Fire Blight Control

Program Contact: Tianna DuPont, Regional Specialist, Tree Fruit
(509) 663-8181 • tianna.dupont@wsu.edu

Seven Steps to Improved Fire Blight Management

By Timothy J. Smith, WSU Extension, 400 Washington, Wenatchee, WA 98801 smithtj@wsu.edu

  1. Sanitize:
    Cut blight out of the orchard as much as possible during the winter. Cut before you prune, so you may remove the blighted cuttings from the orchard. During the winter, you do not need to cut nearly as far below the canker, as bacteria are mostly confined to the canker edge. Just cut at the next “horticulturally sensible” site below the canker. You do not need to sterilize tools when you are cutting on fully dormant trees.Some growers have effectively used blowtorches to kill cankers that were difficult to remove from the tree. As the bacteria are killed at about 150-160 degrees F, charring the wood to kill the canker is not necessary.During the Summer, cut blight when you see it. Removing a strike can greatly reduce further damage on the tree, especially if you catch the strike early. If you have too many strikes to cut out, you have lost the battle. Be certain, before you give up cutting in the Summer, that discouragement over the mess does not cloud your judgment of “too many strikes.”
  2. Manage the orchard environment:
    Heat drives the infection process, and moisture on the blossoms triggers it. You can do little to affect the daily temperature in a way that will reduce the potential for blight infection. You cannot stop the rain from wetting blossoms, but you may influence the potential for dew. When a period of abnormally high temperature comes and goes, without rain, blight outbreaks may occur in low, flat “frost pockets” or draws in the orchard, where dew forms on flowers earlier and stays longer. Data gathered from leaf wetness sensors shows a wide variation in the presence and duration of dew. It appears that as few as two or three hours of wetting is sufficient to trigger infection if the four-day temperature risk value total is over the high risk threshold.What may you do to reduce dew? In Central Washington, relative humidity is naturally low during the common infection period, as rain is uncommon in the Spring. The orchard microclimate may have a higher than ambient RH, due to irrigation, frost control, and the transpiration of trees and cover crop. The higher the RH, the higher the dew point, and the more likely your orchard will reach the minimum conditions for infection.If blossoms are present and the weather has been warm, the light wetting that may occur from sprinkler mist reaching higher in the tree or along the edges of irrigation zones can also trigger blight.Of course, your orchard will need some irrigation during May and June. However, studies have shown that trees are not nearly as stressed for water as we think they are in the Spring. A little soil drying is beneficial, assuming trees are well watered when the really stressful time of year starts. It is unlikely that you will excessively stress trees during the few days that make up peak periods of highest fire blight risk.

    Keep the intervals between sets as long as possible, and let the soil surface dry.

  3. Reduce infection potential of the host:
    Most blight problems start as blossom infections. You may greatly reduce the chance of infection in a young block by hand removing blossoms. I timed the process last year, and it took about two to three hours of labor per acre on second leaf trees, much less on first year trees. As blossom removal is not necessary during the normal cool weather, it may not be necessary most seasons. If the fire blight model says risk is high, and your young trees have scattered blossoms, pulling the blossoms may save you much more than the cost of labor.Many organic growers successfully use the blossom removal method to prevent secondary bloom fire blight on their young pears and apples.On larger trees, any caustic blossom thinning sprays that burn the stigma surfaces are likely to disrupt the continued build-up of the blight bacteria on the treated blossoms. This stigma surface removal may set back the overall infestation level of blossoms in the block for a few days after application.Plant apples on fire blight resistant rootstock, whenever possible. Some good new blight resistant rootstocks are recently available, and more are coming. These will not make the top of the tree much less fire blight susceptible, but they will help prevent tree death from “collar blight.”

    Keep vigor of the tree moderate. This will not prevent infection, but it will reduce damage done to the tree by the blight strikes.

  4. Manage bacterial colonies on the stigma tips:
    Watch for a dangerous warming trend (calculate temperature risk values, using today’s forecasted high temperature, plust the past three days, and project them for the next three days using predicted temperatures). If your trees are likely to be blooming during the upcoming high risk period, apply a biocontrol spray to slow the build-up of fire blight bacteria in the blossoms.
  5. Watch the Model, and Follow Advisories:
    Controlling this disease is difficult unless you apply an effective control product to protect blossoms in the four days leading up to an infection event, and also apply an effective product very close to the infection time. Most sprays provide no long-term protection or kickback, such as we expect with control materials for other important diseases. This is one important disease situation where you cannot rely on advice to take action. Fire blight infection risk evolves much faster than most other pests. Advisers can warn you that a high blight risk period is coming, but day to day decisions during the time that risk is high are up to the on-site decision-maker.Most of the infections that occurred this past decade happened during a time that the fire blight model was indicating high or extreme danger. The major infections that occurred the past two years happened during periods when most advisers were advising careful control. During the past two years, many growers applying sprays as suggested by the “Cougarblight” model have had good control compared to those who sprayed on a schedule, or not at all until too late.Growers did not apply sprays at right time during the 1997 and 1998 seasons for many reasons, including lack of availability of effective materials, and a too-late release of the special label for the one really effective product. Many growers were also surprised by the wide-spread infection of late blossoms on apples.Whenever blossoms are wetted during a time that the model indicates your orchard is in high or extreme risk, infection is likely in your region, but not always in your orchard. You might be lucky, and not have a local blight bacteria source. Your blossoms may be free of contamination. As we have no quick test yet for the presence or absence of E. amylovora in blossoms, you should probably just assume they are there. The economic and emotional punishment for being wrong about this decision is far greater than the expense of a properly timed, justified spray.

