Exploring Fire Blight Management, Part I: Models

Fire blight, a disease primarily of pear and apple, has been recognized as a serious problem for over 100 years. The disease, caused by the bacterium Erwinia amylovora, is a perennial issue for growers in Ontario with some seasons being more conducive to the spread of the disease than others. Some research presented at the 11th International Workshop on Fire Blight in Oregon in August, 2007 may help to explain why the disease continues to elude the best efforts at management, at least in some years.

Models for predicting fire blight infection periods can help determine appropriate timing of antibiotic sprays (streptomycin in our area) or antagonistic bacteria applications (Pantoea agglomerans from "Bloomtime" or "BlightBan). Models such as Maryblyt and Cougar Blight have proven exceptionally useful over the last two decades. Growers should be using these models or at least consulting regional reports that mention infection periods. However, these models do not provide perfect prediction and cannot guarantee disease control. As Lawrence Pusey and Tim Smith (both from Washington) pointed out, there are multiple factors - some still poorly understood - that explain why something as simple as a four day temperature evaluation prior to a wetting period (as used in Cougar Blight) actually manages to usually effectively predict the potential for infection. The four day period was originally incorporated into the model based on an assumption that flower stigmas support the growth of E. amylovora for only a few days after flower expansion. It is now known that flowers can be infected for a much longer period depending on environmental conditions, and that there is a continuum of multiple peaks of infection risk as new flowers open especially in areas where orchards bloom at different times because of gradients in climatic factors. Nonetheless, Cougar Blight and other models definitely assist in predicting peak infection periods and these models should always be used as part of an integrated approach to disease management.

No model can predict all blossom infections though. For one thing, models predict peak infection activity not all infections. In addition, there are sources of infection that the models cannot possibly predict or take into account (see Part 2 of this series on fire blight - "Infection Sources").

The key to remember is that perfect control is unlikely in most years unless chance favours you through ideal environmental conditions for flowering but not for fire blight infection and there are no systemically infected blossoms already present in the orchard. As well, antibiotic sprays must be well-timed by the models (which are imperfect). Applications must be made under ideal conditions and with excellent coverage, follow-up monitoring, and additional application where needed.

Much work on the basic biology of E. amylovora continues and will help to refine the predictive disease models (for example, work at Cornell by Dewdney et al., 2007). Predictive models are only part of the fire blight management puzzle; good management practices in all aspects of the disease are necessary for adequate suppression of fire blight.

References:

Dewdney, M.M., R. C. Seem, and H. S. Aldwinkle. 2007. The effect of apple blossom a ge on populations of Erwinia amylovora on the stigma surface. 11th International Workshop on Fire Blight , Paper P12.

Pusey, L.P. and T. Smith. 2007. Susceptibility of apple hypanthium to Erwinia amylovora in relation to flower age and 4-day temperature evaluation within Cougarblight model. 11th International Workshop on Fire Blight , Paper O3.

For more information on Cougar Blight or BlightBan and Blooomtime:

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

Celetti, M. and N. Carter. 2006. Using Cougar Blight to predict fire blight risk. Hort Matters, Vol. 6, Issue 6, April 12, 2006

Related Links

• Exploring Fire Blight Management, Part 2: Infection Sources
• Exploring Fire Blight Management, Part 3: Antagonists of Erwinia amylovora
• Exploring Fire Blight Management, Part 4: Antibiotics