HortFACT - Fire blight (Erwinia amylovora) in apples and pears - An Introduction to the Disease
What is it ?
Fire blight is a disease caused by the bacterium Erwinia amylovora. It infects pears, apples and quince as well as ornamental plants of the Roseaceae family including cotoneaster, hawthorn and pyracantha.
What does it look like ?
Blossoms are usually infected first and have a water soaked appearance. Eventually the blossoms will wilt, shrivel and turn black.
The most characteristic symptoms are blackening of leaves, shoots and, in severe cases, branches. Sometimes the shoots will bend into "shepherd’s crook". A sticky ooze, which contains millions of bacteria, will seep from the infected area. Cankers may become visible from the summer months onwards. Bacteria can overwinter in the cankers and become active again in the spring.
 A susceptible pear seedling showing typical symptoms of fire blight. |
 Typical shepherds crook symptoms of fire blight on shoots and leaves. |
Bacterial blast on pears (Pseudomonas syringae) can look similar to fire blight, especially early in the season. Bacterial blast usually affects the blossoms and surrounding leaves.
What conditions are needed ?
Fire blight usually flares up in the spring on the flowers when the average daily temperatures are greater than 15.6º C and moisture is present.
Where is it found ?
Outbreaks of fire blight can occur anywhere in New Zealand under the right conditions.
World-wide, the disease is found throughout North America and Canada and much of Europe. New Zealand is the only Southern Hemisphere country where it has been recorded.
Why is it a problem ?
A severe outbreak can lead to loss of flowers and hence no fruit is produced. It can also cause die-back on trees and can potentially wipe out a whole orchard. Severe infections are rare on mature trees in New Zealand and are more devastating on young trees. The main problem is that fire blight is used as a quarantine barrier by fire blight-free countries such as Japan and Australia.
What are the current chemical controls ?
The two main forms of chemical control are copper, which must be applied prior to green-tip to prevent russeting, and streptomycin for use during the growing season. Streptomycin is an antibiotic bactericide. Frequent use of streptomycin can lead to the development of streptomycin-resistant bacterial strains and subsequent loss of control. Therefore calender spraying of streptomycin during blossom is not acceptable.
Are there any other forms of control ?
At HortResearch we are focussing on the three main areas :
1. Overcoming market access and quarantine barriers:
This programme has been underway for many years now, and was largely responsible for the access of New Zealand apples into Japan. The current aim is to gain entry of apples into Australia, by showing that the fire blight bacteria cannot be transmitted on export fruit. Work is also in place to scientifically prove that Japan MAFF regulations are unnecessary with regards to fire blight. Funding is currently being provided by ENZA New Zealand (International) and Foundation for Research, Science and Technology.
See : "Why fire blight shouldn’t be a market access problem" (Dr C.N. Hale).
2.Minimising use of streptomycin by developing disease prediction programmes:
Funded by ENZA New Zealand (International) and Foundation for Research, Science and Technology and MAF Policy, computerised prediction programmes have been evaluated in New Zealand since 1991. The programmes can be used to identify infection risks and assist decision making in disease management.
See : "Computerised weather monitoring and disease predictive systems to predict fire blight outbreaks on pipfruit." (Dr S.C. Gouk)
3. Developing methods of biological control.
Biological control methods are being investigated as alternatives to chemical control with some promising results. This work is funded by ENZA New Zealand (International) and Foundation for Research, Science and Technology.
In the long term, breeding fire blight resistant varieties and rootstocks offers a realistic solution for non-chemical control.
See : " Pest and disease resistance in pipfruit rootstocks" V. Bus. The Orchardist. Oct 1994. Vol 67. No (9) : 57.
See : " Developing pest and disease resistant cultivars" (Dr V. Bus)
General control strategies
Some general pointers for control of fire blight (and other warm weather diseases) are given in the article:
"Pest and disease control pointers" J. Wilton. The Orchardist. Aug 1994. Vol 67. No (7) :17.
Further general information can be found in:
"Fire blight" ENZA Pipmark Technical Bulletin, September 1991 (reprint)