Powdery mildew is different from Downy mildew because it does not require free water or rainfall to infect. Warm temperatures, suitable humidity and the presence of susceptible vine tissue are much more important.

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Viti Pulse uses the Gubler–Thomas Powdery Mildew Risk Index as the foundation for its Powdery mildew risk assessment, combined with vine growth stage and berry susceptibility.

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Weather drives the risk

The Gubler–Thomas model evaluates weather conditions that favour Powdery mildew development, particularly temperature and humidity.

Powdery mildew generally favours:

• warm temperatures

• humid conditions

• active vine growth

• susceptible green tissue.

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Importantly, rain does not automatically increase Powdery mildew risk. In fact, heavy rain can temporarily reduce surface colonies by physically washing spores and mycelium from the vine.

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The disease can develop without rain

This is one of the most important differences between Powdery and Downy mildew.

Powdery mildew does not need prolonged leaf wetness for infection. It can develop under relatively dry conditions provided temperatures and humidity are suitable.

 

Therefore:

Dry weather does not necessarily mean low Powdery risk.

A warm, humid period with little rainfall can actually provide very favourable conditions for Powdery mildew.

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Growth stage matters

Viti Pulse also considers where the vines are in their development.

Powdery mildew risk is particularly important around flowering and early berry development, when young tissues and developing berries are highly susceptible.

The same weather conditions can therefore have different practical significance depending on the vine's growth stage.

Viti Pulse increases the urgency of Powdery risk during key susceptible periods rather than treating every day of the season in exactly the same way.

 

What does the Powdery risk mean?

The Powdery risk represents how favourable the current and forecast environmental conditions are for Powdery mildew development, taking vine phenology into account.

A high reading means conditions are favourable for disease development.

It does not mean that Powdery mildew is already present or that infection is certain.

Disease development will also depend on factors such as:

• whether Powdery mildew is already present

• existing colonies and inoculum

• canopy density

• vine growth

• variety susceptibility

• spray protection

• vineyard microclimate.

 

Why risk can build quickly

Powdery mildew can develop rapidly when conditions remain favourable.

Unlike a disease model that simply responds to an individual rain event, the Gubler–Thomas approach reflects the cumulative effect of favourable weather conditions on disease development.

This means several days of suitable conditions can be more significant than one isolated weather event.

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Why Viti Pulse considers phenology

Weather is only part of the risk.

Viti Pulse combines the Powdery mildew weather risk with the vine's phenological stage to provide a more agronomically useful interpretation.

For example:

Warm, favourable weather + flowering = high priority

The same weather conditions later in the season, when berries have become less susceptible, may have a different practical significance.

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What does a high Powdery risk mean?

A high risk reading means:

The current conditions are favourable for Powdery mildew development, and the vine is at a stage where that risk may be particularly important.

It is a risk indicator, not a diagnosis of disease presence.

 

Why Viti Pulse uses this approach

A simple rule such as "warm weather = Powdery risk" does not tell the whole story.

Viti Pulse instead considers:

Are temperatures favourable?
Are humidity conditions favourable?
How long have favourable conditions persisted?
What growth stage are the vines at?
Are the vines currently in a highly susceptible period?

This provides growers with a clearer indication of when Powdery mildew conditions are becoming important, rather than simply reporting the weather.

 

Scientific basis: The weather component is based on the Gubler–Thomas Powdery Mildew Risk Index, with Viti Pulse adding its phenology-aware interpretation to reflect the changing susceptibility of grapevine tissues through the season.