Fire blight models background information

Model logic sources

"The Development and Use of Cougarblight 1990 - 2010.  A situation-specific fire blight risk assessment model for apple and pear."  Timothy J. Smith, WSU Cooperative Extension, 400 Washington Street, Wenatchee, WA 98801.  http://www.ncw.wsu.edu/treefruit/fireblight/smith.htm 

        The Cougarblight 2010 model uses hourly heat unit accumulation for each 24 hours pre day to assign bacterial growth hours for that day.  The 2010EZ version uses daily high temperature to estimate the hourly growth unit sum.  The author states that using the summed hourly values is clearly preferable to assigning daily bacterial growth units from the estimate derived from daily high temperature alone.  Heat unit values for the 4 days leading up to an infection event are used to rate the potential severity for fire blight blossom infection.

         Excerpts from "The Cougarblight 2010 Fire Blight Risk Model" by Tim Smith.
         Additional descriptors of the risk ratings in italics are from the Excel version of Cougarblight published by Agri-Food Canada in cooperation with Tim Smith, online at http://www.agf.gov.bc.ca/cropprot/tfipm/fireblyt.htm#links

"The proximity and number of active cankers makes a great difference in the contamination percentage of flowers, and the initial number of bacteria that start growing on the stigma. This aspect of infection risk is adjusted according to which of the three initial orchard settings you choose."

 

"This model relates the population growth rate of a fire blight bacteria colony (Erwinia amylovora) on a flower stigma to the average temperature of each hour of the day."

 

"Watch forecasts to plan necessary control measures. If your orchard has flowers, risk is forecasted to be high four days in the future, you may apply biological or other non-antibiotic control measures to help reduce the build-up of blight bacteria on the flowers. When risk is high or above, wetting events are considered an infection event. It is almost certain that infection will occur if an infection event occurs when blight bacteria (E.a.) are present on flowers and the model indicates high or extreme risk."

 

"Antibiotics are most effective when applied within 24 hours before an infection event, and are also effective when applied within 24 hours after infection. Efficacy drops each hour after infection event. Full coverage of the interior of flowers is essential for control sprays to be effective."

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Cougarblight assigns risk according to presumed local inoculum levels.  The three categories are:
        1. No fire blight in your neighborhood last year.  ("Neighborhood" is not defined, but a good guess is to assume it refers to the local area within 1 mile of your orchard in any direction.)
        2. Fire blight occurred in your local area last year, but not in your orchard, and there are no currently active fire blight infections in the local area this year.
        3. Fire blight infections occurred in your orchard last year, or there are currently active fire blight infections in your local area this year.

      Fire blight blossom blight infection potential for each category for each day is assigned one of the following levels.  Note that actual potential is incremental and does not fall into separate groupings so neatly.  The growth unit values for each day are displayed so that growers can see if a day is just barely above the next lowest level, or just short of being classified at the next higher risk level.  There is no "Low" risk level assigned for category 3 orchards because with their presumed high inoculum level, these orchards should be managed at no less than the Caution level.  The risk level statements are as follows.  

     Low:  You can rest easy knowing that the conditions for the development of the disease are poor.  Wetting of flowers during these conditions has not led to new flower blight infections in past years. 

    Caution: You should be aware that the development of fire blight in your orchard is a possibility and you should be prepared to act if conditions reach the “HIGH” or “EXTREME” level.  Wetting of flowers by rain, 3+ 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: You should be actively protecting your flowers during the three to four days leading up to the days of forecasted “HIGH” risk of infection. Protective programs should be maintained until flowers are no longer present, or the risk drops to the “CAUTION” level. You should contact your local advisory network and seek their advice.  Numerous serious blight outbreaks have occurred in past years when 4-day heat unit totals 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: You should be expecting fire blight to appear in your orchard and you should take action according to the advice of your local advisory network. Protective programs should be maintained until flowers are no longer present, or the risk drops to the “CAUTION” level.  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 with high temperatures of 95F (35C) or above reduces the risk of new blossom blight infection.

