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Cougar Blight 2010

Program Contact: Tianna DuPont, Tree Fruit Extension Specialist
(509) 663-8181 •

fbcluster2_000We recommend that you use the new version “CougarBlight 2010.”  Page revised Aug., 2013.

There is a basic Excel spreadsheet version of the model in both Celsius and Fahrenheit:

The F version: CougarBlight 2010 EZ ver.5.1

The C version: CougarBlight 2010 EZ ver. 5.1 

These use estimated total temperature risk values for each day based on the daily high temperatures.   Use this version only if you have no access to hourly temperatures.  If hourly temperatures are available,  the model will function better under variable climatic conditions if the values of each hourly temperature are accumulated for each day.  At the time of a wetness event, the total hour by hour temperature risk values compiled during the 96 hours prior to the 2 hour or longer blossom wetting will best indicate the infection risk. This may be hand calculated using the hour values, but is much more practical when set up to be calculated by computer programs. The WSU Decision Aid System has a very good example of how this can be done. See at: 

This new version was made available at the 12th ISHS Fire Blight Workshop in Warsaw in Mid-August 2010, and is now available in full detail on this web page.  The current version is Ver. 5.1.  If you would like to receive the most current version of the model Excel spreadsheet, or have questions, contact the author at

Revised Fire Blight Flower Infection Risk Assessment Model

The model is intended for use during moderate temperatures ( 10 – 30C) that usually occur during spring and early summer and when flowers are open. New infections are not likely during high summer heat. The lower threshold for assigning risk value has been dropped from 60F (15.5C) to 50F (10C) in recognition of recent research on the growth of E.a. on stigma surfaces. However, the greatest changes relate to the relative value given to the temperatures at which peak bacterial growth (division) occurs. (For details, contact the author.)

The “degree hours” have been altered in their derivation, and now will be referred to as”temperature risk values.”

It is a common misconception that the numerical values assigned to each hour or day in this model are a true degree hour, (the average temperature for that hour, minus a base temperature.) In fact, the numerical “risk value’ assigned to each day (relative to daily highs) was based on the shape of the bacterial growth rate curve above, and was altered at higher temperatures to better fit the physical effect that the high temperatures have on flowers. :

So, each hour, past and present can be assigned a precise “temperature risk value” related to the growth rate of E.a. on apple stigma surfaces at that temperature.  The future forecasted risk level is estimated based on average daily risk values of past daily totals of days with the same daily high temperature. The four-day (96 hour) total risk value prior to the time flowers are wetted is used as a time when infection may have occurred, and is when an infection risk evaluation should occur. The higher the risk value at the time of wetting, the greater the infection risk. This process can be simplified by using an Excel spreadsheet for calculations and record keeping.  Note: This is the least precise method of this model, as an estimated total risk value is assigned to each day, based on the daily maximum temperature. These estimated daily values are used for developing a FORECAST of blight risk, which is perhaps, the most important aspect of any model.

Fahrenheit version       Celsius Version

Below is a graphic illustrating the hourly risk values of three different days, cool, warm and hot:

Infection Risk Categories, an explanation of terms:

Flower infection risk is assessed to be within one of these four categories:  Low, Caution, High or Extreme.  The explanation of those risk categories is as follows:

Low: Wetting of flowers during these conditions has not led to new flower blight infections in past years.

Caution: Wetting of flowers by rain, 2+ hours of dew, or light irrigation under these conditions is not likely to lead to infection, except within a few yards (meters) of an active blight strike. However, you should closely monitor the blight infection risk forecast, and consider applying non-antibiotic sprays to reduce the potential build-up of blight bacteria if High risk is forecast in three or four days.

High: Numerous serious blight outbreaks have occurred in past years when 4-day risk value totals near or exceed this threshold and blossoms are wetted by rain, 2+ hours of dew or light irrigation. The risk of severe damage due to infection increases in later stages of primary bloom and petal fall, and infection risk may return any time that secondary blossoms are numerous. The potential severity of infection is increased by a series of consecutive High risk days.

Extreme: Some of the most damaging fire blight epidemics have occurred during the time from primary bloom through late spring when numerous blossoms are wetted by rain (usually more than a trace), 2+ hours of dew on the flowers, or light wetting by irrigation under these temperature conditions. As the season progresses into consistently hot temperatures, secondary blossoms seem to be less likely to become blighted.  A series of days of 95F (35C) or above reduces the risk of new blossom blight infection even when degree hour totals are very high.

Orchard History Scenarios:

The 4-day total risk values used as thresholds for the above infection risk depend upon the fire blight “history” in the orchard. The percentage of flowers infested and the starting population of E. amylovora on the stigma surfaces seems very highly related to the presence / absence of active cankers and how near they are to the orchard in question. Those orchards that have active cankers that over-wintered in or nearby are in much higher risk of blight infection. To differentiate the problem orchards from those that did not experience significant blight the previous season, this model offers the user three situations that will determine the risk value thresholds:

Orchard Blight History Scenario Settings:

These local blight situation scenarios have been reduced the to three in the new version:

  1. No fire blight in your neighborhood last year. (This is common in the arid Western USA, relatively uncommon in the eastern and mid-western states. Growers in those areas should assume scenario number 2 as a standard.)
  2. Fire blight occurred in your neighborhood last year. (Default setting in areas with a history of blight problems.)
  3. Fire blight is now active in your neighborhood. (A live canker or new blight strike is currently present nearby.)

