Fire Blight Management In the Pacific Northwest USA

Timothy J. Smith, WSU Extension, 303 Palouse St. Wenatchee, WA. 98801 USA

Blight Hazard Factors:

1. How old is the tree? The younger the tree, the more damage is likely to occur when infected. Blight likes younger, rapidly growing portions of a tree, and the wood on a young tree is almost entirely open to attack. Strikes usually stop developing once they reach two or three-year-old apple wood, even on the more susceptible varieties. The older the apple trees are, the less danger that infection will kill the tree by getting into the rootstock. Research (Dr. Herb Aldwincle, Cornell University) has shown that, on very young trees, low numbers of bacteria can move in 21 days from upper tree infections through the trunk to the rootstock. If the rootstock is highly susceptible to blight, (M9, M26, O3. MM106), "collar blight" may then kill the tree. If the rootstock is no more susceptible than the scion variety, (M-7, B9, seedling), then no collar blight occurs. Great numbers of young apple trees on susceptible rootstocks were killed by the May 1997 infection.

Infection during the first three years is the most dangerous; the younger and more vigorous the tree, the more likely tree death will follow infection. There can be some problems with collar blight in the fourth and fifth leaf. After that, tree structure may suffer, but the tree will regrow the lost wood in two or three years.

2. How likely is the variety to become infected? This is strongly linked to the flowering habits of the variety. Does it bloom over a relatively short period, then quit for the season? Does it straggle out the primary bloom, then scatter late bloom during late Spring and Summer? Some varieties primary bloom first on the older wood, next on the two-year-old wood, then on the tips of last years shoots. Then they produce some odd blossoms on the side of older fruiting structures, and the tips of current season shoots. These poor blooming habits greatly lengthen the time for potential infection, making infection much more likely. These late blossoms are the site where most infections occur some seasons. These scattered infections maintain the fire blight bacteria in the orchard.

3. How vigorous is the tree? An over-fertilized, heavily pruned tree with light fruit set is more likely to suffer extensive damage when infected.

What About the Weather?
Severe infections are possible when a region experiences Summer-like weather while many blossoms remained on pear and apple trees. How warm are these dangerous periods? In Central Washington, normal daily high temperatures in early and mid-May are about 68-75F. During May 1997 and 1998, there were extended periods during which daily highs ranged from 80 to 90F, with periodic rain showers and heavy dew in some regions. Severe fire blight resulted, with the worst blight occurring in orchards that had the most bloom present during this Summer-like weather. Generally speaking, daily high temperatures of 80 to 90F are very likely to lead to rapid bacterial build-up in pear and apple flowers. The more of these warm days that occur consecutively, the more danger of infection.

What is The Background Level of Fire Blight Bacteria?
With a few rare exceptions, you cannot have classic fire blight of apple or pear blossoms without a nearby source of E. amylovora bacteria. A single active carry-over canker seems to produce enough bacteria to severely contaminate the blossoms on at least 1/4 - 1/2 acre (.1-.2 Ha). The closer to the canker, the more likely the blossoms will be contaminated, and the more likely strikes will appear, even following poor infection conditions. The more strikes in the close area, the worse the outbreak will likely be following normal, or dangerous infection conditions.
With the ever-increasing incidence of fire blight in all parts of the Pacific Northwest, your source of blight may be your own orchard. If you look closely this winter while pruning, you may see blossom clusters and small twigs with dead fruit and leaves tightly attached, with a small dead area at the base of the dead shoot and blossom structure. It is possible you will be looking at Pseudomonas blossom blast, but it is more likely to be symptoms of fire blight. These small scattered strikes are sufficient to provide for fire blight outbreaks in the Spring, if weather conditions are proper for infection.

Summary: Fire blight leads to increased blight infection risk in the region near the infested orchard. Removing cankers from the orchard during the winter is the most important control measure. Until we go through at least a season of poor infection conditions and low fire blight occurrence, reducing the number of overwintering cankers in the Pacific Northwest, potential for blight will remain very high in the Spring.

