Fire Blight Daily Risk Estimation Model: Version 98F

Timothy J. Smith
WSU Cooperative Extension
400 Washington, Wenatchee, Washington 98801, USA

If your pear or apple blossoms are wetted, the number of fire blight strikes and the extent of damage they cause depends on three major factors:
1) The tree (variety, age, vigor, and the number of blossoms present);
2) The relative presence of fire blight bacteria in the area (did you have blight in the local area within the past several years, last season, or is it present this year?); and
3) The potential for bacterial growth in blossoms during the past few days.

To evaluate the risk of infection, you must evaluate all the above risk factors. If there are no blossoms, there is no risk of blossom ihnfection. The potential number of strikes rises as blossom numbers increase. Younger trees, those growing rapidly, and certain hightly susceptible varieties are at higher risk, as infection may cause extensive damage or death. Risk of blight infection greatly increases if blight has occurred recently in the area, even if the cankers have apparently been removed. Infection risk is especially high if active cancers are present nearby. The risk of infection increases or decreases with rising and falling daily temperatures. Warmer temperaturesl allow rapid bacterial growth in flowers; if bacteria numbers reach a certain minimum, and moisture moves the bacteria into the flower's nectary, fire blight is possible.

To use this risk model, you may estimate the growth rate of the fire blight bacteria by evaluating the temperatures that have occurred over the past three days, plus the present day. A chart was developed to determine the average number of "degree hours" that have occurred on a day with a certain high and low temperature [this chart has been incorporated into the on-line calculator at]. When blossoms are present in the orchard, record this number daily. The total number of degree days that have accumulated over the past three days, plus the number predicted for today, equal the "four-day degree hour total."

EXAMPLE: 3 days ago, 76/45 = 130 degree hours. 2 days ago, 80/51 = 230 degree hours. Yesterday, 80/45 = 195 degree hours. Today's predicted temperature, 70/42 = 52 degree hours. The sum of these four days degree hours equals 607 degree hours.

Every day will have a new "four-day degree hour total." If blossoms are wetted, usually by rain, but sometimes by heavy dew or by light irrigation wetting, the following table will help you to evaluate risk of infection. Consider the "four-day degree hour total," the risk factors described above, and the general potential of active fire blight cankers in your area. Most fireblight occurs after "high" or "extreme" risk infection periods. Sprays are most effective when applied prior to blossom wetting. Sprays may provide adequate control when applied within 24 hours after a "high" risk infection period. Control is difficult during "extreme" risk periods. During "extreme" risk periods, blight sprays must be applied on a pre-infection schedule (as described on the label), as well as within 24 hours after a wetting period.

Potential for
pathogen presence
Low Moderate High Extreme
No Fire Blight in area
past 2 seasons
360-400 450-500 500-800 800+
Fire Blight in your or near
your orchard last year
100-200 300-350 350-500 500+
Active cankers or strikes
are now nearby
0-50 100-200 200-350 350+

Apple Scab Model Interpretation

OSU Plant Path - Scab Spores End

So when do those apple scab ascospores stop flying around? The end of the primary infection period signals the end is in sight for apple scab applications. Determining when this occurs has been difficult. One method uses squash mounts and a microscope while another uses a spore trap to determine how many spores are left. Recent research from New York has found that a simple degree day model relates well to these other methods. The model simply accumulates the number of degree days from bud break using a base temperature of 32 F. When 760 degree days have accumulated over 95% of the ascospores have matured. Bud break (biofix) was defined as when half the (McIntosh) buds are from silver tip to green tip. To calculate degree days, all you need to do is record maximum and minimum temperatures each day. Add them together, divide by 2 and subtract 32 and you are left with the "degree days" for that day. You could buy the needed equipment (rather cheaply) but you can also get the information on the Web. A nifty web site will do all the calculations for you and for many areas around the state (see below). Using this site I can quickly tell that most apple growing areas around the state are at or near the 760 threshold, depending on when you think McIntosh broke bud. (

Now what? The model states that when the next rain over 0.1 inches along with a temperature over 50F will deplete the ascospore supply. In a really clean orchard you could stop spraying after that period.

