Pests | BC Tree Fruit Production Guide

Fire Blight (Erwinia amylovora)

General Description

Introduction

Fire blight is a destructive bacterial disease of apples and pears. It can affect most parts of the tree – blossoms, shoots, limbs, rootstock and fruit. Outbreaks of fire blight occur annually in pear and apple orchards in British Columbia (B.C.).

Causal Agent

Fire blight is caused by Erwinia amylovora, a gram-negative, rod-shaped bacterium. The bacterium grows by multiplying its cells and the rate of multiplication (i.e. bacterial growth) is regulated by temperature. Bacterial growth is minimal below 10°C, and slow at temperatures between 10 to 18°C. At temperatures above 18°C, the rate of bacterial growth increases rapidly and peaks at 27°C. Above 35°C, the bacterial number can decline.

Symptoms

Blossoms

Blighted blossoms appear wilted, shriveled and brown (Figure 1a). Blossoms will remain attached to the tree for most of the season.

Fruit

Young fruitlets appear water soaked and slightly off-coloured after infection. Fruitlets quickly turn brown to black and eventually shrivel up but often remain attached to the spur.

Shoots

Blighted pear shoots are black in colour, while infected apple shoots are usually a lighter shade of brown. Infected shoots (or "strikes") wilt rapidly and often form a shepherd's crook (Figure 1b) at the tips. During warm, humid or rainy weather, drops of milky to amber coloured bacterial ooze frequently appear on the blighted shoots (Figure 1c) and fruit. Blighted leaves may remain attached to the tree throughout the winter. When shoots on scaffold limbs or trunks are attacked, the pathogen may spread into the structural wood causing cankers.

Limbs/Trunk

Cankers appear as slightly darker, water-soaked areas in the wood (Figure 1a & b), which may produce amber-coloured bacterial ooze that runs down the bark. Reddish brown streaks may be seen in the cambium under the bark of diseased branches. Later in the season, the bark often cracks around the margins of the canker.

Rootstock

Fire blight may also spread below the graft union and into the rootstock, leading to tree death. Spread may occur from internal movement of the bacterium in the trunk or from root sucker infection. Rootstock may not exhibit the typical fire blight symptoms. It is sometimes mistaken for crown rot due to brown discoloured tissues under the bark at the base of the tree (Figure 2c). Oozing bark may be observed near the base of the tree.

Host Range

Apple and pear are agricultural crops of primary concern. Erwinia amylovora has a wide host range in landscape plants with reports on about 200 species including crab apple, hawthorn, mountain ash and Bradford pear.

Life Cycle

Fire blight bacterium overwinters primarily in cankers on infected trees. During spring, cankers that were not pruned out may produce bacterial ooze, which may or may not be visible. This ooze, consisting of millions of bacterial cells, is easily disseminated to blossoms by wind, rain and insects such as flies, ants, and beetles. If weather conditions are warm and humid, the bacterium multiplies rapidly on the open blossom. The pathogen can then be spread very efficiently from blossom to blossom by pollinators. When there is a wetting event (rain, dew, overhead irrigation), if the number of bacterial cells on the stigma is great enough, there will be an infection through the floral cup.

Once blossoms are infected, the pathogen can quickly spread into shoots followed by branches. Blossom infections become visible as "strikes" or dying shoots anywhere from 1 to 4 weeks after infection, depending on the temperature.

Infected shoots provide additional sources of fire blight bacterium, which can be spread by rain (especially wind-blown rain) and insects. Secondary infections may continue to occur throughout the growing season. It is possible for twig or shoot infection to occur in orchards where little or no blossom infection was found. The bacterium can enter the host through both wounds and natural openings such as lenticels. Hailstorms often result in severe fire blight outbreaks if the bacterial inoculum is present in an orchard.

Management

Cultural Control

Dormant pruning:

Overwintering cankers should be cut out during the dormant season. Fire blight cankers have either smooth or cracked margins. Both types of cankers should be removed. Active cankers may enlarge in the spring causing further structural damage. They also provide inoculum for new infections.

One or more separate passes through the orchard to prune out cankers are recommended. Since cankers may be hard to locate, it is always best to inspect the orchard several times. Make cuts 15-30 cm below the canker margins.

Summer sanitation:

Remove blossoms on non-bearing trees in young blocks. Physical removal of rattail or secondary blossoms would greatly reduce the risk of fire blight infection. Although labour intensive, this practice is highly recommended, especially on young trees that are severely impacted by infection.

Scout for new fire blight strikes every week. Frequent scouting will aid removal of new infections before they have a chance to invade the structural wood or spread disease.