Products Used in the United States for Control of Fire Blight:

Sprays are generally used to prevent the establishment of successful E. amylovora bacteria colonies on the flowers' stigma surface, or perhaps to halt the division of the bacteria once they have entered the nectary. The flower interior is the target of sprays. Great spray coverage is essential.

All sprayed products are equally ineffective unless applied during a very narrow timing period in relation to the disease development. Fire blight bacteria colonies establish daily on newly opened, untreated flowers. The effective control product must be applied into each flower within a day or two of its' opening to adequately protect it from infection. Spraying this well is difficult, as many other important sprays are being applied at this time, and sprayers may be scarce.

As infection risk varies, infection can result in a few strikes per acre, or many thousands. Even a 95 percent reduction in infection could leave one strike per every 10 trees, or ten strikes per tree. During times of extreme risk, you should attempt to apply the effective products prior to infection.

Biological Control: The only partial exception to the narrow spray timing window is the biological agent (BlightBan - the A-506 strain of Pseudomonas fluorescens and some others that may be available in 2006), which are live bacteria that produce colonies on the stigma surfaces and spreads to protect newly opened flowers. The beneficial bacteria's colony must get to the stigma surface ahead of the blight bacteria to protect it. Colonized stigma surfaces are well protected, but the practical difficulty has been attaining a continuous high level of infestation of flowers by the protective bacteria. If flower numbers are low, as they are late in bloom or post petal-fall, or when the weather is cool, the bacteria do not grow and spread adequately. The biological products will be most effective when applied to a heavily flowering orchard at the beginning of a warming trend, rather than during cool weather during a specific blossoming stage. To date, researchers have shown that biological control agents provide partial reduction of blight infection, as high as 50 percent in field tests, and even higher in the laboratory. If applied two or three days ahead of an actual infection, this 50 percent control will be in place when the more effective control product, discussed below, is applied. The two products used in this timing pattern, not together, are the best control team presently available.

Copper: We have generally had poor results with copper products. People still use them, but mostly by habit, rather than any true indication of effect. At times copper products seem to reduce infection, but they are not reliable under high pressure infection conditions, and should be used only as a supplement to more effective products. In side by side tests, copper is always better than nothing, but is usually much less effective than the other normally recommended products. Effective rates may also russet sensitive varieties fruit during cool, wet weather.

Fungicides + Copper: Recent research has indicated that mancozeb + copper fungicide combinations are promising if used during the three to four days running up to a possible fire blight infection period. These sprays may supress the build-up of fire blight pathogenic bacteria on the stigma tip, increasing the level of control when antibiotics are used to combat infection during the actual infection event. The rate of copper used in this combination is low, and russet seems less common than when copper fungicides are used alone.

Oxytetracycline (Mycoshield, FlameOut) is the only reasonably effective product available to most Pacific Northwest USA pear and apple growers. There have been many instances where the properly timed use of this product has greatly diminished the degree of infection in the treated block, compared to untreated nearby blocks considered, at the time of application, to be less at risk. This product has been fully labeled for use on Washington pears for decades, with no indication of resistance in the bacterial population.

For best effect, you must apply this product within 24 hours before an infection period. Application two days ahead of infection period is possibly helpful, but effect probably drops off rapidly as you extend the pre-infection period application timing to three or, especially, more days. Many growers also attain reasonable control when they carefully apply the product within 12 to 24-hour after an infection. It is unlikely that sprays have significant effect when applied more than 24 hours post-infection.

Other Products: There is a long list other registered products as control options, including streptomycin, which is no longer effective in most of the Pacific Northwest USA. Most other spray materials should be considered experimental or supplemental to the overall control program, as they will not adequately control fire blight, even when infection conditions are marginal. We must be careful not to over-sell the effect of these alternative products until they are consistently shown to be sufficiently effective under commercial use. Regulators may expect us to do without the one or two effective products, as there are so many apparent alternatives. Products recommended for supplemental blight control vary from promising products to some that are of very questionable value.

