Last Update 5/14/2012 | | | | | | Next Update: 5/21/2012 | | | |
Date | Day | Forecasted
High
Temperature | Estimated
RAI* | Estimated
Change
in RAI** | Pressure
| | 5/14/2012 | Mon | 85 | 50 | +20 | Mod | | 5/15/2012 | Tues | 87 | 40 | -10 | Mod | | 5/16/2012 | Wed | 93 | 30 | -10 | Mod | | 5/17/2012 | Thurs | 89 | 20 | -10 | Low | | 5/18/2012 | Fri | 82 | 40 | +20 | Mod | | 5/19/2012 | Sat | 86 | 30 | -10 | Mod | | 5/20/2012 | Sun | 94 | 20 | -10 | Low |
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 calculates the RAI on a daily basis for the eight 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) |
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-to21-day Interval |
21-day Interval or Label Interval |
21-day Interval or 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 85°F 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 95°F, 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.
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Grape Variety Susceptibility to Powdery Mildew -- Grape cultivars vary in their susceptibility to powdery mildew. This fact is known by most growers; however, a listing compiled by Stephen Vasquez (UC Cooperative Extension) is a good reminder. Keep in mind that the Powdery Mildew Index typically applies to “Moderately” susceptible varieties. Varieties that are “Highly“ susceptible require more frequent treatment and those that have a “Low” susceptibility can be treated less frequently. | |
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