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2014 Fire Blight Alerts

Program Contact: Tianna DuPont, Tree Fruit Extension Specialist
(509) 663-8181 •

by Tim Smith, WSU Extension 

Thursday, June 5, 2014

The blight is now being reported from most parts of central Washington. Growers who are turning their attention to cherry harvest may complete harvest and find that while they were not paying attention to the pears or apples, fire blight appeared in their orchard.

It appears that 2014 may be a fire blight year to remember.

Below is a report that I posted at the end of May, which contained three distinct infection periods. When in early May, one in the middle of May, and one of the end of May. I don’t think infection is done with us yet, as there are plenty of pears with true shoot tip blossom clusters forming, on Bartlett especially, but also check the Red Sensations (Red Clapp’s Favorite.)

Tuesday, May 27, 2014 update

Fire blight outbreaks are being reported from various parts of the state, some large and serious.  These infections are likely those from the May 1 – May 6 infection period.   We are now in a short interval between infections, but more high risk days are coming soon.  The usual fire blight season has 0 to 2 important infection events, where flowers, bacteria, warm temperatures and flower wetting all line up on the same day, usually not in more than one or two regions of the state.

There have been three distinct High to Extreme Risk infection periods this season, and all parts of the state have had infection risk on at least once or twice, some have had 4 or 5 infections:

The first, and most dangerous was from May 1 to May 6, in warmer parts of the state or areas that had fire blight last year.  Infection was High+ risk everywhere from May 2 to 5. This period was most dangerous this season due to the high number of primary or secondary blossoms that were on apples and pears all across the state.  Blight started appearing from this one about May 18 – 20, and may continue to be first noticed for a few more days.

The second and third infection periods occurred over an extended number of days, lasting from May 14 to May 26 in most regions of the state, with a dip in temperatures and risk in some areas about the 20th. There were infection events caused by dew and rain on many sites, and the amount of damage from these will vary due to flower presence/absence, bacterial numbers by site, and variable wetting.  The blight from these infection periods will begin to show soon, if it hasn’t already.

The fourth infection period is in the forecast starting about May 31 and we don’t know when that one will end.  I have seen infection of late bloom as late as June 10, so continue to watch for late blossoms in your orchards.

Please contact me with blight news from your region.

In 1985. the weather pattern was very much like this year.  There was a minor warming, but no infections in very early May (2-5).  Then another very warm and dangerous infection that came when D’Anjou and Bartlett 2nd blossoms were present, starting May 16 and lasting until May 28th.  There was rain almost daily from May 23 – 28.   A great amount of infection occurred.  I believe conditions this year are worse than they were in 1985, though that remains to be seen, as we are not finished with the 2014 infection season.

The most recent year during which two important and damaging infection periods occurred was 2012, when the first one occurred in late April and the second one occurred in mid-May. This year, the weather to crop development pattern is similar, early May and mid-May.  In 2012, there was a High to Extreme Risk period that lasted from  May 13 – 19, almost exactly as this season has developed.  There was serious damage to apple and pear orchards in many areas of the state in 2012.

There may be no flowers in your orchard, no wetting of flowers, and hopefully, no blight bacteria in those flowers.  If you expect all three of those factors were met dur8ing high+ risk, the best time to have treated was the day ahead of the infection event.  The day after infection is o.k., but not as effective.  Other years that an unusual number of growers had a problem had the following High to Extreme Temperature Risk periods: 1997, May 10 to 19, and 2013, May  14 to 19.

Here are some numbers that were current as of the time the flowers were wet for 2+ hours during the early May infection. listed for each recorded wetting event: Oroville- 665 and 653, Tonasket- 708 & 720, Loomis Grade- 576 & 577, Orondo- 527, Peshastin- 568, N. Cashmere- 639, WSU TFREC- 522, Malaga- 670 & 537, N. Pasco- 753 & 621, Finley- 758 & 541, Green Bluff- 226

This report stated on May 6:  “The first signs of blight infection from this early May infection should start showing by the end of this next week or so (May 16 – 21).  The Fire blight from this infection may become visible and produce bacteria for further infection of flowers in the orchard by the time this mid-May infection period Is completed.  During the May 1 to 5 infection, there were numerous flowers freshly opened in many apple and pear orchards, late blossoms were very common in the earlier sites, and late primary bloom was still under way in the cooler areas.”

Fire blight never occurs in all blocks in an area, nor does it happen at the same level from orchard to orchard.  The degree of damage depends upon the presence or absence of the Erwinia amylovora bacteria, and how well those bacteria have grown during the previous 4 – 5 days.

Last year was a “fire blight year” in many areas of the state, which increases the chances that blossoms are contaminated with blight bacteria in these regions.

In similar situations in the past, orchards that had blight bacteria present and were not carefully protected with effective control materials have suffered extensive fire blight damage.

The best time to apply preventative materials, such as biologicals, is in the three of four days leading up to the predicted high risk period, and reapplied every 3 days during the high risk period or until freshly opened flowers are no longer present in the orchard. If an infection event occurs during the high risk period, apply an effective antibiotic material (oxytetracycline or terramycin) within 24 hour before the wetting to 24 hours after.

