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Basic Version
of Model in Excel

This model was developed during the 1980's and has been the standard Fire Blight Risk Model in Washington and Oregon since the 1990's. The basic use of the model is simple to learn, requires no computer or other special equipment, and is freely distributed to users. The advanced forms of the model are automated and available for use on web sites such as the WSU Decision Aid System. <--- Click to go there.

Example of how DAS fire blight page appears during the season: DAS Cougarblight

The model requires the user to recognize specific and ever-changing local events and aspects of their orchard that may increase or decrease fire blight risk relative to other orchards in the region. The model requires the user to assume there is a risk of fire blight infection whenever blossoms are present on the trees, especially during the petal fall and "post bloom" period, when scattered blossoms may remain on many apple and pear varieties. The model user is asked to carefully assess the situation on their specific site and to initiate control measures if blossoms are present, risk levels are "High" or "Extreme," and blossom wetting is likely to occur sometime during the next 24 hours.

Model Structure:

Temperatures and Wetness: The key Fire Blight process that must be modeled is the potential for bacterial growth on the stigmas of apple and pear flowers. This growth is temperature dependent, so dependable prediction of infection risk requires the use of a measurement method that most accurately reflects the growth of Erwinia amylovora colonies. The main disagreement among modelers is how this should be done.

The Cougarblight model estimates bacterial growth rate with degree hours based on a specific growth rate curve. This growth curve is based on the growth rate of E. amylovora bacteria in laboratory tests. The degree hour values are accumulated each hour of the day that temperatures are over 60F. The hourly values increase as temperatures rise from 60 to the mid-80's, decline at higher temperatures, and reach zero for any hour with temperatures over 105F. The model user does not actually calculate these hourly values, but uses daily high and low temperatures and a look-up chart to assign daily degree hour values. Estimated degree hour values on the look-up chart and actual degree hour values are similar, and repeated tests have shown no practical differences in risk ratings when automated monitoring systems have produced actual daily degree hour values to the model user.

The Cougarblight model user sums the degree hour values of the four days leading up to the time of potential blossom wetting to evaluate the degree of infection risk. Each day will have a new "four day degree hour total." Risk potential in the near future may be evaluated with forecasted temperatures.

In the Pacific Northwest USA, blossom wetting when degree hour totals are below 500 may lead to light, scattered fire blight outbreaks. These outbreaks of blossom blight are usually in orchards with near-by active carry-over cankers. Fire blight damage increases in the region as degree hour totals rise over 500, and severe, widespread damage can be expected if degree hour totals are near 800 when blossoms are wetted. However, the degree hour risk thresholds vary, depending on the potential fire blight bacteria presence in the orchard. The risk threshold table outlines the various thresholds. In practice, infection has not been at all common in orchards unless the degree hour totals at the time of flower wetting are near the "High" risk level or above. These numbers are not absolute! If degree hour totals are near the threshold, there is some degree of risk.

Date	DAILY HIGH TEMP	LOW TEMP.	DAILY DEGR.Hrs. (CHART)	4- DAY TOTAL DEGR. HOURS	RAIN/ BLOSSOMS WET?	COMMENTS:
4/14	63	49	9	9	.	.
4/15	61	44	2	11	.	.
4/16	68	43	33	44	.	Full Bloom Pear
4/17	69	40	42	86	.	.
4/18	70	44	52	129	.	1st Bloom Apple
4/19	74	48	100	227	.	.
4/20	77	49	146	340	.	.
4/21	82	47	228	526	.	.
4/22	78	45	162	636	Local Rain Shower	Infection occurred
4/23	71	48	62	598	.	Apple Full Bloom
4/24	64	42	10	462	.	.
4/25	60	43	0	234	.	.

Infection did not occur in most of the area apple orchards the situation described above because: 1. most of the region did not have the rain showers, 2. No dew wetted flowers in the orchards outside of the rainfall area, and 3. most flowers were probably not contaminated with the blight bacteria at this early stage of bloom. The area where blight was a problem that season was an early blooming region, and had carry-over cankers in the region from the previous year, ensuring an early start in the blossom contamination process.

Local conditions:

Growers may set personal risk thresholds at lower than those recommended on the model if the site in question has higher than usual risk of fire blight damage. The block may be young, an especially fire blight sensitive variety, on blight sensitive rootstocks, have high flower numbers, and be in an area that seems to be fire blight prone.

Use of the Model:

Starting when blossoms first open in the orchard, write down the degree hour value indicated on the look-up table for the known high and low temperature the prior day, and the predicted high and low for the current day. Write the next three days degree hour values based on temperature forecasts. Then give each day a value based on the current day plus the three previous days. Count only those days where open blossoms are present.

Update the numbers daily. Spray appropriate materials as the degree hour totals approach "high" risk. Maintain control until degree hour totals drop below your chosen threshold, or blooming stops. If degree hour totals are well above the 500 threshold, nearing or over the "extreme" risk numbers, and flowers are numerous in the orchard, carry out all possible control measures to their maximum degree.

The model will most often indicate that sprays are not necessary. If you find that you are spraying very often when infection conditions are not relatively obvious, you are probably using a too-low personal risk threshold.

The most current version of the model may be found at the internet site:

http://www.ncw.wsu.edu/treefruit/index.html