Exploring Fire Blight Management, Part 3: Antagonists of Erwinia amylovora

Fire blight reached epidemic proportions in Switzerland in 2007 and much of the rest of Europe as well (Duffy et al. 2007). Early, warm weather was conducive for fire blight infections during a long synchronous bloom, exposing many more blossoms than usual to the causal pathogen, Erwinia amylovora. Losses were significant across Europe and were compounded by strict regulations on antibiotic sprays. The European Union has a strong stance against the use of antibiotics for horticultural production which has encouraged the search for alternative management strategies. Many areas around the world, including in North America, have witnessed the increase in various antibiotic-resistant strains of E. amylovora. This has also hastened the search for new methods of limiting fire blight.

One area of research has focused on finding organisms that are antagonistic to the E. amylovora bacteria. Antagonistic organisms are intended to out-compete the disease-causing bacteria where they occur in blossoms. There is a limited amount of resources where infection can occur, so bacteria or fungi that can grow quickly and deprive E. amylovora of food or space and, at the same time, not cause disease are helpful for fire blight suppression. It is not nearly as simple a process as that sounds though. Recent research shows that having some virulent E. amylovora present on the nutrient-rich flower stigma actually enhances the yield of other non-disease causing bacteria (Johnson et al. 2007). Johnson et al. suggest that E. amylovora actually modify their habitat by the expression of "pathogenesis-related genes" and increase resources (nutrients) available to themselves and co-occurring bacteria and fungi. A suppression of host resistance could also be occurring, but in any case, it is not just a simple situation of resource competition. The fact that E. amylovora create a "biofilm" (Koczan et al. 2007) which is a complex aggregation of bacteria, other molecules and sugars, also makes the process of antagonistic competition even more complicated. E. amylovora thrive in the biofilm they create but the biofilm also provides a niche for many other bacteria and microorganisms.

One bacterial antagonist of E. amylovora that is already registered for use in Ontario is Pantoea agglomerans. Two different strains of P. agglomerans are registered as "Bloomtime" and "BlightBan" and since I've written about those products recently (Carter and Celetti 2006), I won't go into any great detail on them here. Researchers in Oregon (Stockwell et al. 2007 ) are exploring ways of integrating antoibiotic sprays with antagonists (P. agglomerans and Pseudomonas fluorescens) for improved control of the blossom blight phase of fire blight. Preliminary work suggests that antagonists applied at around 70% bloom followed by antibiotic spray (in this case oxytetracycline) later in bloom may provide reasonable control with reduced antibiotic use from standard management practices.

Another antagonist that is undergoing field tests in Europe is "Blossom Protect", which contains two yeast strains (Aureobasidium pullulans) antagonistic to fire blight bacteria (Ertl et al. 2007). A suitable formulation is now in limited use after many years of work, not least of which was the determination of a fermentation process capable of producing commercial amounts of the yeasts. These particular yeasts must be applied early and frequently through flowering since they propagate in the flower and prevent infections by E. amylovora. Preventative use at 10%, 40%, 70% and 90% open blossoms may be lowered to two applications as more is known about the biology of the yeasts. Work continues to lessen fruit russetting if the product is used too late and to reduce the relatively high cost of the product.

With the number of antagonists under study now, it's very likely that some form of Erwinia antagonists will become a standard recommendation in the future for management of fire blight in Ontario. Integrating their use with antibiotic applications will be an area requiring a great deal of field work in the future.

References

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

Duffy, B., J. Vogelsanger, B. Schoch and E. Holliger. 2007. Swiss situation and the 2007 fire blight epidemic. 11th International Workshop on Fire Blight , Paper P101.

Ertl, C., G. Mogel, S. Kunz, C. Donat and H. Danner. 2007. From the laboratory to the market - The success story of Blossom Protect. 11th International Workshop on Fire Blight , Paper O65.

Johnson, K.B., T.L. Sawyer. V.O. Stockwell and T.N. Temple. 2007. Comparison of the epiphytic fitness of a pathogen and an avirulent strain of Erwinia amylovora on pear and apple flowers as it relates to biological control. 11th International Workshop on Fire Blight, Paper O4.

Koczan, J.M., M.J. McGrath, Y. Zhao, and G.W. Sundin. 2007. Biofilm formation in Erwinia amylovora: Implications in pathogenicity. 11th International Workshop on Fire Blight , Paper O6.

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

• Exploring Fire Blight Management, Part I: Models
• Exploring Fire Blight Management, Part 2: Infection Sources
• Exploring Fire Blight Management, Part 4: Antibiotics