    Exceptional:  This is a new risk assessment added for the 2010 version, for which the minimum threshold is twice that of the "Extreme" rating.  An "Exceptional" rating indicates potential for catastrophic fire blight infection risk.

 

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Maryblyt-v7 for Windows.  Paul Steiner, Gary Lightner, Alan Biggs, and William Turechek.   http://www.caf.wvu.edu/kearneysville/maryblyt/
Excerpts on Maryblyt threshold settings from  "Maryblyt - Questions and Answers" at http://www.caf.wvu.edu/kearneysville/maryblytfaq.html

"Example: You have had and continue to have a serious problem with fire blight (hence, an abundance of inoculum in the orchard) and your decision to spray\not spray hinges on temperature data that gives you high risk at 59.5F and infection risk at 60F -- prudence and experience would favor making the streptomycin application. If, on the other hand, you have done a good job in the past in managing fire blight and have not had a serious outbreak in the last 2 years, your risk here is marginal and little loss if any is likely to occur if you don't spray. Despite the fact that we assign numbers to certain thresholds (i.e. 198 cumulative degree hours, 60F, 0.01 inch or 0.10 inch of rain) does not mean these are ABSOLUTE LIMITS. Wherever possible, we have chosen our program thresholds on the lowest possible, CONSERVATIVE judgment. Because of this making a decision not to spray when these conditions are just marginal is not likely to result in significant levels of infection."

 

"EIP = 100 means that approx 3-5% of the blossom OPEN that day are colonized. If you have 20% bloom, then 3% of 20% = (0.03 x 0.20)100 = 0.6% of open blossoms could be infected if it rains... etc. Maryblyt was built ON THE ASSUMPTION that there was an abundance of inoculum. A light to moderate amount of fire blight, if not properly and promptly attended to can provide an abundant amount of inoculum. Where an aggressive blight management program has been used and the overall number and distribution of potential overwintering cankers is low, then EIP=100= somewhere less than 3-5% flowers colonized."

Additional sources:

Timothy J. Smith, Cougarblight 2009. Washington State University.  http://www.ncw.wsu.edu/treefruit/fireblight/2000f.htm

Timothy J. Smith, Report on the Development and Use of Cougar Blight 98C - A Situation Specific Fire Blight Risk Assessment Model for Apple and Pear.  Washington State University.
http://www.ncw.wsu.edu/fbsmith.htm

Timothy J. Smith, A Risk Assessment Model For Fire Blight of Apple and Pear, Version 1998 Update.  Washington State University.

Timothy J. Smith, Cougar Blight 2002 Fire Blight Risk Assessment Model. http://www.ncw.wsu.edu/treefruit/fireblight/2000f.htm.  Washington State University.

Timothy J. Smith, Fire blight Degree Hour Values.  http://www.ncw.wsu.edu/images/curve.gif.  Washington State University.

Paul W. Steiner and Gary W. Lightner, MaryBlyt 4.1,  A Predictive Program For Forecasting Fire Blight Disease in Apples and Pears, University of Maryland at College Park.  1992

Zoller, B. G., and Sisevich, J.  1979.  Blossom populations of Erwinia amylovora in pear orchards vs. accumulated degree hours over 18.3C (65F), 1972-1976.  (Abstr.)  Phytopathology 69:1050.

Personal communication with the late Paul Steiner and Gary W. Lightner about subsequent updates in MaryBlyt model logic and epidemiology of fire blight.  Personal communication with Timothy J. Smith on fire blight epidemiology.

Management

Dave Rosenberger, Fire Blight Rundown, Scaffolds Fruit Journal 9:15, June 26, 2000, Cornell Cooperative Extension
http://www.nysaes.cornell.edu/ent/scaffolds/2000/6.26_disease.html

Timothy J. Smith, Suggested tactics for fire blight management, Good Fruit Grower, March 15, 1999.

