Introduction

     Fire blight is a very serious disease.  On young trees, an infection can rapidly lead to the complete loss of the plant.  When the initial infection is not removed from a nursery, dozens of trees can be lost in a matter of weeks.

     The situation is not unique to Alberta.  Since 1997, many nurseries in British Columbia, Washington State and Oregon have suffered losses to fire blight.  This was due to the unusually warm weather in the springs of 1997 and 1998, and because commercial apple growers are replanting with apple varieties more susceptible to fire blight.

     In 1998 and 1999, CropHealth Advising & Research worked with nurseries in Alberta where crabapples were showing poor growth.  Two important observations were made in the field:

     -   First, nursery growers and their senior personnel must recognize the very early symptoms of fire blight.  The successful management of this problem depends on a rapid intervention.

     -   Second, fire blight is only one of four disease problems affecting Malus plants. In some situations, the disease apple scab causes more damage than fire blight.  Two other problems seen in nurseries are dieback from poor pruning and stress from nutrient deficiencies.

     This presentation will review the disease fire blight, with a focus on recognition of early symptoms and different approaches of management.

Symptoms of infection

     The most common image of fire blight infection is a young branch with a “sheperd’s crook” appearance: the branch tip is wilting while the leaves remain attached but look as if they were “scorched” by fire, thus the name fire blight.

     This “wilted shoot” is a recognizable symptom of fire blight in the field, but for nursery growers, it may be too late for action.  The actual infection will occur 4 weeks before the “strike” becomes visible in the spring, and 2 weeks in the summer.  During that period, the disease will spread inside of the tree and cause important, permanent damage.

     The earliest symptom of fire blight infection is the blackening of leaf petioles and leaf veins.  Think of fire blight as a disease that infects from the inside out: the first visible infection is in the veins, then the leaf tissue becomes discolored and turns brown.

     There are other ways to confirm a fire blight infection:

     - A flower head or a fruit spur has a stem that appears water-soaked, later turning brown or black, and then wilting.

     - There is a small droplet of thick, yellow to amber-colored “ooze” near the infected tissue.  The "ooze" is produced by the disease pathogen multiplying rapidly in the infected tissue.

     - For the most accurate diagnosis, send an infected branch to a laboratory for culturing.  This method is especially important when many trees have to be removed after a sudden outbreak.

Seasonal development

     Fire blight is a disease caused by the bacteria Erwinia amylovora.  It is most common on apples and crabapples, but is also seen on mountain ash, hawthorns and pears.

     Fire blight cannot be controlled.  There is no pesticide on the market that will eliminate the problem from an infected tree.  At best, the problem is “managed” to cause as little damage as possible.  Good management comes from understanding the disease and its seasonal development.

Winter: branch cankers

     The bacteria overwinters in cankers on limbs infected the previous season.

     The cankers are commonly found on small branches and around pruning cuts made the previous year on infected trees.  Cankers typically have a depressed center with a rough margin between the infected area and the healthy part of the branch.  With warm temperatures in early spring, the bacteria in the canker will multiply and be transmitted around by wind or flying insects. 

Spring: infection of flowers

     Four conditions must be present for a fire blight infection to occur in the spring:

     1- Fire blight must have been present in the area in the previous 2 years.  The bacteria can survive on weeds and non-susceptible trees and travel up to 250 meters on the wind.

     2- The flowers must be open.  The bacteria responsible for fire blight needs to find a point of entry into the tree to successfully establish and grow.  Open flowers are a perfect “open door” into the tree.

     3- The temperature must be warm.  The bacteria becomes active at temperatures above 18^oC (64^oF).  Warmer temperatures result in more rapid development, with a disease explosion when the temperature is above 24^oC.

     4- There is a wetting event, such as rainfall, overhead irrigation or heavy dew.  This water will move the bacteria down into the flower.

     In Alberta nurseries, growers should pay particular attention to the newly-planted Malus trees.  Typically, these trees are planted in late May or early June in the field and in containers.  They will bloom in mid to late June, when day-time temperatures can be above 18^oC.  Any rainfall at this time may trigger an infection, and the infected tree will carry the bacteria for years to come.

     If there was fire blight on the property the previous year, the grower should consider a preventative measure and spray the trees with the pesticide fixed copper in the spring between the green tip and pink bud stages of flower development.

Summer: infection of shoots

     The strikes from fire blight become more obvious during the summer.

     When flowers are infected in the spring, but not removed, the bacteria will progress into the supporting branch, eventually killing cell tissues and girdling the branch.  Bacteria from infected flowers can be transported to growing shoots and trigger infections throughout the summer.  Any “trauma” that will open the tree bark, such as hail or pruning, makes the tree more susceptible to infection and new strikes.

