Overview

Fire blight (Erwinia amylovora) can be the most damaging pathogen to apple trees in Montana (Zidack et al.).  Fire blight is a bacterial pathogen that infects flowers of pear and apple and can rapidly spread through the tree killing both the scion and the rootstock of susceptible cultivars and rootstocks.  Young trees are particularly vulnerable to the disease which thrives under warm (70-90F) and humid conditions.  In addition to infecting pears and apples it can also be found on several other members of the Rosaceae family including native mountain ash, serviceberry, raspberry, contoneaster and hawthorn (Koetter and Grabowski).  It is spread between trees primarily by pollinators and other insects but can also be spread to flowers by wind and rain. 

Identification

Once infected blossoms may appear shriveled or frost damaged. As the pathogen spreads, branch tips will appear scorched and droop with the characteristic “sheperd’s crook.” Later infected fruit begins to ooze and cankers form on branches.  The ooze can then be spread by insects in the orchard and lead to further infections through wounds.  Early detection is an important aspect of controlling fire blight.  WSU Fire Blight guide offers comprehensive identification information in addition to management resources.  Michigan State University provides a thorough guide on scouting for blossom blight symptoms of fire blight in apples for early detection. MSU Schutter Lab can also assist growers with identifying and confirming fire blight in orchards.  Samples can be submitted through local Extension offices in Montana.  The photos above and below are of early spring infections in flower buds and cankers where infection has over wintered providing inculom for future infections.

Fire blight on new flower budOld Canker

Risk Factors Contributing to Fire Blight Infection

In all liklihood fire blight is present in some form in many areas of Montana, however, a specific orchards risk of damaging infection depends on several risk factors. The risk of fire blight in an orchard depends on the susceptibility of the cultivars present, the length of the bloom period (typically increases with the number of cultivars planted), the age of the trees and their vigor, the presence of fire blight in the neighborhood and the risk of infection based on heat and humidity during bloom.  This fire blight risk tool can help growers determine their general risk based on the various factors contributing to the presence and spread of fire blight in a given orchard. More information about each factor is provided below.

Cultivar susceptibility

Some cultivars are resistant to fire blight.  Even if resistant cultivars are exposed to fire blight it does not spread to other areas of the tree.  More susceptible cultivars can be severely damaged or even killed by the pathogen as it spreads to limbs, the trunk, and if grafted to a susceptible rootstock, the roots as well.

Ways to reduce risk

When possible, plant cultivars and rootstocks that are resistant to fire blight.  If you must plant cultivars that are susceptible try to select rootstocks that are not and be diligent to prevent fire blight infections when trees are young. For example, Montana cider apple growers have found that several of the cultivars grown for hard cider production are very susceptible to the disease including Harry Masters Jersey, Brown Snout, Sommerset Redstreak, Muscadette de Dieppe, and Porter's Perfection.  By removing flowers when the trees are young they can reach an age when they are less susceptible to being permanently damaged by the pathogen. 

Additional Resources

Disease Susceptibility of Common Apple Cultivars

Orange Pippin Fire Blight Resistant Apple Varieties (includes some cider apple cultivars)

Resistent Rootstocks Key to Surviving Fire Blight, Good Fruit Grower

Length of the bloom period and late blooming cultivars

The longer the bloom period in an orchard the higher the risk for fire blight infection.  As the pathogen builds with warmer weather it is more easily spread.  Orchards with a mix of cultivars are more at risk for the spread of fire blight as they often have a long stretched out bloom period.  Similarly, later blooming varieties are at higher risk as a result of favorable fire blight conditions later in the spring.

Ways to reduce risk

Orchardists can select for cultivars with early bloom times and keep their cultivar selection to a minimum.  Secondary or “rat-tail” blossoms can also be removed with lime sulfur sprays or hand thinning to reduce opportunities for infection when risk is high.

Additional Resources

Cultivars are often classified as having early, mid or late bloom times.  WSU has several years of data tracking full bloom in several cider apple cultivars.  This information is available in their annual reports located on the WSU Varietal Evaluations page.

Tree age and vigor

One of the primary factors for the extent of infection in trees is their age and vigor.  Fire blight will spread more easily to vital areas of the tree including the trunk and roots if trees are young and vigorous.  A grower with young trees will want to protect that investment more aggressively if the trees are yet to bear and have several years of potential income ahead if managed properly.  For similar reasons, an orchard large in size may warrant more aggressive protection than a small orchard.

Ways to reduce risk

During the establishment phase and early production, growers should take an aggressive approach to managing fire blight especially if they are pushing trees to grow fast with an intense nutrition program.  Young trees should be protected with a weekly copper spray program from spring through summer in their first through third leaf and blossoms from very young trees can be hand thinned.  If fire blight is contracted, trees should be pruned quickly and aggressively to remove infections.

Additional resources

Taking the Fight to Fire Blight, Good Fruit Grower: includes several tips on protecting young trees.

 "Dealing with fire blight once it is in the orchard" provides helpful tips on how to scout for fire blight, effective pruning techniques and what to do with infected material.

