Exploring Fire Blight Management, Part 4: Antibiotics

Fire Blight, the most serious disease of pear and apple in Ontario, is normally suppressed by the combination of cultural management techniques and antibiotic sprays. Some antagonists are now registered (Carter and Celetti 2006) and the plant growth regulator prohexadione-calcium ("Apogee") has also been added to our short list of fire blight management products (Cline 2006).

Antibiotics are a traditional approach to managing bacterial diseases of horticultural crops but there is strong pressure against their use in some areas including the European Union. The United States has had other antibiotics registered for use against fire blight for some time (e.g. oxytetracycline) but there are continuing concerns with their use in horticulture. Since streptomycin and oxytetracycline have uses in human medicine, there are concerns with development of resistance by human pathogens. More disturbing from a horticultural perspective is that there has been widespread resistance developed in many regions to streptomycin by Erwinia amylovora, the causal agent of fire blight (McGhee et al. 2007, Russo et al. 2007, Adaskavag 2007, Evans 2007, Sholberg and Boulé, 2007).

Even though there are streptomycin resistant E. amylovora in some regions, streptomycin is often still the standard against which other control measures are evaluated. However, in all cases and for all products used to manage fire blight, one should be very cautious of putting too much stock in any single field trial or experiment. Stockwell et al. (2007) presented an excellent summary of 16 years of fire blight trials in Oregon at the 11th International Workshop on Fire Blight in Portland, Oregon in August 2007. All products, whether anatagonists or antobiotics - including streptomycin, showed a wide variability in level of control when examined over the long term. This summary of trials was a good reminder that seasons, plants, bacteria and applications are all variable, and you can't expect anything to work perfectly in every case. The other side of that coin is that limited field trials that show success should be viewed cautiously since there may have been low levels of E. amylovora to start with, or environmental conditions that did not favour infection, or any number of other reasons for unusual or unexpected results.

A relatively new antibiotic that is not yet available in Ontario holds good promise as an alternative fire blight management tool. Kasugamycin ("Kasumin") is a fermentation product of Streptomyces kasugaensis and can be a useful adjunct to other antibiotics for several reasons. Kasugamycin has no uses in human medicine, so concerns over human pathogen resistance are immaterial. Also, it appears that almost all streptomycin-resistant strains of E. amylovora are not resistant (at present) to kasugamycin. This fact would make kasumin an excellent rotational tool for antibiotic-based fire blight management (Evans 2007). The integration of kasugamycin with bacterial antagonists (see part 3 of this series of articles) in fire blight management programs will require further investigation since it appears that kasugamycin may be a stronger inhibitor of bacterial biocontrol agents than streptomycin (Johnson et al. 2007).

Other antibiotics continue to be studied for use against E. amylovora as do various plant extracts such as sea buckthorn juice (Sholberg and Boulé, 2007). For now though, streptomycin remains as our primary chemical method for fire blight control and following resistant management strategies is critical to maintain its effectiveness. The addition of new antibiotics to our available products would be an excellent way to keep streptomycin effective and help to manage fire blight sustainably.

References

Adaskaveg, J.E., W.D. Gubler, H. Forster and L.M. Wade. 2007. Detection of Erwinia amylovora strains with reduced sensitivity to currently registered antibiotics and management of fire blight of pome fruits using kasugamycin in integrated strategies. 11th International Workshop on Fire Blight , Paper O62.

Carter, N. and M. Celetti. 2006. 11 questions about using "Bloomtime" and "BlightBan" to suppress fire blight. Hort Matters, Vol. 6, Issue 30, Dec. 7, 2006

Cline, J.A. 2006. Apogee® - A New Plant Bioregulator for Apples. Ontario Ministry of Agriculture Food, and Rural Affairs. Agdex 24/211. Order No. 06-045. [August 2006]

Evans, K. 2007. Survey results of Erwinia amylovora in Utah for resistance to streptomycin and investigations comparing kasugamycin (Kasumin) to streptomycin and oxytetracycline for control of fire blight. 11th International Workshop on Fire Blight , Paper P74.

Johnson, K.B., V.O. Stockwell and T.N. Temple. 2007. Evaluation of kasugamycin as a component of an integrated biological and chemical strategy for suppression of blossom blight. 11th International Workshop on Fire Blight , Paper P75.

McGhee, G.C., L. Bellomo, S.E. Blumer and G.W. Sundin. 2007. Emergence and progression of streptomycin resistance in Erwinia amylovora in Michigan. 11th International Workshop on Fire Blight , Paper O61.

Russo, N.L.. D.I. Breth, T.J. Burr and H.S. Aldwinckle. 2007. Spread and identification of streptomycin resistant isolates of Erwinia amylovora in New York. 11th International Workshop on Fire Blight , Paper P71.

Sholberg, P. and J. Boulé. 2007. Evaluation of antibiotics and plant extracts for control of streptomycin-resistant Erwinia amylovora. 11th International Workshop on Fire Blight , Paper P72.

Stockwell, V.O., T.N. Temple, K.B. Johnson and J.E. Loper. 2007. Integrated control of fire blight with antagonists and oxytetracycline, 11th International Workshop on Fire Blight , Paper O63.

Learn more

• Apogee® - A New Plant Bioregulator for Apples, OMAFRA Factsheet Order No. 06-045
• Exploring Fire Blight Management, Part I: Models
• Exploring Fire Blight Management, Part 2: Infection Sources
• Exploring Fire Blight Management, Part 3: Antagonists of Erwinia amylovora