Fire Blight Season is Approaching – Preventative Copper is Recommended

As apple flowering-season approaches, growers should begin thinking about management of fire blight.  This bacterial disease can cause severe damage on apples, pears, and related ornamental plants during warm, rainy spring weather.

 

There is no single method that will provide consistent and reliable control. Management of fire blight requires an integrated approach that relies primarily on cultural practices and is supported by the judicious use of bactericides.



Fig 1 – Blossom blight phase of fire blight in which bacteria infect blossoms during bloom.

Disease Development: The fire blight bacterium overwinters primarily in cankered or diseased branches and trunks. During spring, bacteria-laden ooze is exuded from canker margins. Splashing rain and insects carry the pathogen to blossoms (Fig 1), and bees further spread the pathogen as they pollinate.

 

If weather is warm and rainy, populations of the causal bacterium (Erwinia amylovora) double every few hours, and more than a million bacterial cells can colonize a single floral stigma. Rain or dew then washes the bacteria into openings at the base of blossoms. Resulting symptoms are called blossom blight.  Infections can spread from blossoms to supporting spurs and branches, causing cankers that eventually kill entire branches (Fig 2).

 

Even if there is no blossom infection, shoot infections may occur. Bacterial cells infect externally through shoot tips, as young, succulent tissue is susceptible during periods of rapid growth. This phase of fire blight is called shoot blight or shepherd’s crook.  

 

Fig 2 – Flower and shoot infections can spread to branches, causing cankers that eventually kill entire limbs.  The fire blight bacterium overwinters in cankers and dead wood.

 

Bactericides:  During bud swell (late dormancy), an application of copper fungicide (e.g. Kocide or other fixed copper) should be applied, especially if fire blight was severe last year.  This copper application should reduce amounts of bacterium present on the surfaces of branches and spurs, reducing risk for disease development.  Do not apply copper after ¼ inch green leaf stage, as can be phytotoxic (cause foliar burn). 

 

During bloom, beginning at the first sign of open blossoms, a bactericide such as streptomycin (e.g. Agri-strep) should be applied at 4- to 5-day intervals through petal fall.  A minimum of two applications is recommended.  Another type of bactericide, oxytetracycline (e.g. Mycoshield) may be substituted, but it is not as effective as streptomycin.  Oxytetracycline may be mixed with streptomycin bactericides to help reduce the risk for resistance development.  Disease risk assessment sites (see below) may be used to improve timing and efficacy of bactericide applications.  Note:  Home orchards are usually not sprayed with antibiotics, so the preventative copper spray is critical.

 

After bloom, certain weather conditions can increase risk for shoot infections.  This shoot blight phase can be severe during rapid shoot development, especially under warm, rainy conditions.  The growth regulator prohexadione calcium (e.g. Apogee) reduces terminal growth, reducing succulent tissue that is most susceptible to infection.

 

Pruning: Growers should remove all damaged, dead, or diseased wood from trees during dormancy, before bacteria become active this spring.  This will help eliminate large amounts of infective inoculum.    

 

Disease Risk Assessment & Weather Models: Plant disease prediction models utilize weather data to analyze disease risk. The University of Kentucky maintains weather stations and incorporates this data into disease risk predictions models. Models can be found at http://wwwagwx.ca.uky.edu/plant_disease.html

  

More information:  See also our newest fact sheet Fire Blight http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-FR-T-12.pdf

Midwest Tree Fruit Spray Guide http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/MW_tree_fruit_spray_guide_ID-92_(2013)[1].pdf

Disease and Insect Control Programs for Homegrown Fruit in Kentucky http://www.ca.uky.edu/agc/pubs/id/id21/id21.pdf

Fire Blight Damage on Flowering Pear

Homeowners continue to question me concerning management of fire blight in flowering pear, crabapple, cotoneaster, hawthorn, and pyracantha.  Shepherd’s crooks and spur dieback occurred in March or April, as pathogenic bacteria infected flowers or young shoots.  Often, homeowners do not notice damage until later in the season, and inquiries continue for weeks after damage occurred.

There is no management option during this time of year, as the fire blight bacterium is not currently active.  Hot summer temperatures suppress bacterial growth, and plants are able to compartmentalize and wall off spread.  Thus, visible symptoms are the result of early infections. 

Current recommendations indicate that pruning of blighted twigs and cankered branches should be delayed until winter when risk of disease spread is lowest.  Under certain circumstances, homeowners or commercial landscape contractors may choose to prune infected branches during the growing season.  Only young, vigorous trees should be considered, and care should be taken to prevent bacterial spread.  Always avoid working with wet plants.  Cut branches at least 6 to 8 inches below cankers, disinfesting pruners between each cut (10% Lysol disinfectant, 10% bleach, or rubbing alcohol).  Ideally, winter pruning is recommended. 

