Subject: Adjustments to this Fire Blight Flower Infection Risk Assessment Model.

To:  Anyone who is using this model, regarding a new version, “CougarBlight 2009."

”CougarBlight 2000" will be altered somewhat to a new version this year, and those of you who are working with the current version may be interested in some of the alterations, and possibly include them in your current working versions.  The current changes may simplify the use of this model, and may better explain the model output to the user, while maintaining, or perhaps improving its accuracy under the variables of this complex biological situation.

The degree hour values for this year continue to be the same as those in the CougarBlight 2000 version; however, I plan this spring to re-evaluate them, and there will be some changes in the final 2009 version.  There has been some very valuable research done relating to the growth rate of Erwinia amylovora on the stigma surface since this model was first designed in 1992.  I also have a large volume of historical hourly temperature records available, which will perhaps lead to a more refined analysis of the temperatures that occurred during the days leading up to known fire blight infection events.   The general structure of any automated model or spreadsheet you may have been using will not change, but the numbers used to calculate the degree hour totals and the 4-day totals that relate to the four risk categories may be changed.   Meanwhile, use the current numbers.

Risk levels, 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 levels 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 degree hour 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 as a series of High risk days occur.

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, 2+ hours of dew, or light irrigation under these 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.

I have reduced the local blight situation scenarios to three in the new version: 1. No fire blight in your neighborhood last year. 2. Fire blight occurred in your neighborhood last year. (Default setting) 3. Fire blight is now active in your neighborhood.

Risk setting #1 has the following F degree thresholds: Low 0 – 299. Caution 300 – 499. High 500 – 799. Extreme 800+.
Celsius degree thresholds:  Low 0 – 166. Caution 167 – 277. High 278 – 443. Extreme 444+.

Risk setting #2:   F degree thresholds:   Low 0 – 149. Caution 150 – 300, High 300 – 499. Extreme 500+
Celsius degree thresholds:  Low 0 – 79. Caution 80 – 166. High 167 – 277. Extreme 278+.

Risk setting #3:  :   F degree thresholds  Low 0 – 99, Caution 100 -199, High 200 – 299, Extreme 300+
Celsius degree thresholds:  Low 0 – 55. Caution 56 – 109. High 110 – 166. Extreme 167+.

As mentioned above, these 2000 version thresholds may be altered in the final 2009 version.

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, 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 degree hour portion of this model is intended to give you some relative measurement of the amount of relevant heat that has been experienced by the oldest bacterial colonies on the flowers.

Observation of numerous infection events over the past 25 years in the state of Washington and Oregon has led to the degree hour 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 degree hour threshold is 500. Due to likely variations in the initial levels of bacteria that are transfered to the flowers, the actual threshold is probably 450 to 550.

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 has not apparently triggered blight in the Pacific Northwest. 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 wetted, infection is possible. For adequate control, antibiotic materials must be applied within 24 hours before or after the infection (wetting) event.

Please feel free to contact me for further details.

Blossoms: 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 ternds 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 usuallyt 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.

Recent Blight History: Flowers must be first contaminated by Erwinia amylovora 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 most of the time.

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 80 - 88F range (27 - 31 C). These sorts of warm days can occur during primary bloom, and should alert you to the possibility of blight infection when they occur, especially when it is 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 no 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 duriong flowering periods, you should probably assume wetting occurs every night, and pay most attention to the temperatures .

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 version of this 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.

(See the link to this free-access system on the "Current Models" page on this web site. Click here. 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.

Look-up Chart Version of the Cougarblight model:

The look-up chart method described below has served the users well, despite the fact that the values are estimates, and can vary by 10 percent, but usually track the actual hourly values relatively well. If you use the look-up table, take this 10 percent uncertainty into account when deciding risk threshold levels. There is not likely to be much difference between 480 and 520 when 500 degree hours is considered a "threshold" in a biological system such as this.

Use the degree hour look-up chart or this Excel spreadsheet to assign a degree hour value to each day. The total of the degree hours for the four full days prior to blossom wetting helps you assess risk of blossom infection. If blossoms are wetted during the day or evening, total the number of degree days that have accumulated over the past three days, plus the number predicted for the current days high temperature to equal the "four-day degree hour total." If blossoms are wetted in the early morning hours, use the degree hour total from the past four days.