Fire blight models background information
Model logic sources
"The Development and Use of Cougarblight 1990 -
A situation-specific fire blight risk assessment model for apple and pear." Timothy J. Smith, WSU Cooperative Extension,
400 Washington Street, Wenatchee, WA 98801.
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
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."
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
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."
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."
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
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.
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.
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.
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
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.
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
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.
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
Maryblyt-v7 for Windows.
Paul Steiner, Gary Lightner, Alan Biggs, and William Turechek.
Excerpts on Maryblyt threshold settings from
"Maryblyt - Questions and Answers" at
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."
Timothy J. Smith, Cougarblight
2009. Washington State University.
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.
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
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
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
Dave Rosenberger, Fire Blight Rundown, Scaffolds
Fruit Journal 9:15, June 26, 2000, Cornell Cooperative Extension
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.
Paul W. Steiner, Managing Fire Blight in Apples,
Illinois Horticultural Society Meeting, January 2000.
Paul W. Steiner, The Biology and Epidemiology of Fire
Blight, Illinois Horticultural Society Meeting, January 2000.
Paul W. Steiner, A Philosophy for Effective Fire Blight
Management, State Horticultural Association of Pennsylvania Annual Meeting,
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.
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.
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
Orchard Radar uses hourly temperature values to
accumulate heat units according to the Cougarblight 2010 hourly heat unit
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.