Fire blight in the neighborhood

If trees had fire blight last year, chances are fire blight will be a problem the following year.  This not only goes for a particular orchard but for the general neighborhood, or the distance a pollinator will forage.  Growers should be aware that the pathogen may be present even in orchards with few if any symptoms.  For example, an old Macintosh may show few signs of fire blight damage, however, inoculum on the tree may still be present and can be carried to more susceptible trees in the area by pollinators. 

Ways to reduce risk

Growers can take a proactive approach if they know fire blight is present nearby.  Fixed copper sprays can provide some sanitation but are only necessary if fire blight was in the area the previous season.  Additionally, growers should pay attention to the cougarblight model to determine high risk times and apply their treatments accordingly.  The model provides different recommendations based on the activity of fire blight in the area presently and the previous year.

Orchard Management Approach

The effectiveness of spray options that provide anti-biotic control decrease with organic controls options (Cueva, Serenade and Previsto) in the following order from most effective to least: streptomycin > Kasumin > Previsto > oxytetracyline > Cueva > Serenade. Options like Blossom Protect can be effective in both conventional and organic spray programs when applied at 70% bloom to the whole row.  Antibiotics, in particular streptomycin should be used with caution to aviod bacteria developing resistence.

Ways to reduce risk

Use an integrated approach to pest management by planting resistent varieties, practicing good sanitation, keeping humidity low in orchard during bloom and timing spray applications strategically.

Additional Resources

For additional information on using Blossom Protect effectively visit Fire Blight Management-Tips for Using Blossom Protect. For more information on organic control of fire blight download the Organic Center's report "Grower Lessons and Emerging Research for Developing an Integrated Non-Antibiotic Fire Blight Control Program in Organic Fruit."

Heat and humidity during bloom

The optimal conditions for fire blight growth are temperatures between 75-90F coinciding with a rain or dew event.  In order for flowers to be infected they must be open. 

Ways to reduce risk

Watch the cougarblight model for information on high risk events.  In addition, avoid any sprinkler irrigation.  Even drip irrigation can be avoided during bloom and petal fall with little consequence to fruit production.

Management Options

Several management options exist to prevent catastrophic fire blight damage in orchards.  Taking an integrated approach is essential beginning with planting resistant cultivars and rootstocks, practicing good sanitation, keeping humidity low in your orchard by minimizing irrigation during bloom, and limiting vigor of trees by avoiding high fertility applications. 

Additionally, young trees should be protected with an aggressive, proactive approach when trees are young and most vulnerable.  This includes removal of blossoms in very young trees, scouting to identify infections early and removing them quickly as well as using a preventative spray program based on tree phenology rather than relying solely on weather conditions to determine high-risk periods.   For backyard growers, chemicals are often unnecessary if they employ good cultural management practices.  Antibiotics should be avoided by back yard growers, in particular streptomycin as fire blight bacteria can develop resistence to these products. Find out more about options for backyard growers.

Chemical options

Several chemical and biological options exist for both conventional and organic growers.  These chemicals vary in their mode of action from copper sprays providing dormant season sanitation to biological sprays containing yeasts that compete with the bacteria and antibiotics which directly kill the pathogen.   The WSU Fire Blight guide explains in detail these control options and provides a suite of organic and conventional management options. 

The use of the products listed in the WSU guide, are applicable to Montana growers, with the possible exception of early lime sulfur and oil treatments to thin blossoms.  This is not always necessary in Montana where frost can provide this service, however, later and lingering "rat-tail" blossoms may be thinned with lime sulfur to reduce the risk of infection in cultivars with such tendencies.  As with any chemical growers should confirm the product is labeled for use in Montana and follow the label carefully. 

Regardless of age or management approach, timing is crucial to the success of antibiotic and biological sprays to control fire blight.  According to the Crop Protection Guide for Fruit Trees in Washington, sprays should be applied with in a 24 hour period prior to a wetting event during a high risk period.  The cougarblight model, helps growers time sprays appropriately based on forecasted heat and humidity and their risk based on the presence of fire blight in the area.  In addition, this chemical cost calculator for products labeled in Montana, can help growers weigh their risks and fire blight management options against the cost of spraying controls. 

As with any pest management program it is important to take a multi-faceted approach and not rely solely on chemicals to manage a pest.  While planting resistant rootstocks and cultivars is a good foundation, identifying infection early and aggressively pruning out infected plant material is an important factor in reducing risk during infection periods and can reduce the need for using sprays.  "Dealing with fire blight once it is in the orchard" provides helpful tips on how to scout for fire blight, effective pruning techniques and what to do with infected material.  In addition, Growing Produce’s article titled “Integrating Control Methods to Tackle Fire Blight” provides several helpful tips on how to maximize the benefits of available management options to control fire blight.

For more information on managing fire blight in Montana you can view this presentation, Fire blight in Montana, given by Dr. Ken Johnson as part of a workshop on IPM in Montana Orchards.

 

Disclaimer:

These recommendations are provided only as a guide.  It is always the pesticide applicator’s responsibility, by law, to read and follow all current label directions for the specific pesticide being used.  Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them.  If any information in these recommendations disagrees with the label, the recommendation must be disregarded.  No endorsement is intended for products mentioned.  The authors and Montana State University assume no liability resulting from the use of these recommendations.