Powdery Mildew Risk Assessment Indices 

for Fresno & Madera Counties
The Powdery Mildew Risk Assessment Index (RAI) is a system of using temperatures in the vine canopy to help adjust the intervals between control for powdery mildew.  

The University of California Statewide Integrated Pest Management (IPM) Program is now calculating the RAI on a daily basis for the eleven Pest Cast Stations in Fresno & Madera Counties. By clicking on the tab below, you will be taken direct to their web page.

 Use your browser’s “Back” button to return to the AgQuest web page.  

Or Click on an item in the far the Left Column.

Go to the UC IPM Risk Assessment Indices Web Page

Risk Assessment Index Forecast
In order to assist grape growers in their planning, AgQuest has estimated changes in the RAI for the next week. These forecasts are based on the forecasted temperatures in Fresno & Madera Counties and are updated each Monday. (How the RAI is Calculated)

Last Update: July 13, 2009

Next Update: March 2010
  Date   Day

Forecasted

High Temperature

Estimated RAI*

Estimated Change in RAI**

Pressure

7/13/09

 Monday  97 0 0 Low

7/14/09

Tuesday 100 0 0 Low
7/15/09 Wednesday 102 0 0 Low
7/16/09 Thursday 103 0 0 Low
7/17/09   Friday 103 0 0 Low
7/18/09   Saturday 102 0 0 Low
7/19/09   Sunday 102 0 0 Low
** Estimated RAI and disease pressure/pathogen status report obtained for the Easton Pest Cast Station.
** Add or subtract this number to the Estimated RAI

The Powdery Mildew RA Indices 

and Effects on Spray Intervals
RAI Index Disease
Pressure		 Pathogen
Status		 Biological
and SARs		 Sulfur
 		Sterol-
inhibitors		 Strobilurins
 0-30  low  present 7-to 14- day   interval 14-to
21-day
interval		 21-day
interval or
label interval		 21-day
interval to
label
Interval
30-60 intermediate reproduces
every 15
days		 use not
recommended		 10-to
17-day
interval		 21-day
interval		 21-day
interval
60 or
above		 high reproduces
every 5
days		 use not
recommended		 7-day
interval		 10-to
14-day
interval		 14-day
interval
How the RAI is Calculated -- The index is calculated by adding 20 points whenever the temperature in the canopy is between 70 and 85oF continuously for 6 or more hours in one day.  If there are less than 6 hours in this range or if the maximum daily temperature is greater than 95oF, then 10 points are subtracted from the index.  The index never goes below zero nor above 100.

Adjusting Treatment Intervals -- A short interval for dusting sulfur is 7 days, while 10 to 14 days would be a longer interval.  For a sterol inhibitor type of material with a label interval of 15 to 20 days, a severe RAI would mean that retreat is needed after 15 days and a low RAI would mean that the material would be effective for 20 days.

 

California AgQuest Consulting, Inc.
4545 N. Brawley Avenue
Fresno, CA  93722
Ph. (559) 275-8095  *  FAX (559) 275-5301
e-mail:  consult@calagquest.com

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