Fire Blight Control for Organic Orchards: Moving Beyond Antibiotics

eOrganic author:

David Granatstein, Washington State University

Introduction

Fire blight is a serious disease of apple and pear caused by the bacterium Erwinia amylovora. It originated in the United States and is now found in many parts of the world. Most domesticated apple and pear cultivars have some degree of susceptibility to infection. Damage is not cosmetic, but reduces crop yield and may kill entire trees. Prior to implementation of the National Organic Program (NOP), a number of U.S. certifiers allowed the use of the antibiotics oxytetracycline and streptomycin for control of this disease, as they are naturally occurring molecules produced by soil microorganisms. However, the NOP classified them as synthetics but allowed their use for fire blight control only. The National Organic Standards Board has voted to remove these antibiotics from the national list of allowed materials, and their use will be prohibited after October 2014. Due to the potentially devastating damage from this disease, organic apple and pear growers are looking for viable non-antibiotic control measures.

Breeding for Resistance

The ideal solution to fire blight is genetic resistance bred into both the scion (fruit-bearing portion) and rootstock of the tree. The ‘Geneva’ series apple rootstocks do exhibit a high level of resistance to fire blight, but do not confer resistance to the scion grafted onto them. No highly resistant scion cultivars have been identified that also have the requisite fruit quality characteristics needed for commercialization. Several cultivars of pear developed by the USDA and Agriculture and Agri-Food Canada do exhibit increased resistance. Breeding programs are looking at sources of resistance in other wild apple species that have better fruit quality potential, and progress can be expected over the next 10-15 years.

Biological and Chemical Control

Application of antibiotics has been the primary practice used to manage fire blight for more than 50 years. Antibiotics are effective and fast-acting, and can be used in concert with disease prediction models (e.g., COUGARBLIGHT, MARYBLYT) so treatments may only be made when risk of infection is high. Research on biological control practices has been conducted since the 1980s, and several products have been commercialized such as Blight Ban®A506 (Pseudomonas fluorescens strain A506). However, until recently, no products exhibited efficacy similar to antibiotics. In 2012, the yeast product Blossom Protect™ (Aureobasidium pullulans) debuted in the U.S. market and has performed well for the past two seasons. Other materials such as Serenade® MAX (Bacillus subtilis), Double Nickel 55™ (Bacillus amyloliquefaciens), and soluble copper (e.g., Cueva®) are also available and organic-compliant, providing growers with several options to combine into an integrated fire blight management program. In addition, lime sulfur, commonly used by apple growers as a blossom thinner to reduce crop load, has been shown to exert control of fire blight when applied during bloom.

IMPORTANT: Before using any pest control product in your organic farming system:

  • Read the label to be sure that the product is labeled for the crop and pest you intend to control, and make sure it is legal to use in the state, county, or other location where it will be applied

  • Read and understand the safety precautions and application restrictions

  • Make sure that the brand name product is listed in your Organic System Plan and approved by your USDA-approved certifier. If you are trying to deal with an unanticipated pest problem, get approval from your certifier before using a product that is not listed in your plan—doing otherwise may put your certification at risk.

Note that OMRI and WSDA lists are good places to identify potentially useful products, but all products that you use must be approved by your certifier. For more information on how to determine whether a pest control product can be used on your farm, see the article, Can I Use This Input On My Organic Farm?

Time to Test Alternatives

Organic growers exporting fruit to the European Union were prohibited from using antibiotics, and they tested various approaches and products that were successful in certain locations and with certain cultivars. More recently, an organic fire blight control project for Oregon, Washington, and California, led by Dr. Ken Johnson of Oregon State University, has made significant progress in testing many of these materials and evaluating their efficacy as well as potential combinations and timings for best results. Growers need to test alternative controls on their own sites with their specific cultivars to prepare for the loss of antibiotics.

The following resources are available to help organic growers learn more about fire blight control alternatives. These include eOrganic webinars by Dr. Johnson, a new publication Grower Lessons and Emerging Research for Developing an Integrated Non-Antibiotic Fire Blight Control Program in Organic Fruit from The Organic Center, a recent journal article (Johnson and Temple, 2013) Evaluation of strategies for fire blight control in organic pome fruit without antibiotics), and an annotated powerpoint by Dr. Johnson summarizing the research progress to date. The applicability of the information presented in these various sources will undoubtedly vary by region, crop, orchard age, training system, and cultivar, so growers should be conducting some simple field evaluations of their own while the research proceeds.

One issue not fully resolved is that of fruit marking or russetting—a cosmetic defect on the skin of fruit caused by phytotoxicity of control materials when applied at a susceptible fruit stage. If a material provides a high level of fire blight control but russets fruit and renders it unmarketable in commercial channels, then it will not be an acceptable material for most growers.

Conclusion

Significant progress has been made in the past several years on non-antibiotic fire blight control methods that would be compliant on organic orchards. Well-vetted recommendations are not yet available, and thus growers need to be testing these new materials and ideas on their own orchards in the meantime. Ultimately, genetic resistance to the disease will provide the most sustainable alternative but this is likely decades away. However, growers can test small plantings of some of the reputedly more resistant cultivars now, observe their resistance, fruit quality, and horticultural needs, and develop their own markets for those new cultivars. The demand for organic apples and pears continues to increase, and growers need well-proven fire blight control approaches to allow them to respond to this demand while minimizing risks from the disease.

References and Citations

Additional Resources

Published May 15, 2014

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.

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