Monitor Streptomycin Resistance in Erwinia Amylovora Populations in New England

Progress report for ONE22-431

Project Type: Partnership
Funds awarded in 2022: $30,000.00
Projected End Date: 12/31/2024
Grant Recipient: Connecticut Agricultural Experiment Station
Region: Northeast
State: Connecticut
Project Leader:
Quan Zeng
Connecticut Agricultural Experiment Station
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Project Information

Project Objectives:

This project seeks to understand the status of fire blight pathogen E. amylovora's susceptibility to the commonly used management materials, streptomycin. Two objectives proposed are:

Objective 1: Conduct a survey evaluating the streptomycin resistance of E. amylovora isolates in New England apple and pear orchards.

Objective 2: Disseminate the findings to the New England apple and pear growers and provide education about the causes and prevention of streptomycin resistance.

Anticipated impacts: Findings from these objectives will provide increased knowledge in tree fruit management area regarding the accurate use of antibiotic streptomycin to control fire blight.  Streptomycin susceptibility in E. amylovora populations in New England will be obtained.  This will provide necessary knowledge to the New England apple and pear growers about whether streptomycin is still an effective bactericide for fire blight management.  In case streptomycin resistance is observed, the geographical distribution of streptomycin resistant E. amylovora populations in the New England region will be acquired.  If resistant populations are at the emerging stage, eradication of orchards will help to restrict the spread of the resistant population to other parts of the region. 


1.1 Fire blight is a devastating disease of apples and pears

Apples and pears represent economically important commodities in the United States and New England.  According to recent U.S. census data, apples and pears are grown on 142,000 and 24,300 hectares nationally, with cash receipt values of $2.6 billion and $372 million, respectively (USDA-NASS 2014)  .  In New England (CT, MN, MA, NH, RI and VT), cash receipt values of apple and pears exceed $54 million and $6.1 million, respectively (USDA-ERS).

Fire blight, caused by a bacterial pathogen Erwinia amylovora, is one of the most serious diseases currently limiting apple and pear production in New England and in the United States (Malnoy 2012; Zeng 2013; Zeng 2014).  Fire blight can result in both yield reduction, when infection occurs on the scion, and tree death, when infection occurs in the rootstock. Annual losses to fire blight and costs of control in the United States are estimated at over $100 million (Norelli 2003).  In recent years, severe fire blight outbreaks occurred throughout the New England region. 

1.2 Timely application of antibiotic streptomycin is an indispensable component of fire blight IPM

Most apple and pear varieties sought after by consumers, such as “Gala”, “Fuji”, and “Barlett”, are either susceptible or highly-susceptible to fire blight.  Although multiple fire blight resistant rootstocks such as “Geneva” and “Vineland” have been developed, grafting the susceptible cultivars to the resistant rootstocks only prevent rootstock infections but does not prevent scion infections.  In practice, the fire blight susceptible scions are often grafted to fire blight susceptible dwarfing root stocks for the new industry standard, high-density orchard systems, making these orchards highly vulnerable to fire blight.

The lack of effective cultural control makes chemical control an indispensable component of fire blight IPM.  The most critical step of fire blight management is to limit the multiplication of E. amylovora on stigmas of apple flowers to prevent blossom blight (Malnoy 2012).  With appropriate disease forecasting models (e.g. Maryblyt or CougarBlight), properly timed application of bactericide during bloom can effectively reduce bacterial number in flowers and control blossom blight.  Streptomycin is by far the most effective (up to 90% control) and widely-used bactericide for fire blight control in the United States .  Other control alternatives include the biological control agents Serenade MAX , BlightBan A506, and the systemic acquired resistance (SAR) inducers such as Actigard.  However, these control alternatives have not provided consistently high levels of control compared to streptomycin (Sundin 2009).

1.3 Streptomycin resistance raises critical challenges to fire blight IPM

The intensive, long-term use of streptomycin led to the evolution of streptomycin resistance in E. amylovora populations.  Since the first report in California in 1971 (Miller 1972), streptomycin resistance in E. amylovora has been commonly detected in almost all apple producing regions in the United States, such as Washington (1972), Michigan (1990), and has recently been detected in New York (2003) (Coyier 1975; Chiou and Jones 1993; Russo 2008).  Currently, the New England is the only region in the United States where streptomycin resistance in E. amylovora has not been reported.

The impact of streptomycin resistance is dramatic, both from the economic loss point of view and the environmental impact and human health point of view.  First, because streptomycin is no longer effective in regions with streptomycin resistant E. amylovora populations, thousands of acres of valuable apple and pear crops are exposed to fire blight, which has caused catastrophic losses.  In 2000, the fire blight epidemic in southwest Michigan resulted in the death of 400,000 apple trees, and over $42 million in losses.  In addition, the loss of streptomycin led the growers to adopt other antibiotics to control fire blight.  Oxytetracycline and kasugamycin have been used in fire blight control in California and Michigan due to streptomycin resistance.  Introduction of these antibiotics into agriculture raises concerns of facilitating the development of antibiotic resistance in the environment, with a potential impact to human health. 



