Chelan - Douglas County WSU Extension

Fire Blight StrikeFire Blight Management in the Pacific Northwest USA

Timothy J. Smith, WSU Extension,
400 Washington St. Wenatchee, WA. 98801 USA

Updated as of Dec. 2012

The Infection Process:

Erwinia amylovora, the bacteria that cause fire blight, overwinter only in the blight strikes remaining on host trees. The bacteria may die out in many of these strikes, but from twenty to fifty percent of these cankers reactivate around blossom time and ooze bacteria to their surface. This ooze is attractive to many insects (especially flies), which feed on the ooze, then gather the pollen or nectar of nearby apples or pears, transferring the blight bacteria to the flowers sigma. The bacteria multiply on the stigma during the four to eight days the flower is open. If the weather is warm, the bacteria grow rapidly, form the necessary large colony, then may be washed gently into the flowers' nectary by water (usually from rain or heavy dew). If the colony is then successful in attacking the small fruitlet, the bacteria spread into the phloem (just between the bark and the wood) of the tree, killing any young, tender parts of the nearby host tree structures.

Blighted Apple FlowerAbout five to 14 days after the infection takes place, symptoms become easily seen by the casual observer. A fully developed "strike" will often take up to 21 days to appear. During this time of symptom development, the bacteria stream inside the tree, well ahead of the visible symptoms. They often move into other more sensitive portions of the tree, such as the nearby shoot tips or the susceptible rootstock, causing more blight strikes and bacterial build-up.

As the tree goes dormant in the fall, the bacteria form a dormant mass along the living edge of the current season strike, and overwinter until the next spring.
The cankers that developed relatively later during the growing season, and those on younger wood are the most likely to survive to ooze bacteria.

An Overview:

Fire blight in the Pacific Northwest United States was once a concern to pear growers only, with most outbreaks occurring only in regions that seemed unlucky that season. The weather is generally cool (70F- 21C or cooler daily) in this region during pear bloom, so primary blossom infections, while they are possible, have been very rare (until 2012). The infection of primary blossoms has increased over the past decade, and became a serious problem in 2012, when thousands of acres of older Gala and Fuji apple trees were blighted during a late April extra-ordinary warm temperature event, which was followed by region-wide rain.  Most seasons, however, infections take place on blossoms that appear during or after petal fall, the "side bloom" or "secondary bloom" that appeared on the pear or apple tree shortly after bloom as the weather begins to warm.  This usually happens during May and June, while weather is warm, but not hot.

Most apple secondary bloom occurs during the two or three weeks following petal fall. However, some of the newest cultivars have the dangerous habit of producing flowers randomly throughout the growing season. The various pear varieties produce secondary blossoms at different times of season, and more or fewer from one season than the next. Infection most often occurs on the side bloom, produced on the fruiting spur during petal fall to fourteen days past petal fall, or on the "rat-tail" blossoms, produced on the end of short shoots throughout the summer. Rat-tail infections are rare after early July, though the blossoms may be present. It takes the unfortunate coincidence of blossoming, a period of warm weather, a close-by source of fire blight bacteria, and (usually) rain or heavy dew to cause a flurry of blight in the orchard. As the blight strikes are usually scattered about the tree and orchard, it is often at least ten to 14 days after the infection day before the blight is noticeable. This disconnects the infection conditions from the appearance of blight. Pear and apple growers may have difficulty relating their spray control efforts to the actual presence or absence of blight in their blocks.

Many growers consider blight an apparently random curse, and part of the game a pome fruit orchardist must play. As outbreaks do not occur yearly, damage always seems to be somebody else's problem, until it suddenly appears in your orchard. As strikes are usually scattered thinly across a block, cutting the strikes from the tree may appear to be the best control measure, though cutting blight is probably the most demoralizing of all orchard operations. Cutting blight often forces you to remove random large portions of the tree, and you are butchering trees that may have taken ten to twenty years to grow to their peak condition. Young orchards (less than 8 years old for apple, less than 20 years old for pear) are especially vulnerable. However, the mature orchard often grows back very well from this cutting, and within three to five years may be as good as ever.

Older apple trees may not need the same degree of cutting to remove the individual blight strikes from the tree, as older apple wood seems relatively resistant to damage. However, apples may have many strikes per tree, which leads to extensive damage, or may die from the eventual infection of the more sensitive rootstocks.

Young pears or very wood-sensitive varieties, such as the Beurre Bosc (Keiser Alexander) and Red Sensation varieties, have been special cases, as visible infection damage often travels into the trunk rapidly, eventually killing the tree.  Bosc pears generally do not produce flowers after the primary bloom period is over, but Red Sensation (Clap's Red) produce secondary blossoms commonly, making them a serious blight risk.  Boscs have usually escaped infection during the usual May and June infections experienced by other pears. However, as they bloom later than other pears, the latest viable primary flowers remaining on the tree are at higher risk.

