FIRE BLIGHT DAILY RISK ESTIMATION MODEL:

VERSION 2002C ( Celsius )

Timothy J. Smith, WSU Extension
400 Washington St., Wenatchee, WA 98801 e-mail smithtj@wsu.edu

More Fire Blight information:

Fire blight danger varies by orchard, 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:

Flowers on the Tree: The potential number of strikes is greatly affected by the number of blossoms in the orchard, but the model user should not discount the relatively light late blossoms produced after primary bloom on many susceptible varieties. Start counting degree hours as the first blossoms open, and continue until few remain. Younger trees, those growing rapidly, and certain highly susceptible varieties or rootstocks are at higher risk, as infection may cause extensive structural damage or tree death.

Recent Blight History: Blight may appear after being absent for many seasons. However, bacterial contamination of blossoms occurs much more rapidly if there is a near-by active canker. Experience shows the risk of blight infection greatly increases if blight has occurred recently in the area, even if the cankers have been (apparently) removed. The model user is asked to take in to account the recent history of blight in the area around the orchard, and set degree hour risk thresholds accordingly. No orchard is entirely free of infection risk, but those in an area with recent blight are at greater risk.

How the weather Influences the Infection Process: The blight bacteria can live only a few days (3-5) on individual flowers. Warmer temperatures allow more rapid bacterial growth in flowers. Infection is most likely if the following events occur in sequence: 1. bacteria contaminate flower stigma tips, 2. warm daily temperatures allow bacteria numbers to exceed dangerous thresholds during the four days the flower is in good condition, 3. moisture from rain, dew or light irrigation wetting moves the bacteria into the flower's nectary. Infection can occur on a "cool" day if temperatures during the 2-3 days leading up to the cool, wet day were warm. Watch temperatures over time.

Blossom Wetting: Blossom wetting alone does not cause fire blight, but if recent temperatures have allowed rapid bacterial development, blossom wetting is the infection "trigger." Infection is uncommon when degree hour totals are "Low" or "Caution." Spray controls are often necessary when blossoms may be wetted while risk of infection is near or above the "High " threshold. Severe outbreaks may occur without apparent prior-season infection when risk of infection is near or over "Extreme."

Most sprays are most effective if applied prior to blossom wetting, especially during "extreme risk" temperature conditions. The biological materials and other sprays that are intended as preventative (such as fungicides and copper) must be used to suppress the build-up of bacteria on the stigma tips of flowers during the four days prior to the potential infection period. These "softer" products are much less effective after the bactirial colonies have developed on the young flowers. Antibiotic sprays are less effective when applied after an infection period, but are very beneficial for about the first 12 hours post infection, beneficial if applied within 12-24 hours after infection, and may be of some effect during the 24-48 hour post infection period.

Evaluating Risk: Most accurate degree hour values are totaled using houry temperatures for the 96 hours leading up to a three + hour wetting of flowers. These hourly temperature based assessments are best when done by automated monitoring systems. The hourly degree hour values are available from the author, contact smithtj@wsu.edu.

If you have no access to hourly temperatures, use the degree hour look-up chart to assign a degree hour value to each day. (These estimated daily degree hour values may vary from actual by 10 percent, so risk threshold numbers should be considered as approximate.) 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 day 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.

Example:

EXAMPLE: 3 days ago, 21/9 = 28 degree hours. ( See table below for DHr. values) 2 days ago, 28/11 = 150 degree hours. Yesterday, 28/8 = 129 degree hours. Today's predicted temperature, 25/10 = 94 degree hours. The sum of these four days degree hours equals 401 degree hours. If blossoms are wetted, fire blight infection risk is "High."

Every morning it is a relatively quick procedure to calculate a new "four-day degree hour total." You should also use temperature forecasts to watch for future risk levels. This is the most useful aspect of this model. To accurately evaluate potential infection periods, you must have a way to monitor for blossom wetting. A leaf wetness sensor is probably the best method. If blossoms are wetted, usually by rain, but sometimes by heavy dew (3+ hours) or by light irrigation wetting, the following table may help you evaluate infection risk. Evaluate your "four-day degree hour total" for the four days leading up to the wetting period, the risk factors described above, and the potential of active fire blight cankers in your area.

Sprays are most effective if applied prior to blossom wetting, especially during "extreme risk" temperature conditions. Antibiotic sprays are less effective when applied after an infection period, but are very beneficial for about the first 12 hours post infection, beneficial if applied within 12-24 hours post-infection, and may be of some effect during the 24-48 hour post infection period.

INFECTION RISK RELATIVE TO 4-DAY DEGREE HOUR TOTAL:

ESTIMATED NUMBER OF DEGREE HOURS FOR A DAY, BASED ON DAILY HIGH TEMPERATURE