During dry conditions, remove current season infections as soon as possible. Prune out strikes and cankers at least 30 cm below the visibly diseased part. This is necessary as bacteria are active beyond the discoloured area as they move down the branch. Disinfect pruners between cuttings, especially if cuts are not made the recommended distance away from the diseased tissue (over 30cm). Prunings should be removed from the orchard and buried or burned, when possible. Where infections occur on shoots attached to scaffold limbs or the trunk, it is not always possible to cut back 30 cm without sacrificing the limb or even the tree. Trees should be removed if there are cankers on the main leader.

During severe epidemics, give priority to young trees and high density plantings. Concentrate on salvaging as much of the tree structure and bearing surface as possible while ensuring pruning cuts are made 30cm below infected tissue. Excessive pruning during the summer will encourage a late flush of growth, which may make the tree more susceptible to continued infections.

Summer pruning (other than removal of strikes) should be avoided during a severe outbreak, due to the danger of spreading the disease. Avoid pruning during wet weather or when storms are expected within the next 24 hours.

Root suckers and rootstock sprouts may put the entire tree at risk if they become infected. Common dwarfing rootstocks such as M9 and M26 are highly susceptible to blight. Do not cut root suckers or rootstock sprouts during a blight outbreak, because the wounds may become infected. They may be safely removed during the dormant season.

Varietal Resistance:

Varieties rated as susceptible are more likely to have fire blight. See below for susceptibility ratings of apple and pear varieties. You can minimize the chances of introducing fire blight into your orchard by selecting nursery stock from a reliable source. Ask your supplier if they have had fire blight problems in the nursery.

Varieties which have a late or prolonged blossom period are more likely to have bloom on the tree when the weather warms up and are, therefore, more prone to blossom blight. Even relatively resistant trees, such as Red Delicious, may get fire blight after a hailstorm.

Table 2. Relative resistance ratings of commercial pome fruit against fire blight

	Apple	Apple rootstock
Resistant	Red Delicious, Dayton, Early MacIntosh, Empire, Enterprise, Freedom, Liberty, Novamac, Pristine	MM.111, all Geneva series
Tolerant	Crimson Crisp, Goldrush, Honeycrisp	M.27, M.7
Susceptible	Ambrosia, Braeburn, Cameo, Cortland, Golden Delicious, Fuji, Gala, Ginger Gold, Gravenstein, Idared, Jerseymac, Jonagold, Jonamac, Lodi, Macoun, McIntosh, Mutsu, Northern Spy, Paulared, Pink Lady, Silken, Stayman, Wealthy, Zestar!	M.9, M.26, B.9, MM.106, Ottawa 3
	Pear	Pear rootstock
Resistant		Old Home (OH), Old Home x Farmingdale (except OHF 51)
Susceptible	Anjou, Bartlett, Bosc, Cascade, Flemish Beauty, Starkrimson	Bartlett seedling, quince
	Asian pear
Resistant	Seuri, Shinko, Singo
Tolerant	Kosui, Chojoro, Shinsui
Susceptible	Hosui, Shinseiki, 20th Century
	Crabapple
Resistant	Dolgo
Susceptible	Manchurian, Snowdrift

Chemical Control

Products labelled for fire blight control in Canada include antibiotics, several biopesticides, growth regulator, and copper-based chemicals. These products are only effective when used preventatively. There are no treatments that can cure fire blight after symptoms are present.

Timing of fire blight sprays is critical. Blossoms that are rapidly opening during a warm spring pose high risk of infection as they are not protected. Additionally, secondary or rattail bloom that occurs when conditions are warm and wet will pose a large risk.

Table 1. Products Registered for fire blight control or suppression in Canada (the pesticide label is the legal document and must be consulted before use).