Organic growers who do not wish to utilize oxytetracycline products may opt for alternatives, such as Serenade Optimum which seems safe to fruit finish in our recent trials or Blossom Protect, a more effective product (in our recent trials), but which comes with a slight risk of causing russet, especially when applied to pears.  Both of these products may be used in organic or conventional orchards.

Overview:  A more than usual percentage of orchards in the state have active cankers in the neighborhood, so fire blight may occur on flowers in those orchards even though temperatures may not be al the levels that start general fire blight outbreaks (75-90F over 3 – 5+ days.)  Fire blight will occur after warm, then wet conditions where blossoms are contaminated with high numbers of blight bacteria.  These high populations of the blight bacteria growing on the stigma surface may have started relatively few in number, but grew to large colonies during warm weather, or they may have been introduced to the stigma in high numbers, so need no warm weather to increase to levels that could infect the blossom if wetted.

These warm, then wet conditions have become more frequent in the past decade. Recently, these prime infection events have occurred only once per season, but these infections have led to serious problems that remained for the rest of that growing season.

Some seasons, there may be two or more infection periods, each with its unique mix of host flower stage development, daily temperatures and presence/absence of blight bacteria.  The potential for damage often is more serious in the second infection period.

Last year, (2013) the warm period followed by rain occurred in mid-May, which coincided with the blooming of apples and pears in the latest  districts (such as Okanogan County and Spokane) and with the late blossoms on Bartlett pears and some apples.  There were numerous reports of blight appearing in June from these mid-May infections.

These spring 2013 infections were made worse by the numerous violent thunderstorms that crossed the state from south to north numerous times during the summer.  The wind and rain led to wounding of young, growing shoot tips contaminated with blight bacteria produced by early season infections. leading to wide-spread reports of late season outbreaks of blight.  This is abnormal for the usually dry central Washington growing region.  These late infections are important because they are more likely to survive the winter to ooze this spring.

Previous comments:

On the WSU DAS model, when infection risk is indicated at “marginal” the likelihood of infection is actually low. Marginal is a risk word used to alert you to possible rising risk, and that risk may approach an important threshold of “high”during the next few days. There is no expectation of infection until the risk level is high. If you are trying to be extra careful about fire blight infection, you may wait at least until the risk level rises to “marginal – high.”

The fire blight bacteria need warm weather to thrive and infect host trees. The minimum temperatures that enable the bacteria to grow fast enough to cause infections are in the mid-70’s, and they become especially dangerous in the 80’s and low 90’s. Daily temperatures in the 50’s and low 60’s will prevent fire blight infections, regardless of treatment, or no treatment.

Rain does not cause blight infection. Rain can trigger blight infection only if the threshold of heat has occurred prior to the wetness. Cold rain during a series of cold spring days is not dangerous.


The Washington State University CougarBlight 2010 fire blight risk analysis model has been significantly changed, especially in the way that temperatures are measured and related to risk of flower infection. Cougarblight 2000 remains useful, but users are requested to transition to this new version.

On the WSU DAS model page you will find the current Temperature Risk Value, the recent wetting events, record of rainfall, risk of rain, forecasted risk, and risk level at the current hour available. Be certain to set the “Your Risk Setting” situation first, or most of this information will not display.

The model structure uses the same principles as in the past. There are four major aspects of infection that this model evaluates to estimate risk.

Use of the model, the basics:

A. Set the “Your Risk Setting” relating to the recent history of fire blight in your neighborhood.

B. When flowers are on your trees, watch the “Model Status” forecast for predicted risk levels for today and the next four days.

C. If risk is low, infection is unlikely.

D. If infection risk is “marginal” infection is not likely, but you should be alert for changes predicted in the next few days. If the prediction of risk is rising into “high,” you could start protecting your blossoms with biological products (Bloomtime, Blight Ban, Serenade) during the 3 or four days leading up to the “high” risk date to help prevent the build-up of blight bacteria on the apple or pear stigmas.

E. When the risk level is “high,” infection is possible whenever the blossoms are wetted for 2 or more hours. These events are recorded for four days under the “wetting events” tab. Antibiotic sprays must be applied within 24 hours before or after the infection event to be most effective.

More Details:

