primary and secondary risks together to create an arbitrary score.

Instead, it assesses both pathways independently and identifies which pathway is currently driving the disease risk.

The dashboard therefore shows:

• Primary infection probability

• Secondary infection risk

• Overall risk

• Driver — Primary, Secondary or Neither

This tells the grower why Downy risk is elevated, rather than simply saying that conditions are favourable.

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What does secondary risk mean?

Secondary risk is based on the daily increase in infection pressure, derived from the model's SEV calculation.

It is not a probability and should not be interpreted as "a 50% chance of infection."

It represents the amount of new infection pressure generated by the existing Downy mildew population under the prevailing conditions.

 

Weather drives the process

The secondary model responds to changing vineyard conditions, including:

• temperature

• humidity

• rainfall

• leaf wetness

• sporulation conditions

• survival of infectious material

• the presence of existing infectious lesions.

As conditions dry out or become unsuitable, infection pressure can fall rapidly.

When wet and suitable conditions return while viable inoculum remains present, secondary infection pressure can increase again.

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Why risk can remain after a rain event

Downy mildew does not necessarily start and stop with each individual rain event.

Existing lesions can continue to contribute infectious material, and viable inoculum can remain available for subsequent infection periods.

Therefore, a dry day does not necessarily reset the vineyard to zero risk.

Likewise, a wet day does not automatically mean high risk if temperatures are unsuitable or viable inoculum is absent.

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What does a high Downy risk mean?

A high risk reading means that the model has identified biologically favourable conditions for a new Downy infection, given the current state of the vineyard.

It does not mean that disease is already present everywhere in the vineyard.

The actual outcome will also depend on factors such as:

• whether Downy mildew is present in the vineyard

• the amount and age of infectious lesions

• canopy conditions

• vine growth and susceptibility

• spray protection

• local vineyard conditions.

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Why Viti Pulse uses this approach

A simple rule such as "rain + warm weather = Downy risk" misses an important part of the disease cycle.

Viti Pulse instead asks:

Are conditions suitable for primary infection?
Is viable Downy inoculum already present?
Can existing lesions produce infectious material?
Are conditions suitable for a new secondary infection?
Which pathway is currently driving the risk?

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This allows Viti Pulse to distinguish between weather that could start an infection and weather that could drive an existing Downy mildew population into another infection cycle. 

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The most valuable financial finding of this research and models is the 87% certainty of a "nil-risk" window. If the model says there is no infection risk, you can safely save money on chemicals.

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• Establish a protocol where you withhold fungicide applications during periods where the DSS shows zero risk—even if your traditional calendar schedule says it is time to spray.

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• Identify Zero-Risk Windows: Look for hot, dry spells (temperatures >29°C and high Vapor Pressure Deficit) where the model mathematically proves that any existing airborne sporangia have dried out and died.

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Scientific basis: Primary infection is based on the mechanistic model of Rossi et al. (2008). Secondary infection follows a Viti Pulse adaptation of the weather-driven epidemiological architecture described by Brischetto et al. (2021).