Apples are generally susceptible to fire blight. Some are more resistant than others (Red Delicious), others seem very susceptible (Pink Lady, Jazz). Since 1993 the apple growers of Washington have had good reasons to become truly concerned about fire blight, as most newer apple cultivars have had a blight problem. 

Now that there are extensive acreages of highly susceptible apple varieties on super-susceptible rootstocks in warmer parts of the state, the possibility of serious fire blight damage in Washington apples has evolved into a reality. It has become obvious that some apple orchards are in more danger from blight than the average pear block. Despite the potential risk, most orchards of susceptible apple varieties continue to have no problem with blight. There have been minor, limited acreage outbreaks of blight in apples almost every season since 1991, and serious damage in about 5-10 percent of the orchards in 1993, 1997, 1998 and 2012. 1997 and 2012 were, by far, the worst fire blight seasons ever in Washington orchards. 1998 brought serious damage, but mostly in orchards that were not affected in 1997. Since the 1990's, blight has damaged apple orchards randomly, some years worse than others, but no year seems trouble free.

The blossoms of most pear varieties are either highly or extremely susceptible to fire blight. The wood of various pears is variably sensitive to infection. The pear is a wonderful fruit, with a grand future, but all commercial varieties should be considered sensitive to blight. Perhaps "sensitive" does not describe the situation properly, and we should outline some factors that affect the importance of fire blight on a specific variety.

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 Aldwinckle, 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. There are some "Geneva" series rootstocks entering the market that, while as dwarfing as M9, are resistant to fire blight. The infected young tree may still be badly damaged by blight, depending on cultivar, age, vigor, etc., but it may be much more likely to survive and regrow the bearing surface, rather than die from rootstock blight.

    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 serious problems with collar blight up through the fifth leaf, and sometimes as late as the seventh season. After that, Collar blight is much less common. The tree structure may suffer due to loss of younger productive wood, but the tree will usually survive an d 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 there are sometimes extended periods during which daily highs range from 80 to 90F, with periodic rain showers and heavy dew in some regions. Severe fire blight may result, with the worst blight occurring in orchards and trees that had blight the previous season and have 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. A series of days where temperatures reach 95F or more seems to greatly reduce the new infections of secondary flowers. This may be due to the effect of higher temperatures on the flowers, the pathogen, or both.

What is The Background Level of Fire Blight Bacteria?
With a few rare exceptions, you cannot have classic, serious fire blight of numerous 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 one acre (0.4 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, even if weather conditions are marginal 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.

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 flower's stigma, 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 of the flower 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. Repeat applications may be necessary to protect blossoms that opened after the previous treatment.

Biological Control: The only partial exception to the narrow spray timing window are the biological agents.  These living organisms are applied to newly opening flowers during the 2 - 4 days of warming weather leading up to an actual infection event.  In order to use these products wisely, you must anticipate potential infection periods.  This is not always possible, but it is greatly aided by daily monitoring of weather and fire blight model forecasts.  Biolopgicals and caustic flower thinning sprays are generally incompatible, but if a biological is applied after the flower thinning spray, it may thrive on newly opened flowers. Biologicals are also not compatible with copper fungicides.

The following "biologicals" are now registered for use:

Blossom Protect, which is a combination of two strains of Auriobassidium pullulans, a yeast that occurs naturally in Pacific Northwest pome fruit flowers.  This organism infests the nectary and stigmas of treated flowers, and appears to change the condition of the flower so that the blight bacteria don't thrive, greatly reducing the infection risk (theory, yet to be proven).

Bloomtime Biological, is the E-325 strain of the bacterium Pantoea agglomerans. This bacterium competes with the blight bacteria for space and resources on the stigma, and appears to exude natural antibiotic substances that inhibit other bacterial growth.

BlightBan - the A-506 strain of Pseudomonas fluorescens, which seems to be competitive with the blight bacteria for resources and space on the stigma surface.
 
When applied to open flowers, these micro-organisms produce colonies on the stigma surfaces and nectary, and spread by insect to protect newly opened flowers. The beneficial bacteria's colony must get to the stigma surface ahead of the blight bacteria to fully protect it. Colonized stigma surfaces are usually 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 beneficial bacteria do not grow and spread adequately. The biological products will be most effective when applied every two or three days to a heavily flowering orchard starting four or five days prior to a forecasted high infection risk period, rather than at random 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 50-85 percent in field tests, and even higher in the laboratory. If applied two or three days ahead of an actual infection, this 50-85 percent control will be in place when the more effective control product, usually an antibiotic, is applied. The two product classes used in this timing pattern, not together, are the best approach to control presently available.