Disease management includes both cultural practices and preventative bactericides.  Because the fire blight bacterium overwinters in cankered branches, removal of diseased plant tissue before bud break (mid to late winter) is critical.  Copper applied as buds swell (late dormancy or silver tip) reduces build-up of bacterial cells, especially during warm rainy spring seasons.  Streptomycin applications are only recommended for fruiting apple and are not labeled for use in the landscape. 

See earlier posts for more information on the biology of the fire blight bacterium and for management in orchards.

What You May Not Know about Fire Blight

Fire blight can be a devastating bacterial disease of apple, crabapple, pear, and flowering pear, but disease epidemics are often sporadic.  In fact, optimal conditions must be met for severe disease to occur.  Our current conditions are an indication that fire blight may be severe this spring.

Erwinia amylovora infects trees through flowers.  However, large numbers of bacterial cells must be present during flowering in order for the disease to develop into an epidemic.  The fire blight pathogen favors rain and temperatures above 60˚F.  Under these conditions, bacterial cells multiply quickly.  Thus, if conditions are favorable during flowering, infection can be severe.

Predictive systems are available for growers.  University of Kentucky’s Cougarblight model evaluates the potential for infection by analyzing temperature and leaf wetness data from the previous four days in order to estimate potential risk for infection.  Trees must be in bloom for this predictor to be effective.  Cougarblight is an excellent decision-making tool for growers and can be accessed at http://wwwagwx.ca.uky.edu/plant_disease.html .

Most growers are familiar with shoot blight, the most obvious fire blight symptom in which infected shoots die quickly, causing branch tips to form a distinct crook (photo).  Shoot blight, however, does not result from infection of blossoms.  Direct penetration of bacteria into green shoots or the upper leaves of young shoots after bloom typically causes shoot blight symptoms.

Blighting of shoots, also called shepherd's crook, is the most recognized symptom of fire blight on apple.

Initial fire blight infections occur through flowers.  We call this symptom blossom blight.  Petal browning is the first sign of petal blight, but many growers do not notice it.  Browning of pedicels (stems that attach flowers to stems) follows.  Often, droplets of bacterial ooze can be seen coming from pedicels (photo).  Bacteria quickly travel down the spur and into the twig.  Cankers that form around the spur-attachment site girdle branches, and then branch parts above the canker also die (photo). 

Initial infection by the fire blight pathogen occurs through blossoms.  Notice bacterial ooze come from the pedicel.

Bacteria can spread through flowers and spurs into twigs.  Resulting cankers can girdle entire limbs and branches.

Fire blight control measures include anti-bacterial pesticides applied during bloom.  Applications made after bloom are ineffective.  When fire blight risk is high (warm temperatures combined with rain) during bloom or if fire blight was a problem last year, the following spray schedule should be followed:

1.       Apply fixed copper at silver tip.  Homeowners should not skip this step, as it is their only tool available to combat fire blight.  Do not use copper fungicides after bud break.

2.       Apply streptomycin beginning at pink stage, repeating every 4-5 days, through petal fall.  At least 2 applications are required, but up to 4 sprays may be applied, depending on rain and temperature conditions.  Pay extra attention to susceptible varieties (i.e. Gala, Jonathan, and Rome).  Utilize Cougarblight or MARYBLIGHT predictive systems for assistance.  Mycoshield (oxytetracycline) is also available for management of fire blight but is not as effective as streptomycin.  Neither product is recommended for homeowner use.

Various cultural practices may be implemented to aid in disease management.  Combine these practices with bactericide sprays above for best control.

1.       Select disease resistant or disease tolerant varieties.  Liberty, Pricilla, SirPrize, Enterprise, Gold Rush, and Sundance are recommendations from ID-21.

2.       Prune last year’s cankers and dead wood before bud break.  Burn, bury, or completely remove prunings from the orchard to eliminate the possibility of bacterial cells being carried back to healthy tissue.  Monitor predictive systems.  Know your risk.  Cool temperatures or no rainfall will result in low disease incidence.

3.       Remove flower/fruit spurs immediately after symptoms develop so bacteria cannot continue infection into branches.  Dip tools in 10% bleach, 10% Lysol concentrate, or pure rubbing alcohol after each cut to keep from spreading bacteria.

4.       Do not prune limbs or branches during the growing season.  Trees natural defenses wall off infection sites and stop disease spread.  Remove these branches during the dormant season, instead, when threat of disease spread is lowest.  Removal of all infected wood is critical to prevent spread of inoculum.

More information on fire blight can be found in PPA-34 and PPFS-FR-T-7. 

Spray recommendations for commercial growers ID-92 and homeowners ID-21 are also available online.