    Model resources: The simple to use, most recent version of “Cougarblight 2010F” (or 2010C for Celsius users) is available, e-mail me and I will send you the latest version. smithtj@wsu.edu

  6. Automated versions are available on the Washington State University Decision Aid System. Several other tree fruit production regions in the USA and other countries provide an automated blight infection risk advisory. Take advantage of this service, and check the model status daily.
  7. Apply preventive sprays when necessary:
    One or two applications of oxytetracycline (Mycoshield) applied at just the right time during each of the last two serious “blight seasons” would have prevented most of the fire blight damage. (Streptomycin is even more effective where resistance is not common. Resistance of the bacteria to this product is very common in the Pacific Northwest USA.) Unfortunately, while it is relatively easy to use the fire blight model to look back at orchard conditions and determine when infection occurred, it is much more difficult to always guess correctly that infection will happen sometime during the upcoming 24 hours. The model evaluates the risk of infection caused by temperatures, but cannot predict blossom wetting. The blossom wetting aspect must be left to you, the model user. In more humid climates, wetting from heavy dew should be assumed during periods of high infection risk. There are serious blight outbreaks that have occurred because growers believed that rain was necessary for infection.Be certain to pay close attention to the “neighborhood scenarios” that are critical to the Cougarblight model when determining degree hour thresholds. The presence or absence of blight in your neighborhood makes a significant difference in the amount of degree hours necessary to reach the low, moderate, high or extreme thresholds.When risk is “Low,” relax. Take care of other business.When the risk level rises to “Moderate,” pay more attention to the weather forecast, and insert forecasted highs into your daily blight calculations. As the 4-day degree hour total rises near the “High” threshold, keep a sharp eye on orchard conditions. ( By now, you should have applied the biocontrol product.) Remember that the numerical degree hour thresholds are guidelines, not absolute numbers indicating infection. Foe instance, it does not make sense that infection will not ever occur at 490 F degree hours, but always will at 501. Nature just does not operate so precisely that we can have absolute model thresholds. If your orchard is blooming, young, a more susceptible variety, in an area with a “blight history”, and on blight sensitive roots, you should have a lower risk threshold than someone without any of the above concerns.

    If risk levels are “High.” If your orchard is developing degree hour thresholds higher than you wish to tolerate, and you believe that blossom wetting is a possibility sometime during the next day, you might choose to apply a preemptive preventive spray. If blossom wetting does occur within the day, congratulate yourself on a wise choice, based on the best available information. Sit back and watch everybody else rushing to get their blight spray on. Mark the day on the calendar, and expect all heck to break loose in some of the local untreated blocks in about seven to twenty-one days, when strikes appear from the current infection period.

    If blossom wetting does not occur, console yourself that you made a wise choice, based on the best available information.

    If you decided that degree hours were at dangerous levels, but that infection was not likely because dew points were low, then a rain shower crosses your orchard, all is not lost. As soon as possible, begin spraying the recommended rate of an effective antibiotic product in adequate water to wet the interior of all blossoms. Apply the sprays on alternate rows across the entire block, then immediately spray the skipped rows. Both coverage and time are important, so try to get the job done as quickly as possible, without driving the sprayer too fast. Sprays applied within 24 hours of infection seem to be effective.

    When Risk Levels are “Extreme.” These are the weather conditions that lead to wide-spread, serious fire blight damage. Monitor the orchard situation carefully. Are flowers present? How numerous are they? Is rain or dew likely? All possible blight control practices and legal spray products and intervals should be maintained until these “Extreme” conditions pass. This is usually after only a few days.

  8. Scout for and Remove Current Season Strikes:
    If you have identified a likely infection period, begin scouting the orchard about five to seven days after infection. Symptoms usually show within this period, then become ever-more obvious about ten to fourteen days after infection. The earlier you see the strike and remove it, the less damage may be done to the tree. Waiting until all the strikes are easily seen may lead to increased collar blight in dwarf apples, and excessive structural damage in pears.
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