  1. The presence of flowers. If there are no flowers present, ignore risk warnings. This is a blossom infection model.
  2. The level of disease pressure in the orchard. This “Your Risk Setting” must be sellected before the model will work. Has blight been a problem recently? What degree of confidence do you have that there are no active cankers in or near your orchard? 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. The recent or current presence of blight in your orchard or neighborhood (adjacent orchards) lowers the temperature risk value (TRV) thresholds automatically in the DAS model interface.
  3. The temperatures that have occurred over the last four days (96 hours). 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. This temperature relationship is expressed as Temperature Risk Value (TRV), and is reported to you as the total TRV units that have occurred over the past 96 hours. The past and current forecast of 4-day TRV from day to day are reported in the “Model Status.” The number of TRV that have accumulated over the past 4 days (96 hours) before the current hour is reported under “Today’s Current Data as of…(hour).” These are not the “fire blight degree hours” as they were in Cougarblight 2000, but play the same role in assessing bacterial growth rate per hour. The number of TRV that indicate risk of infection (low, marginal, high, extreme) depends on which of the three “Your Risk Setting” modes you choose. The higher the chance that active blight cankers are in the area, the lower the TRV thresholds. These are automatically chosen relative to your Risk Setting choice.
  4. The documentation of a two hour or more blossom wetting event. This two hours or more of wetting is considered the possible infection event, as wetting is a necessary aspect of blossom infection. At the time of 2+ hours of wetting, as detected by leaf wetness sensors, or in the event of measurable rain, the DAS model “time stamps” and records the time and date, and the temperature risk value that was current for the 96 hours prior to that hour that the 2 hours of wetting occurred. This information can be found by clicking on the “Wetting Events” tab. This tab records the wetting events that have occurred during the past four days. As these wetting events are difficult to accurately detect by remote monitor, the DAS system will not be able to document a potential infection event if the wetting is caused by dew in more humid portions of the orchard or by light wetting of flowers by poorly timed irrigation.

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. It is no currently practical way to rapidly determine if your flowers are actually contaminated with blight bacteria.

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.

For blight infection to occur, you need four factors to occur, and they must occur in the following order.

  1. First, you need flowers open on a potential host. This is usually obvious, except during the secondary bloom period, which is the most dangerous time. We now have this, and will have flowers in some blocks into early June. Some cultivars scatter bloom off and on all spring and early summer, which makes them more likely to be blighted.
  2. Secondly, the blight bacteria must contaminate the flowers. This you can not see. This is possible anywhere, and any year, but is even more likely when your orchard neighborhood had blight last year.
  3. Third, and most critical to the understanding of the infection process, you need warm weather. During primary bloom and petal fall, you need ABNORMALLY warm weather. After primary bloom, the normal temperatures are often warm enough to increase infection risk on secondary blossoms. The sort of daily high temperatures we must be wary of in most orchards start in the mid to high 70’s F, and are especially dangerous in the 80’s F. These sorts of warm days can occur during primary bloom, and should alert you to the possibility of blight infection when they occur, especially when it is warm for two or more days in a row.
  4. And finally, after you have those first three factors occur, you must have blossom wetting. Rain, heavy dew, irrigation. The wetting moves the bacteria from the stigma tips down into the nectaries, which then may lead to infection of the fruitlet and the tree.

A few random observations about fire blight:

TREATING TREES THAT HAVE FIRE BLIGHT: There are no products that lessen the damage to trees after infection, though there are some that claim to, so your efforts are best spent preventing infection, and/or removing strikes as soon as possible after you see them. There are no studies done under controlled conditions that show any benefit from any spray applied post infection, though many products have been sold for this purpose over the years. There is usually at least one product being sold for this purpose on any given year, but they come and go. When a well-tested, registered product is actually found to have the ability to “stop” bight or reduce damage to the infected tree, I’ll let you know as soon as possible.

Hail: Hail does not cause fire blight. A block that is free of fire blight does not have the bacterial contamination necessary for infection. Most of the time, blight DOES NOT follow hail.

However, if a hailed-on block has fire blight present at the time of the wounding and wetting caused by the hail, the bacteria that are on the tree gain access to the shoot tip and fruit tissues that are highly susceptible to infection, and blight follows within one to three weeks. This is one reason we recommend removing blight whenever you see it.

There are no good treatments that prevent this infection, but it is worth a try. As soon as possible after the hail, a treatment of oxytetracycline (Mycoshield, FireLine) might reduce the number of strikes. It is probably best to have this treatment on within a few hours, but it might be helpful within 24 hours. Copper bactericides are also likely to help reduce blight after hail.

It is not at all likely that any other product would have an effect on reducing the degree of infection, including  biologicals, or any other of the so-called post infection treatments.

Newly planted apples:

Apple trees planted four or five weeks prior to the mid-May infection period may have blossom buds when planted, so they may bloom far later than the usual date for the area.

A first year apple will almost always die from this early infection. You will also have a blight problem in that affected block the next year, and perhaps for several seasons after. It is very simple to remove these blossom clusters, and it is less expensive to do it by hand than to spray the block. Complete removal of blossoms leads to complete control of the current fire blight infection. You should carefully assess the blossom situation in your blocks prior to deciding not to do any blight prevention.

Blossom wetting: “Wetting of blossoms” does not include spray wetting, as that does not wet the blossom long enough to induce infection.  There have been reports of spay-wetting induced blight infections in more humid regions of the USA, especially when applications occurred at night under slow drying conditions.

Aerial Application:  Most blight control products work directly on the blight bacteria as it grows on the stigma tip or, after an infection event, within the nectaries. If you do not get very good spray solution coverage of the interior of the flower, your terramycin antibiotic will not work. It is not likely that many potential flower infections are stopped by aerial application of oxytetracycline. The label allows aerial application without any scientific validation of efficacy demonstrated by scientific testing.