Timothy J. Smith, Cutting Fire Blight From Infected Apples and Pears, Washington State University Extension, 1998.  http://www.ncw.wsu.edu/treefruit/blightcut.htm

Paul W. Steiner, Managing Fire Blight in Apples, Illinois Horticultural Society Meeting, January 2000.  http://www.caf.wvu.edu/kearneysville/articles/FB-MANAGE00.html

Paul W. Steiner, The Biology and Epidemiology of Fire Blight, Illinois Horticultural Society Meeting, January 2000.   http://www.caf.wvu.edu/kearneysville/articles/FB-BIOLOGY00.html

Paul W. Steiner, A Philosophy for Effective Fire Blight Management, State Horticultural Association of Pennsylvania Annual Meeting, January 2000.   http://www.caf.wvu.edu/kearneysville/articles/PHILOSOPHY2000.html

Paul W. Steiner, Problems Managing Fire Blight in High Density Orchards on M-9 and M-26 Rootstocks, State Horticultural Societies of Virginia and West Virginia, Annual Meeting, January 1998.   http://www.caf.wvu.edu/kearneysville/articles/SteinerHort100.html

Paul W. Steiner, How Good Are Our Options With Copper, Bio-Controls, and Aliette for Fire Blight Control?, State Horticultural Societies of Virgina and West Virginia, Annual Meeting, January 1998.   http://www.caf.wvu.edu/kearneysville/articles/SteinerHort200.html

Comparing Cougarblight and MaryBlyt 
     Both models track heat units on days leading up to a wetting event, using slightly different methods to do so.  Only days with open blossoms count towards heat unit accumulation thresholds. 

       Both models require some type of wetting to initiate infection, and allow for heavy dew in the absence of rain as sufficient to cause such wetting.  Dew that only affects the orchard grass is not enough to cause infection.  As there is no independent measure of heavy dew, an observation of 2 or more hours of leaf wetness is used as a proxy to represent the possibility of heavy dew.  This is an imperfect measure, but it is the best we have.  MaryBlyt also allows for infection on a day with no rain or dew if the previous day had more 0.1” rain.

       Under most conditions, airblast spray water is not considered sufficient to promote fire blight infection.  However, adding airblast spray coverage to flowers already wet with due could increase risk of blossom blight infection.  Overhead irrigation can provide enough moisture to create devastating fire blight infections.

       For Cougarblight, Orchard Radar uses hourly temperature values to accumulate heat units according to the Cougarblight 2010 hourly heat unit assignment table published by Dr. Smith.  These are not calculated using a degree hours formula, but are an association of different temperatures with risk of fire blight blossom infection.  In Cougarblight, the number of accumulated heat units on the 3, 4, or 5 days before a wetting event (5 from King Bloom to Full Bloom, 4 from Full Bloom to 30 days later, 3 from 30 to 60 days after bloom) are added to those added on the day of wetting.  Temperatures between 12am and 8am are assigned to the previous calendar date.  The heat unit total is compared to thresholds defined according to the orchard and regional fire blight history during the past two seasons.

      MaryBlyt also accumulates heat units for as many days as fall within 80 degree days base 40 prior to the wetting event.  This usually turns out to be 4 days just like Cougarblight, but it does allow adjustment for longer or shorter persistence of individual blossoms under unusually cool or warm conditions.  MaryBlyt also requires that the average temperature on the day of infection be at least 60F, a condition that Cougarblight does not require.  While no formal study has been done, there is anecdotal experience in Maine that an average temperature of 60F is not required for fire blight infection to occur.

       Despite their differences, comparisons of the MaryBlyt and Cougarblight models have shown that the two models are more alike than they are different in identifying possible fire blight infection periods, and that neither is a perfectly sensitive forecaster to correctly identity conditions as positive or negative true fire blight risk, and neither is perfect specific at avoiding falsely saying infection would occur, or falsely saying infection would not occur.

      The choice of model seems less important than using one or both models to keep a watch for possible infection conditions in the forecast and in consideration of block history and local inoculum level treating when infection conditions seem hazardous.  For a detailed though not definitive statistical comparison of MaryBlyt with an earlier version of Cougarblight, see Dewdney, M. M., Biggs, A. R., and Turechek W. W. 2007. A statistical comparison of the blossom blight forecasts of MARYBLYT and Cougarblight with receiver operating characteristic curve analysis. Phytopathology 97:1164-1176.,  http://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-97-9-1164