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. By this time, the blight bacteria have had ample opportunity to be carried to the flowers, spread, and build in colony number. While primary blossoms are most numerous, temperatures during this time are usually lower than those that lead to infection. The model user should not discount the relatively light late blossoms produced on certain apple/rootstock

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 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. Bacteria also tend to infest an increasing percentage of the flowers as the primary bloom period advances toward petal fall.

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 "Extreme." Never assume that E. amylovora is not present.

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 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. Mist from sprinkler irrigation or dew are the most common, and difficult to identify wetting events.

When flowers are present, and the temperatures have been warm, you are often left trying to deteermine 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.