Attention to these scenarios above is critical when using this model.

Risk Value Thresholds:

Risk values were adjusted so that thresholds occurred at easy to remember numbers. They are the same for both F and C versions. The thresholds are not exact, they are guidelines.  Nature does not function in sudden jumps from one level to another.  If the specific risk threshold is 100, then 99 is not likely to indicate a much different risk of infection.  The threshold line between risk catagories is variable by at least 10 percent.

For scenario 1, risk categories and temperature risk value ranges are:

Low 0 – 150, Caution 150 – 500, High 500 – 800, Extreme 800-1000, and Exceptional 1000+.

For scenario 2, (The default setting) risk categories and temperature risk value ranges are:

Low 0 – 100, Caution 100 – 200, High 200 – 350, Extreme 350 – 500, and Exceptional 501+.

For scenario 3, risk categories and temperature risk value ranges are:

Low 0 (there is no low risk in an infected orchard), Caution 0 – 100, High 100 – 200, Extreme 200 – 300, and Exceptional 301+.


Erwinia amylovora may infect the host in numerous ways, but the most important infection site is the flower. Once a small colony of the bacteria have become established on the host flower stigma surface, there are a number of factors that determine success or failure to infect the host leading to fire blight. Most often, one or more of the necessary factors is not met, and the infection does not occur.

Orchard growers often fear fire blight so much that they apply spray control materials on a schedule, regardless of infection risk. The labels of blight control products usually describe application timing relating to tree blossom stage, without reference to high or low infection risk.

Studies have improved understanding of both the pathogen and the host, illustrating how dangerous this pest can be during the times that all of the infection factors are met or exceeded at the same time. Rather than attempting to manage this disease by constant application of sprayed control materials during bloom time, growers are much more successful controlling this disease if they recognize and monitor the risk factors, (host flowers, pathogen presence, conducive temperatures, wetting of blossoms.)  They can intensify control measures during the most critical days. This model, when used properly, will aid in the recognition of the more critical infection risk time periods.

Below is a short “using this model” text written for the WSU Decision Aid System:

Fire blight infection occurs when blight bacteria are carried to the flower stigma tip, grow to a large colony over two to four days of warm weather, then are gently washed into the flowers’ nectaries.  Infection does not occur unless the bacteria are in high numbers. The development of high numbers of bacteria on the stigma tip requires warm daily temperatures, the most dangerous in the range between 78 and 90°F (25 and 32C), but infection may occur during slightly cooler conditions if there is a recent history of blight infections in the orchard neighborhood. Blight is much more likely if there was fire blight infection in the area the previous year. You must consider your local blight history carefully when using this model. If there was blight in the neighborhood last year, or your orchard seems to have blight on a regular basis, set the blight history to “blight in the neighborhood last year” even if you are not aware that any occurred. The model will then help you determine if dangerous temperature conditions have occurred under your local conditions.

The risk value portion of this model is intended to give you some relative measurement of the amount or effect of heat that has been experienced by the oldest bacterial colonies on the flowers.

Observation of numerous infection events over the past 30 years in the state of Washington and Oregon has led to the risk value thresholds for the various orchard blight history scenarios listed in this model. Under no circumstances should these thresholds be treated as if they are absolute numbers. Use them as guidelines. For instance, a common high risk threshold is 200. Due to likely variations in the initial levels of bacteria that are transferred to the flowers, the actual threshold is probably 175 to 225.

Flower wetting is a critical aspect of infection. Wetting that triggered flower infection has occurred from rain, dew of about two hours or longer, misting from nearby irrigation, and light wetting from any form of sprinkler irrigation. Great quantities of irrigation water that directly strikes the blossom does not trigger infection, perhaps because the blossoms are actually washed relatively free of bacteria colonies. Wetting from sprayers does not seen to trigger blight in the Pacific Northwest, but night spraying might if the wet blossoms dried slowly. While the decision aid system makes every effort to assist you to determine that wetting has occurred on the weather monitoring site, you must be aware that the temperature and wetting measurements are taken in non-irrigated sites outside of the more humid orchard, and undocumented wetting may have occurred in the orchard, especially in frost pockets, draws or similar areas with poor air drainage leading to higher humidity and dew.

Watch the model forecast. If high risk is predicted, check your orchards for flowers, including the secondary flowers that develop in May on apples, or into June on Pears. If flowers are numerous, you may choose to protect them with biological control products during the 3 – 4 days leading up to the high risk period, and every 2 – 3 days during the high risk period. If high risk actually occurs, and unprotected flowers are wet, infection is possible. For adequate control, antibiotic materials must be applied within 24 hours before or after the infection (wetting) event.