     One word can describe the most effective treatment for fire blight: pruning.  When a new strike is seen, it should be pruned out, the faster the better.  Making a cut 12 inches below the visible part of the strike is generally sufficient.  However, researchers in Virginia have found the bacteria in the internal bark tissues of branches 3 to 9 feet below any visible symptom!

     After pruning out a fire blight strike, it is common for the bacteria to colonize the pruning cut and create a canker.  If the cut is made close to the main trunk, the canker will infect the whole tree.  Thus, it is better to leave an “ugly stub”, up to 12 inches long.  The bacteria will travel down a few inches but will not enter the trunk.

Management recommendations

If there was no fire blight on the property last year:

   - Be alert.  An outbreak of fire blight can happen any year when the conditions are favorable.  Be especially vigilant when the flowers are open and there is hot weather with rainfall.

   - Scout the Malus blocks once a week during bloom and for one month afterwards.

   - As soon as a strike is seen, prune, prune, prune.  Cut about 12 inches below the visible part of the strike, using the “ugly stub” method.  Disinfect the pruning tool after each cut with Lysol, PineSol or bleach.

Fire blight was present on the property last year:

   - Assume you will have fire blight strikes again this year.  Make it a priority to manage the problem on a regular basis.

   - In late winter, but before March 15, prune out all cankers and “ugly stubs” from the year before.  Cut about 4 inches below any sign of dead bark, and, if possible, remove the pruned branch from the site.

   - Between green tip and pink stage, make an application of fixed copper 50WP at 1 kg of product in 500 liters of water.  This application will reduce the viability of the fire blight bacteria and give some protection against new infections.

   - During the bloom period, avoid overhead irrigation, which could trigger an infection.  Pay attention to weather conditions: 4 days in a row of 18^oC or more can trigger new infections.

   - Scout the Malus blocks twice a week for one month after bloom.  As soon as new strikes appear, prune, prune, prune.  Use the “ugly stub” method and disinfect the pruning tool after each cut.

   - In the summer, following hail or pruning, immediately spray with fixed copper 50WP at 500 grams of product in 500 liters of water.  Note that fixed copper will cause damage to the fruit of apple and pear trees.

When using pesticides,
always read the label for rate of application and safety instructions

To disinfect pruning tools:

     Researchers in California have compared various products to disinfect pruning tools tainted with the fire blight bacteria.

     Clorox (bleach), PineSol and Lysol gave “consistent protection” but not 100% control when the blades were soaked in the products for one minute, or the blades were sprayed with the products.  Clorox (bleach) and PineSol caused the blades to rust or discolor.

Registered pesticides:

     Pesticides currently registered for fire blight include streptomycin and fixed copper compounds.

     Streptomycin is very effective to prevent bloom infection, but it has stopped working in many areas of North American because the bacteria have developed a resistance to the product.

     Many fixed copper products are available in commercial formulations.  They contain copper oxychloride and are registered for use on crabapples, pears and ornamentals.  Copper will not kill the fire blight bacteria, but will reduce its ability to survive and reproduce on the plant surface.

     Again, if there was fire blight on the property the previous year, the grower should consider a preventative measure and apply fixed copper between the green tip and pink bud stages.

For more information

     - T. van der Zwet and S.V. Beer, 1999  Fire blight – Its nature, prevention and control  U.S. Department of Agriculture, Agriculture Information Bulletin no. 631.
An excellent reference publication on all aspects of fire blight.  Sold by the U.S. Government Printing Office, SSOP, Washington, D.C., 20402-9328.
     - P.W. Steiner and A.R. Biggs, 1998  Fire blight, Erwinia amylovora.
Available from West Virginia University, at the internet Web site http://www.caf.wvu.edu/kearneysville/disease_month/fireblight.html.
     - G. Jesperson, 1995  Fire Blight of Apple and Pear  B.C. Ministry of Agriculture, Fisheries and Food, Pest Management Note 95-02.
An excellent summary of management practices for Canadian apple growers.
     - T.J. Smith, 2000  Principles of Fire Blight Control in the Pacific Northwest  This article, and other articles on pruning for fire blight, are available from the University of Washington, http://www.ncw.wsu.edu/fireblt6.htm
     - Al Jones and H.S. Aldwinckle, Editors, 1997  Compendium of Apple and Pear Diseases  American Phytopathological Society, St. Paul, Mn.  A technical review of important diseases of Malus and Pyrus.  Contact the APS at www.shopapspress.org