Various Minerals: Over the past 10-15 years there have been mineral fertilizer products that are said to have reduced or eliminated fire blight when used in orchards elsewhere. These fertilizers are usually phosphorus or calcium based, but are sometimes mixtures of various macro and micronutrients. The reports of efficacy are almost always based on observations that growers had a problem with fire blight one year, used the product the next season, and, to everyone's surprise, the blight was not a problem. No nutrient spray program has done at all well in carefully documented efficacy screening trials.

Copper Fungicides: Copper is quite toxic to bacteria, and has been used for fire blight management for many years.  The various old standard products with "fixed" copper as the active ingredient have not performed well in our trials. The level of control when these products are applied multiple times to open blossoms varies from 20 to 60% compared to a non-treated inoculated check. The level of control when they are applied to a dormant tree or pre-bloom is low, but somewhat significant as part of a control program.  At times copper products seem to reduce infection, especially when applied to open blossoms, but they are not reliable under high pressure infection conditions, and should be used only as a supplement to more effective products.  Effective rates may also russet sensitive varieties of fruit, especially when applied during cool, wet weather. There is promising data showing very good efficacy with a newer formulation of copper fungicide that may be safely applied to the flowers during bloom. 

Fungicides + Copper: 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 suppress the build-up of fire blight pathogenic bacteria on the stigma, 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 relatively low, and russet seems less common than when copper fungicides are used alone.

Antibiotics:

Oxytetracycline (Mycoshield, FireLine) 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 labeled for use on Washington pears since 1975, with no indication of resistance in the bacterial population. In local efficacy trials, antibiotics provide 85-95 percent control, and in actual practice, seem to perform at a level much better than that. Streptomycin, kasugamycin and gentamycin have all performed very well in efficacy trials where flowers were inoculated with a streptomycin-susceptible laboratory strain of E.a. bacteria. Oxytetracycline has performed well in trials, probably because treatments were applied within one hour of a known time of infection.

For best effect, you must apply tetracycline 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. Due to lack of coverage of the interior of flowers, applying this product by aircraft, as is allowed by the label, is not recommended. (Ask yourself: how well do you think caustic flower thinners, such as lime sulfur and oil, would thin apple fruit if applied by air?  The target of the two different sprays, thinners and blight sprays, is the same: the interiors of flowers must be treated.)

Streptomycin (AgriStrep, FireWall):  The bacteria that cause fire blight have been more or less resistant to streptomycin in Washington orchards since about 1975.  Recent tests have indicated that this resistance level has dropped, and that this product may be of use as a suppliment to oxytetracycline.  The product should not be used unless a serious infection period is under way, and it should not be used more than once per season.  You should assume that the bacteria that remain in the orchard will be mostly resistant to streptomycin.


SAR Products:  SAR = "specific acquired resistance"   Several products that claim to make the treated host more resistant to blight.  Some have been tested, and more seem to appear every season. These products seem to perform in some crops and with certain diseases, but have not met with any great success in the orchard. Those tested often have a measurable effect, but the range of control varies from 0 to 30 percent, a control level which would be difficult to see outside of a highly replicated trial. The more effective materials of this class of product may play a role in supplementing the control level of antibiotic or biological sprays when applied several days prior to infection.

Other Products: There is a long list other registered products sold as control options.  Most other spray materials should be considered experimental or supplemental to the overall control program, as there is often no legitimate research results backing up the claim of efficacy. Some are somewhat effective in controlling blight, but are likely to damage fruit.  We must be careful not to over-sell the effect of these alternative products until they are consistently shown to be sufficiently safe and effective under commercial use. Regulators, taking note of these questionable products, 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.

Post-infection Treatments: A unfortunate amount of money has been spent on truckloads of post-infection treatments over the past thirty years. This is understandable, as any grower would try anything to reduce damage in a block being devastated by blight. Unfortunately, to date, no careful scientifically valid trials have yet demonstrated 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 with the patent medicine, the disease has almost run its' course, and stops "running" naturally. This leads to the false assumption that the sprayed concoction did the job as advertized. There is always hope that a product will prove to reduce damage to the tree post-infection, but watch for results in carefully designed research trials, and treat testimonial evidence with skepticism.

There is a tree growth regulator (Apogee) that often is used to 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 in-effective on pears. To work properly, the Apogee must be applied very 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 categories. 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 2009F and 2009C" are explained and available on the web site- http://www.ncw.wsu.edu/treefruit/

    Examples of automated versions of this model are available on the Washington State University Decision Aid System at http://das.wsu.edu/index.php 
  6. Apply preventive sprays when necessary: One or two applications of oxytetramycin (Mycoshield, FireLine) applied at just the right time during high infection risk periods would prevent 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 or the presence/absence of blight bacteria. Those factors must be judged by 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 a 299 risk value, but always will at 301. 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 the above concerns. Dealing with blight after infection is very expensive. 

    If your orchard is developing risk values 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 risk values 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. Fire BlightScout 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.

     
 
 
 
 
   
 

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