Chelan - Douglas County WSU Extension
Chelan - Douglas County > Agriculture > Tree Fruit > The CougarBlight 2010 Fire Blight Risk Model

The Washington State University CougarBlight 2010 fire blight risk analysis model has been significantly changed, especially in the way that temperatures are measured and related to risk of flower infection.  Cougarblight 2000 remains useful, but users are requested to transition to this new version.

For an extensive explanation on the model and its use go to the following page: https://county.wsu.edu/chelan-douglas/agriculture/treefruit/Pages/Cougar_Blight_2010.aspx

 

On the WSU DAS model page you will find  the current Temperature Risk Value,  the recent wetting events, record of rainfall, risk of rain, forecasted risk, and risk level at the current hour available.  Be certain to set the "Your Risk Setting" situation first, or most of this information will not display.

 Use of the model, the basics: 

A.  Set the "Your Risk Setting" relating to the recent history of fire blight in your neighborhood.

B. When flowers are on your trees, watch the "Model Status" forecast for predicted risk levels for today and the next four days.

C. If risk is low, infection is unlikely. 

D. If infection risk is "marginal" infection is not likely, but you should be alert for changes predicted in the next few days.  If the prediction of risk is rising into "high," you could start protecting your blossoms with biological products (Bloomtime, Blossom Protect, Serenade) during the 3 or 4 days leading up to the "high" risk date to help prevent blossom infection.

E. When the risk level is "high," infection is possible whenever the blossoms are wet for 2 or more hours.  These events are recorded for four days under the "wetting events" tab.  Antibiotic sprays must be applied within 24 hours before or after the infection event to be most effective.

 More Details:

The model structure uses the same principles as in the past.  There are four major aspects of infection that this model evaluates to estimate risk.  

1.  The presence of flowers.  If there are no flowers present, ignore risk warnings.  This is a blossom infection model.

2. The level of disease pressure in the orchard.  This "Your Risk Setting" must be selected before the model will work.  Has blight been a problem recently?  What degree of confidence do you have that there are no active cankers in or near your orchard?  The proximity and number of active cankers makes a great difference in the contamination percentage of flowers, and the initial number of bacteria that start growing on the stigma.  This aspect of infection risk is adjusted according to which of the three initial orchard settings you choose.  The recent or current presence of blight in your orchard or neighborhood (adjacent orchards) lowers the temperature risk value (TRV) thresholds automatically in the DAS model interface.

3. The temperatures that have occurred over the last four days (96 hours).  This model relates the population growth rate of a fire blight bacteria colony (Erwinia amylovora) on a flower stigma to the average temperature of each hour of the day. This temperature relationship is expressed as Temperature Risk Value (TRV),  and is reported to you as the total TRV units that have occurred over the past 96 hours.  The past and current forecast of 4-day TRV from day to day are reported in the "Model Status."   The number of TRV that have accumulated over the past 4 days (96 hours) before the current hour is reported under "Today's Current Data as of...(hour)."  These are not the "fire blight degree hours" as they were in Cougarblight 2000, but play the same role in assessing bacterial growth rate per hour. The number of TRV that indicate risk of infection (low, marginal, high, extreme) depends on which of the three "Your Risk Setting" modes you choose.  The higher the chance that active blight cankers are in the area, the lower the TRV thresholds.  These are automatically chosen relative to your Risk Setting choice.  

4. The documentation of a two hour or more blossom wetting event.  This two hours or more of wetting is considered the possible infection event, as wetting is a necessary aspect of blossom infection. At the time of 2+ hours of wetting, as detected by leaf wetness sensors, or in the event of measurable rain,  the DAS model "time stamps" and records the time and date, and the temperature risk value that was current for the 96 hours prior to that hour that the 2 hours of wetting occurred.  This information can be found by clicking on the "Wetting Events" tab.  This tab records the wetting events that have occurred during the past four days.  As these wetting events are difficult to accurately detect by remote monitor, the DAS system will not be able to document a potential infection event if the wetting is caused by dew in more humid portions of the orchard or by light wetting of flowers by poorly timed irrigation.

Watch forecasts to plan necessary control measures.  If your orchard has flowers,  risk is forecasted to be high four days in the future, you may apply biological or other non-antibiotic control measures to help reduce the build-up of blight bacteria on the flowers.  When risk is high or above, wetting events are considered an infection event.    It is almost certain that infection will occur if an infection event occurs when blight bacteria (E.a.) are present on flowers and the model indicates high or extreme risk.  It is no currently practical way to rapidly determine if your flowers are actually contaminated with blight bacteria. 

Antibiotics are most effective when applied within 24 hours before an infection event, and are also effective when applied within 24 hours after infection.  Efficacy drops each hour after infection event.  Full coverage of the interior of flowers is essential for control sprays to be effective.

 

 

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