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  • Stephen Wood - Producer


Materials and methods:

Objective 1. Conduct a survey evaluating the streptomycin resistance of E. amylovora isolates in New England apple and pear orchards.

1.1 Collect fire blight tissues from New England orchards and isolate E. amylovora

Fresh fire blight infected apple and pear tissues will be collected from New England orchards in 2023 and 2024.  The sample collection will be through a collaborative effort of extension agents in the area, including but not limited to, Mike Basedow, Anna Wallingford, Win Cowgill, Kristy Grigg-McGuffin, Dan Cooley, and Jeremy Delisle. Fresh shoot and fruitlet samples with ooze droplets will be collected and put in a plastic bag with a damp paper towel and shipped to Connecticut Agricultural Experiment Station using a prepaid envelop. Special attention will be given to orchards that have used streptomycin in the past, observed inconsistent control of fire blight using streptomycin, or had severe fire blight symptoms in the past years.  Once receiving the samples, researchers from CAES will perform pathogen isolation and resistance testing. Diseased samples will be surface sterilized followed by removal of the outer bark.  Bacteria from infected internal plant tissue / ooze droplets will be resuspended in sterile water and plated on an E. amylovora differential medium CCT.  Pale violet, mucoid E. amylovora colonies will be selected and preserved as putative E. amylovora strains.  The identity of E. amylovora will be confirmed by polymerase chain reaction (PCR) using E. amylovora diagnostic primers that amplify a region from the ubiquitous E. amylovora plasmid pEA29 (Bereswill, Pahl et al. 1992).

1.2 Evaluate the streptomycin resistance of E. amylovora isolates

The confirmed E. amylovora isolates will be tested for streptomycin resistance using the Kirby-Bauer antibiotic diffusion method.  E. amylovora isolates will be cultured in Luria-Bertani (LB) broth at 28°C overnight.  100µl of 108 cells/ml bacterial suspension will be evenly spread onto LB plates.  The growth of bacteria will be challenged by placing filter paper disks impregnated with streptomycin sulfate solutions at 0, 10, 50, or 100 μg/ml.  A streptomycin resistant strain and a streptomycin susceptible strain will be included as positive and negative controls.  We expect that zone of inhibition will be observed on plates with streptomycin susceptible strains but not with streptomycin resistant strains.  The levels of resistance will be determined by measuring the sizes of inhibition zones at 24 and 48 hours post incubation.


Research results and discussion:


A list of collaborators have been identified, and sampling methods and collection bags have been delivered accordingly. Sampling of streptomycin resistance in E. amylovora will start in May 2023, when the infected plant materials are available. 


In 2023, we tested 48 samples collected from 13 farms in New England states.  This includes 7 farms in New Hampshire, 5 farms in Connecticut, and 2 farms from Massachusetts.  Most of the samples were mailed by growers with prepaid Fedex services provided by the grant. Erwinia amylovora strains have been isolated from the infected tissue, mostly bacterial ooze droplets, or infected immature fruits and occasionally shoots. The isolates were streaked on LB agar plates supplemented with streptomycin at 100ug/ml. All E. amylovora isolates were tested negative in resisting streptomycin. Results are provided to growers / extension agents typically within 10 days. 

Research conclusions:

There is currently no E. amylovora that can resist streptomycin at 100ug/ml at the tested locations in New England.  Measures to prevent streptomycin resistance include restricting transporting plant cuttings, nursery materials from regions where streptomycin resistant E. amylovora is endemic. Alternating the use of streptomycin with other control measures (e.g. Blossom Protect) may help prevent the development of streptomycin resistance from the native Ea populations. 

Participation Summary
13 Farmers participating in research

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

Objective 2 Disseminate the findings to the New England apple and pear growers and educate them about the causes and prevention of streptomycin resistance. 

We will disseminate the survey results to growers through extension bulletins (CAES fact sheets, New England Fruit Pest Management Guide), presentations at regional grower meetings (CT Pomological Society Meeting, MA fruit grower association meeting et al), at the Northeastern Tree Fruit Management Working Group meetings (2023 and 2024), the New England Vegetable and Fruit Conference (NEVF), and one-on-one consulting with growers will be used to further reach fruit growers in New England. Usage of disease forecasting models, appropriate timing and dose of spray will be discussed during the grower education.

Project Outcomes

Project outcomes:


Sampling will start in May 2023, the next growing season when infected plant materials are available. 


Please see result section above. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.