The UC Davis Powdery Mildew Risk Assessment Model (Gubler -Thomas Model) is based on laboratory and field
epidemiological studies of grapevine powdery mildew inCalifornia.
The UC Davis powdery mildew risk assessment model forecasts ascospore release based on temperatures and
leaf wetness periods to predict initial disease onset.

Once infection has occurred, the model switches to a disease risk assessment phase and is based entirely on
the effects of temperature on the reproductive rate of the pathogen.
Ascospore Infection Forecasts Predictions of ascospore release and initial infections are based on average temperatures during extended leaf wetness events.

Approximately 2.5mm of rainfall are required to release ascospores followed by at least 8-12 hours of leaf wetness are required after ascospore release at temperatures between 10 -15°C for infection to occur.

Risk Assessment Index:

Following ascospore release and germination, the subsequent development and reproduction of powdery mildew is influenced primarily by ambient temperatures. The UC Davis model evaluates ambient temperatures and assesses the risk of powdery mildew development using a 0-100 point index.
To initiate the risk assessment index, after budbreak and after disease onset, there must be three consecutive days with a minimum of six consecutive hours of temperatures between 21 and 30°C for the powdery mildew epidemic to be initiated. Throughout the calculation of the index, for each day with 6 or more consecutive hours between 21 and
30 °C, the index increases 20 points. Thus after 3 consecutive days, the risk assessment index climbs to 60, with each of the 3 days contributing 20 index points.
If three consecutive days at these temperatures are not met during the beginning of the season, the index reverts to zero. Once the three consecutive day requirement is met, it is no longer a function of the model. The model then fluctuates between 0 and 100 index points based on duration of daily temperatures. The index gains 20 points for each day that meets the requirement of six consecutive hours of temperatures between 21 and 30°C,
and it loses 10 points for each day that does not meet the six hour requirement, or if at any time during the day, the temperature aceeds to 35°C for at least 15 min.
The index can not exceed 100 points nor go below zero.
Low index values of 0-30 indicate the pathogen is not reproducing. Growers may consider postponing fungicide applications during extended periods with low index values. An index of 40-50 is considered moderate and would imply a powdery mildew reproductive rate of approximately 15 days. Index values of 60-100 indicate that the pathogen is reproducing rapidly (every 5 days) and that the risk for a disease epidemic to occur is great.

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Powdery Mildew in CanopeGraphical Presentation of the Gubler Thomas Model

The spread out of Uncinula necator during the summer (when conidia are produced) is influenced primarily by temperature. The optimal temperatures for the Powdery Mildew are between 21°C and 32°C. The greater the number of hours daily that the temperature is within this range, the higher is the risk for Powdery Mildew. Days with more than 6 hours of temperature in this range increase the risk about 20 points. Days where the temperature is not within this optimal range reduces the risk about 10 points. These are days with temperatures under 21°C the whole day long or days where the temperature exceeds 32°C for 6 hours. If the Powdery Mildew risk is less than 20 points spraying interval can be extended. With 20 to 60 points the normal spraying interval is valid. If the risk is more than 60 points you should shorten the spraying interval. 

  • Risk increases: With every day with equal or more than 6 hours of 21°C <= Temperature < 32°C ==> +20 Points
  • Risk decreases: With every day when temperature is 32°C or above or when 6 hours of at least 21°C are not reached ==> -10 Points

(in FieldClimate.com it is called : The Powdery mildew California risk)The PM CA- Gubler model in strawberry has the same conditions for infection (just other temperature thresholds).