A mountain of money has been spent on shiploads of post-infection treatments. This is understandable, as any grower would try anything to reduce damage in a block being devastated by blight. Unfortunately, to date, no careful trials have shown that any commercially available product will affect damage done to the tree once infection has occurred. Often, by the time blight has been noticed, then sprayed, the disease has almost run its' course, and stops "running" naturally. This leads to the false assumption that the sprayed concoction did the job.

There is a tree growth regulator (Apogee) that may reduce shoot growth on apple trees, thereby reducing the potential for shoot tip infections. This product has been effective for reducing shoot strikes on apples, but remains relatively difficult to use on pears. To work properly, the Apogee must be applied near blossom time, well prior to the known infection of the tree. The use of this growth regulator as a fire blight management tool is confined to areas with high risk of post bloom shoot tip infection.

Most sprays are used to prevent the establishment of successful E. amylovora bacteria colonies on the flowers' stigma surface, or perhaps to halt the division of the bacteria very soon after they have entered the nectary. As the flower is the target, spraying the tree once infections have become established is of no value. No research or field trial has shown good evidence that presently available sprays do much, if anything, to reduce the potential for shoot tip infections that often occur on or near infected trees.

Control Principles:

1. Sanitation: 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 160-180 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. Managing 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 valleys 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 degree hour total is over the high risk threshold.

What may you do to reduce dew? 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. Keep early season irrigation, cover crop and weed growth to a minimum.

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 may 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 overly stress trees during the few days that make up peak periods of highest fire blight risk. Keep the intervals between irrigations 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 this year, and it took about five or six hours of labor per hectare on second year 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 pears and apples.

On larger trees, any caustic blossom thinning sprays that burn the stigma tips are likely to prevent the continued build-up of the blight bacteria on the treated blossoms. This stigma tip 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. Management of bacterial colonies on the stigma tips: Watch for a dangerous warming trend (calculate degree hour potential using the past four 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.

5. Watch the Model. (F Version) (C Version)Controlling this disease is difficult unless you apply an effective control product very close to the infection time. Most sprays provide no long-term protection or kickback, as we expect with other important diseases. This is one important situation where you cannot rely on slow advice to take action. This disease evolves much faster than most other pests. Advisors 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 you.
Most infections happen during a time that the fire blight model indicates high or extreme danger.

During the past ten years, many growers applying sprays as suggested by the Pacific Northwest USA "Cougarblight" model have had good control compared to those who sprayed on a schedule, or not at all until too late.

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 for the presence or absence of E. amylovora in blossoms, you should probably just assume they are there. Recent presence of blight in your block puts your orchard into special high risk catagories. The economic and emotional punishment for being wrong about fire blight control decisions is far greater than the expense of a properly timed, justified spray.

Model resources: The simple to use, most recent versions of "Cougarblight 2000F and 2000C" are available on the web site- http://www.ncw.wsu.edu/treefrt.htm.

Examples of automated versions of this model are available on the Washington State University Public Agric. Weather System (http://frost.prosser.wsu.edu/) or at Oregon State University Integrated Plant Protection Center (http://www.orst.edu/Dept/IPPC/wea/weamodpp.html). Other resources are coming soon (Spring 2007)

6. Apply preventive sprays when necessary: One or two applications of terramycin (Mycoshield) applied at just the right time during each of the last two seasons would have prevented most fire blight damage. 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 clearly shows the degree of potential risk caused by temperatures, but cannot predict blossom wetting. That aspect must be left to you, the model user.

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 this time, you should have applied the biocontrol product.) Remember that the numerical degree hour thresholds are guidelines, not absolute numbers indicating infection. It does not make sense that infection will not ever occur at 499 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 with many of the above concerns.

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 pre-emptive 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 fourteen days, when strikes appear form 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 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.

7. Scout for and Remove Strikes: If you have identified a likely infection period, begin scouting the orchard about five to seven days after infection. Symptoms of blossom cluster blight usually show within this period, then become ever-more obvious about ten to fourteen days after infection. During cool seasons, the full development of near-by shoots, or a "strike," may take 4 to six weeks. 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.