Further details:

Fire blight danger varies from orchard to another, and over time within each orchard. To assess the risk of fire blight blossom infection, the model user must consider the factors below throughout the Spring and early Summer:


The potential number of strikes is greatly affected by the number of blossoms in the orchard, and late in the primary blossom period is most dangerous. The percentage of contaminated blossoms tends to rapidly rise in orchards near a carry-over source of E. amylovora as the number of days that blossoms have been open increases. While the potential for tree damage is at its highest during the primary bloom period, temperatures during this time are usually lower than those that lead to infection. Late in the primary blossom time, and for the following two to four weeks, secondary blossoms are common on many pear and apple cultivars, and the weather usually warms to levels that may lead to blight infection. The model user should not discount these relatively light late blossoms as significant damage has occurred due to their infection.

Start evaluating 4-day total risk values as the first blossoms open, and continue until few remain. Younger trees, those growing rapidly, and certain highly susceptible cultivars or rootstocks are at higher risk, as any infection may cause extensive tree damage or death.

Recent Blight History

Flowers must be first contaminated by bacteria before they can be at risk of infection. Therefore, many orchards do not experience fire blight even when blight infection weather conditions occur. The risk of blossom contamination leading to blight infection greatly increases if blight has occurred recently in the area near the orchard, even when the cankers have been (apparently) removed. Bacterial contamination of blossoms occurs much more rapidly if there is a near-by active canker.

The model user is asked to take in to account the recent history of blight in the area around the orchard, observe the stage and number of bloom, and set appropriate situation-specific degree hour thresholds.

Severe blight outbreaks may occur without apparent prior-season infection in the region when risk of infection is “High” or “Extreme.” Never assume that E. amylovora is not present, as you will be correct only some of the time.

blight temperaturesTemperatures

The relationship between the development stage of each individual flower and the growth rate of the blight bacteria is complex. During the cooler weather common during primary bloom, an individual flower might last 6 – 8 days from first opening to petal fall. At the temperatures under which blight bacteria colony growth rate is dangerous, the flower in the orchard stays in condition to support that colony growth for about four days. The bacteria must develop to dangerous numbers during the immediate three or four days leading up to blossom wetting. Warmer temperatures induce rapid bacterial growth in flowers.

If bacteria numbers exceed a certain minimum while the flower is in good condition, then the flower is lightly wetted, infection is possible. The sort of daily high temperatures we must be wary of in most orchards start in the mid to high 70’s F (24 C), and are especially dangerous in the 78 – 90F range (25 – 32 C). These sorts of warm days are rare, but can occur during primary bloom, and should alert you to the possibility of blight infection when they occur, especially when it is that warm for two or more days in a row.

Both flower condition and bacterial growth rate degrade as the daily temperatures rise to 95F and over (35 C), especially if these temperatures continue for three or more days.

Infection can occur on a “cool” day if temperatures during the three days leading up to the cool, wet day were warm. Blight bacterial colonies that developed to dangerous size on the warm days do not suddenly go away on the first cool day after the warm period. Watch temperatures over time.

Blossom Wetting

Blossom wetting alone does not cause fire blight. Rain during cold or cool weather does not lead to infection, or blight would be common everywhere, every year.

A blight bacteria colony may grow to the numbers that could lead to infection, but the infection process is not complete without water. The gentle washing of the bacterial colony into the flower nectary is a critical step. Under dry conditions, this factor may be lacking, and infection is avoided.

Rain is the most common wetting event, but there are other equally dangerous ways to wet flowers. While it does not seem that sprayer wetting triggers blight under normal drying conditions, it is possible that it would if high volumes of water were applied (to drip) and the trees were sprayed under very slow drying conditions, at night, for instance. Mist from sprinkler irrigation or dew are the most common, and difficult to identify wetting events. In some production regions, dew is so common during flowering periods, you should probably assume wetting occurs almost every night, and pay most attention to the temperatures and risk value thresholds.

When flowers are present, and the temperatures have been warm, you are often left trying to determine if wetting has happened or will occur. This is a difficult factor to determine, as environmental conditions are quite variable, and remote weather monitoring stations are not always set up to accurately identify wetting in low areas of the orchard, nor do they know when irrigation may have been applied. Wetting may be obvious, but you can never safely assume that no blossom wetting occurred.

There is an automated version of the Cougarblight fire blight infection risk model on the WSU Decision Aid System. This CougarBlight 2010 version 5.0 model is automated, and totals the hourly fire blight degree hour value each hour of the three days leading up to “today’s morning,” and adds to that measured number of degree hours the estimated degree hours for the current day, based on the predicted high temperature and the look-up chard described below.

This model uses the WSU AgWeatherNet data to run a fire blight model for all monitored sites, and is updated hourly. Set the situation relative to blight around your orchard last year, watch the degree hour totals and forecasts, and watch the rain, wetness, and dew point monitors on the upper left part of the page. When the degree hour total is near or over the threshold, flowers are present and wetness is indicated, blight is possible.