Conida (asexual spores) from lesions inside or outside the orchard complicate that strategy. Conida can start new infections (with shorter wetting periods) and keep the disease cycle going. Scab lesion can be observed now and a good scouting program will help make decisions. We also cannot forget about control of powdery mildew and other pest problems.

**** This Information provided (on May 4) by Dr. Jay W. Pscheidt, OSU Extension Plant Pathology Specialist, (E-Mail), (voice) 541-737-3472, (mobil) 541-740-6621 (FAX) 541-737-2412.

Recent Developments in the Epidemiology of Apple Scab

D. M. Gadoury, R. C. Seem, A. Stensvand, and S. P. Falk

Dept. of Plant Pathology, Corvell University
New York State Agricultural Experiment Station

Available electronically via email from

Hop Downy Mildew Emergence Model

Dave Gent - May, 2010
Management of downy mildew is dependent on regular application of fungicides, supplemented by sanitation practices. The downy mildew pathogen, Pseudoperonospora humuli, survives in systemically infected crowns, and appears the following season in systemically infected shoots. The emergence of infected shoots can be predicted based on temperature, providing a means to determine when scouting for the disease should or when the first fungicide application should be made in highly susceptible cultivars.

Oregon Model:
The Oregon degree-day model was developed by
Gent et al. (2010) to predict the probability (%) that shoots with downy mildew have emerged based on air temperature. The accumulation of 115.7 degree-days Celsius base 6 C (208.3 degree-days Fahrenheit base 42.8F) corresponds to the 50% probability level that shoots with downy mildew have emerged.

The purpose of the Oregon degree-day model is to:

(1) Time the first fungicide application for downy mildew in yards planted to highly susceptible varieties and with a history of the disease; or

(2) Begin scouting for downy mildew in yards planted to moderately susceptible varieties or no history of the disease.

This model uses a single sine function to calculate degree-days. The base temperature is 6°C (42.8°F ) and the biofix is 1 February. Temperature measurements are taken at standard meteorological height (1.5 m). To calculate degree-days based on a single sine function, the minimum and maximum temperatures for a day are used to produce a sine curve over a 24-hour period. Degree-days for that day are then obtained by calculating the area above the threshold and below the curve.

Table of probabilities associated with degree-day accumulation for the Oregon model.

Degree-days (C) Commentary
0.000000001 90.6065
0.00000001 90.6178 
0.0000001 90.6375
0.000001 90.6734
0.00001 90.7426
0.0001 90.8879
0.001 91.2303
0.01 92.2073 Begin scouting for downy mildew in highly susceptible varieties
0.02 92.8401
0.03 93.3643
0.04 93.8377
0.05 94.2816
0.06 94.7069
0.07 95.1199
0.08 95.5247
0.09 95.9242
0.1 96.3204
0.2 100.349
0.3 104.914
0.4 110.474
0.5 117.606 (1)Apply first fungicide spray in highly susceptible varieties with a history of downy mildew;
            (2)Begin scouting in less susceptible varieties
0.6 127.295
0.7 141.538
0.8 165.371
0.9 217.916
0.91 227.37
0.92 238.437
0.93 251.642
0.94 267.788
0.95 288.189
0.96 315.177
0.97 353.459
0.98 414.633
0.99 541.215

As an alternative to look up table, the probability of emergence of shoots with downy mildew can be calculated using the following formula:

Probability of emergence (0 to 1) = -6.086 + 7.0571 + [dd x 52.415 - 3.226] (equation 1)

where dd = current degree-days calculated as described above for the Oregon model (bio-fix February 1, base 6C, single sine calculation method).

[Home] [Intro] [Data Table] [Data Map] [DD Calc] [DD Models] [Links]

This project funded in part by a grant from the USDA-Western Regional IPM program.

This page on-line since July 1, 1997
Last updated May 18, 2010
Contact Len Coop at if you have any questions or comments about these web pages.