Fire blight

Written by E. Becker Revision 2026

• French disease name: Brûlure bactérienne
• Other disease names: Fireblight, twig blight of apple
• Pathogen name: Erwinia amylovora (Burrill, 1882) Winslow et al., 1920 emend. Hauben et al., 1998
• Kingdom: Bacteria
• Phylum: Proteobacteria
• Class: Gammaproteobacteria
• Order: Enterobacterales
• Family: Erwiniaceae

General information and importance

Erwinia amylovora, one of the most widely studied plant pathogens, is a Gram-negative enterobacterium historically significant as the first bacterium proven to cause disease in plants. Fire blight, caused by E. amylovora, is a destructive disease that severely affects commercial pome fruit production and native Rosaceae species.

During the past two decades, significant advances have been made in understanding E. amylovora and fire blight disease, particularly in genetic and genomic diversity, host-pathogen interactions, host resistance, and disease management.

Distribution and hosts

Erwinia amylovora affects members of the rose family (Rosaceae). Most of its natural hosts are in the apple subfamily (Maloideae), which includes apples (Malus) and pears (Pyrus). Some hosts are also found in the rose subfamily (Rosoideae), like strawberries (Fragaria) and raspberries (Rubus), and the stone fruit subfamily (Amygdaloideae), which includes cherries and plums (Prunus).    

Tree parts affected

Leaves, branches, bark, flowers, and fruits of susceptible hosts.

Symptoms and signs

Erwinia amylovora is a short, rod-shaped bacterium with rounded ends and numerous flagella surrounding its surface. Infection by E. amylovora affects leaves, shoots, branches, bark, blossoms, and fruits of apple, pear, and many other plants in the Rosaceae family. All symptoms appear above ground and are generally easy to recognize.

Infected blossoms wilt and turn brown abruptly in the spring. Symptoms on blossoms include a “water-soaked” appearance of the floral receptacle, ovary, and peduncles. The affected tissues develop a dull grey-green appearance 1 to 2 weeks after petal fall, eventually shrivelling and turning black. In high humidity, opaque white or amber droplets of bacterial ooze often appear on infected tissues. The bark of branches and stems turns reddish and appears water-soaked at the advancing edge of the infection, eventually cracking and turning black. The wood beneath the bark develops streaks of black discolouration. Infected twigs and leaves turn brown and appear scorched by fire, which gives the disease its common name. A distinctive shape of twigs with a hook on one end, often referred to as a “shepherd's crook,” may form when the tip of a shoot wilts. The leaves on affected shoots often blacken along their mid-veins.

Young, infected fruits appear watery or oily and exude droplets of clear, milky, or amber-coloured ooze. These fruits later become leathery and turn shades of brown or black, depending on the species. The shrivelled, leathery fruit typically remain attached to the tree.

Disease cycle

Fire blight typically spreads via bacteria that overwinter in holdover cankers on the main stem, branches, or infected twigs. In spring, as blossoms open, these cankers release bacterial ooze, which spreads via rain, heavy dew, or windblown mist to infect blossoms and young leaves. Fire blight can also be spread by pollinators like bees, sucking and chewing insects, boring insects, and contaminated pruning tools. Optimal conditions for infection and disease development include warm temperatures (24–28 °C) and high humidity.

The bacterium can enter a host tree through its blossoms, leaves, or stem wounds. In blossoms, bacteria multiply rapidly in the nectar and then enter the flower tissue. From there, they spread into nearby branches. Besides flowers, the bacterium can also enter through stomata and lenticels. Lesions from insect feeding or physical damage, such as cuts from infected pruning tools, are also highly susceptible to infection.

Once the bacterium enters the plant’s xylem or cortical parenchyma, it causes black, necrotic lesions that may exude a viscous bacterial ooze. This bacteria-laden exudate can spread to other parts of the same plant to cause secondary infections or infect new plants, through rain splash, birds, or insects.

Young branch tips can become infected through stomata, through wounds from pruning, insect damage, or hail, all common entry points. Bacterial ooze can appear on infected twigs within three days. Fruit may also be infected through insect wounds. Cankers eventually form from infections in branches. From the initial point of infection, the pathogen spreads through the tree’s vascular system, eventually reaching the roots or graft junction. Once the roots are affected, the tree or shrub often dies. Excessive pruning and fertilization, especially with nitrogen, can promote water sprouts (weakly attached, fast-growing vertical shoots from the trunk or branches; also referred to as epicormic shoots) and other midsummer growth, increasing the tree's susceptibility. Fire blight spreads rapidly during hot, wet weather but becomes dormant in winter when temperatures fall.

Damage

Fire blight ranks among the most destructive diseases affecting fruit trees in North America. The disease occurs sporadically and unpredictably, with occasional outbreaks reaching epidemic levels. Annual losses in fruit production due to fire blight are estimated to cost the United States about $100 million (there are no corresponding statistics for Canada). The ability of E. amylovora to form biofilms reduces the effectiveness of copper-based pesticides, underscoring the need for alternative disease management strategies.

Fire blight causes a range of injuries, including twig and branch dieback, cankers, and the blackening and wilting of leaves and fruit. Severe infections can result in significant growth loss, reduced fruit yield, and, in extreme cases, the death of the entire tree or shrub. The socio-economic impact of fire blight is substantial, affecting not only commercial fruit production but also ornamental trees in urban landscapes. Ecologically, the disease can reduce biodiversity in affected areas where Rosaceae species are dominant.

Prevention and management

To reduce risks of fire blight to ornamental trees, trees should be planted in well-drained soils with moderate pH (range, 5.5–6.5). Certain cultivars can be selected for resistance to fire blight. Additionally, avoid applying excessive nitrogen (such as some lawn fertilizers). If the tree becomes diseased, then infected wood should be removed before the disease spreads because the remainder of the tree that is unaffected can often be saved. Thus, infected twigs and branches should be pruned at least 30 centimetres beyond the last visible signs of E. amylovora infection. All gardening tools used for pruning should be disinfected to prevent further spread of the pathogen to other susceptible trees. Ensure that tree care specialists do the same if they are pruning your tree or trees.

In orchards, regular inspections of trees and shrubs are essential to detect new infections early. Although there is no cure for fire blight, its spread and impact can be reduced through integrated pest management approaches. These include:

• cultivar and rootstock selection for resistance to the disease;
• proper site selection;
• management of tree and overall orchard vigour;
• manipulating fruit load and vegetative growth of orchard trees;
• chemical thinning during bloom;
• managing the orchard microclimate;
• managing other insects and diseases in the orchard;
• careful and timely pruning;
• and applying registered pesticides where and when appropriate.

Further details on these various approaches can be found at Agriculture and Agri-Food Canada’s website on Integrated Management of Fire Blight on Apple and Pear in Canada.

Research into biological control agents began more than 100 years ago and continues in many countries, particularly in regions where fire blight causes significant economic losses. Several commercial products based on bacterial biocontrol agents have been developed to help manage fire blight.

Pest management strategies for a particular pest vary depending on several factors. These include:

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches; and
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

Pesticides registered for use against E. amylovora under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against the pathogen. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Photos

Showy mountain-ash showing symptoms of fire blight disease caused by Erwinia amylovora, including dried wilted leaves and twigs. 

André Carpentier

American mountain-ash showing symptoms of fire blight disease caused by Erwinia amylovora, including wilted twigs and shriveled fruit.

André Carpentier

American mountain-ash exhibiting fire blight symptoms caused by Erwinia amylovora, including trunk lesions. Wounds and lenticels on stems and branches can serve as entry points for the pathogen, which then spreads through the tree's vascular system.

André Carpentier

Selected references

Beer, S.V.; Norelli, J.L. 1977. Fire blight epidemiology: factors affecting release of Erwinia amylovora by cankers. Phytopathology 67(11): 1119–1125. https://doi.org/10.1094/phyto-67-1119

Holtappels, M.; Noben, J.P.; Van Dijck, P.; Valcke, R. 2018. Fire blight host-pathogen interaction: proteome profiles of Erwinia amylovora infecting apple rootstocks. Scientific Reports 8(1): 11689. https://doi.org/10.1038/s41598-018-30064-x

Johnson, K.B. 2000. Fire blight of apple and pear. The Plant Health Instructor. 2015: 43–69. https://doi.org/10.1094/PHI-I-2000-0726-01

Malnoy, M.; Martens, S.; Norelli, J.L.; Barny, M.-A.; Sundin, G.W.; Smits, T.H.M.; Duffy, B. 2012. Fire blight: applied genomic insights of the pathogen and host. Annual Review of Phytopathology 50(1): 475–494. https://doi.org/10.1146/annurev-phyto-081211-172931

Norelli, J.L.; Jones, A.L.; Aldwinckle, H.S. 2003. Fire blight management in the twenty-first century: using new technologies that enhance host resistance in apple. Plant Disease 87(7): 756–765. https://doi.org/10.1094/PDIS.2003.87.7.756

Puławska, J.; Mikiciński, A.; Sobiczewski, P. 2024. The history of fire blight biocontrol with Gram-negative bacteria and bacteriophages. Journal of Plant Pathology 106(3): 839–851. https://doi.org/10.1007/s42161-023-01554-3

Rezzonico, F.; Emeriewen, O.F.; Zeng, Q.; Peil, A.; Smits, T.H.M.; Sundin, G.W. 2024. Burning questions for fire blight research: I. Genomics and evolution of Erwinia amylovora and analyses of host-pathogen interactions. Journal of Plant Pathology 106(3): 797–810. https://doi.org/10.1007/s42161-023-01581-0

Thomson, S.V. 1986. The role of the stigma in fire blight infections. Phytopathology 76(5): 476–482. https://www.apsnet.org/publications/phytopathology/backissues/Documents/1986Articles/Phyto76n05_476.PDF

Van Der Zwet, T.; Keil, H.L. 1979. Fire blight: a bacterial disease of rosaceous plants. United States Department of Agriculture. Agriculture Handbook Number 510. Washington, D.C., United States. 200 p.

Vanneste, J. (2008). Erwinia amylovora (fireblight). CABI Compendium. https://doi.org/10.1079/cabicompendium.21908

Wall, R.E. 1983. Fire blight of wild raspberry on clear-cut forest areas. Canadian Forestry Service, Maritimes Forest Research Centre. Fredericton, New Brunswick. CFS Research Notes. 3: 2–3.

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