Product	FRAC Group, type	Crops	Timing	Notes
Kasumin 2L (kasugamycin)	Group 24, antibiotic	all pome fruit	open blossom only	Preventative. Provides 2-3 days of protection. Maximum 4 applications/ season.
Streptomycin 17 (streptomycin)	Group 25, antibiotic	apple, pear	open blossom only	Preventative. Provides 2-3 days of protection. Maximum 3 applications/ season.
Blossom Protect (Aureobasidium pullulans)	Not classified (NC), biopesticide	all pome fruit	open blossom only	Preventative. Apply up to 4 times at 10%, 40%, 70% and 90% open blossoms, or 5 times when model indicates risk of infection. Do not tank mix with fungicides. Russeting may occur on sensitive cultivars. Do not apply more than 2 times on Golden Delicious, Idared and Jonagold or to control rat-tail blossom.
Serenade Opti (Bacillus subtilis strain QST 713) /	Group BM02, biopesticide	all pome fruit	open blossom only	Preventative. Suppression only. Apply at early (1-5%) bloom. Repeat at 4-7 day intervals during high risk periods during bloom.
Double Nickel (Bacillus amyloliquefaciens strain D-747)	Group BM02, biopesticide	All pome fruit	blossom and post-bloom	Preventative. Suppression only. Compatible with copper. Can be applied post-bloom for shoot blight management.
LifeGard WG (Bacillus mycoides)	Group P06, Plant defense activator	apple	Post-bloom	For suppression of shoot blight, start applications after petal fall and re-apply every 7 to 14 days during stem elongation. Do not apply during the bloom period (from pink stage to petal fall).
Apogee / Kudos (prohexadione calcium)	NC, growth regulator	apple	2.5 – 7.5 cm of new shoot growth	Suppresses fire blight by decreasing host susceptibility. Apply at 2.5-7.5 cm shoot growth; repeat at 14-21 day intervals up to 4 times/season. Will also reduce shoot growth. Do not use on pear.
Cueva Commercial (copper octanoate)	Group M01, fungicide/ bactericide	all pome fruit	Dormant, bloom and in-season	Preventative. Less likely to cause russet injury than other coppers. Use highest rate for dormant application.
Copper Spray Fungicide (copper oxychloride)	Group M01, fungicide/ bactericide	apple, pear	silvertip, post-harvest	May cause fruit russet. Dormant copper applications have been shown to reduce or delay the production of inoculum in overwintering cankers.
Parasol Flowable (copper hydroxide)	Group M01, fungicide/ bactericide	Apple, pear	Silvertip and after harvest with 50% leaf drop	Apply as a dormant application in sufficient water for complete coverage. Use two applications per year.
Buran (garlic powder)	NC, biofungicide	All pome fruit	open blossom	Suppression only at blossom blight stage. Do not tank mix with streptomycin. Use of a non-ionic surfactant at a rate of 0.1% could help improve coverage.
Cyclone Plus (Citric Acid + Lactic Acid)	NC, biofungicide	Apple	Not specified	Suppression only. Apply before wetting events when conditions are conducive to infection. Do not mix with alkaline products
Oxidate 2.0 (hydrogen peroxide + peroxyacetic acid)	NC, sterilant / general biocide	Apple, pear	Not specified	Partial suppression only. Does not have activity on infected blossoms or shoots, helps prevent spread of bacteria to other susceptible tissue.

Resistance Management:

Both kasugamycin and streptomycin are at high risk of resistance development in the pathogen and other non-target bacteria. In the late 1990s, there were concerns of fire blight resistant populations of the pathogen to streptomycin. In a study conducted by Agriculture and Agri-Food Canada in Summerland, BC (Survey of Erwinia amylovora isolates from British Columbia for resistance to bactericides and virule (tandfonline.com), a high number of streptomycin resistant isolates (46%) of Erwinia amylovora were detected. Since this time there have been no further studies in BC. In Washington, USA, a recent survey of antibiotic resistance showed that tested populations of Erwinia amylovora were not resistant to streptomycin (Fire Blight of Apple and Pear | WSU Tree Fruit | Washington State University). Using resistance management strategies including product rotation and limited use of antibiotics per season for fire blight management has been successful in maintaining the effectiveness of streptomycin.

Forecasting Fire blight:

There are several models available which can be used to forecast fire blight blossom infection based on the weather. They can be used to determine whether sprays are needed to protect open blossoms.

The CougarBlight model was developed by Dr. Tim Smith, Washington State University to predict blossom infections. CougarBlight is integrated into the BC Decision Aid System (BCDAS). To use BCDAS, set up a free account, select a weather station near your orchard, and add the fire blight model. BCDAS Spray recommendations are linked to the BC Tree Fruit Production Guide.

Summary

Fire blight poses a serious threat to new and established orchards of apple and pear. Successful fire blight control requires diligent management. The following summary lists the major management practices used to control fire blight:

Dormant

Remove cankers and dead branches
Delayed dormant copper spray

Bloom

Monitor temperature during bloom
Apply preventative sprays during bloom if blight-favourable conditions are forecasted in BCDAS
Consider Apogee for shoot blight suppression in blocks at risk of fire blight. Must be applied early.

Summer

Scout for and remove strikes at regular intervals
Disinfect tools between pruning
Use a products with fire blight control or suppression if wounds are created on the tree and inoculum is present (ex. After a hailstorm)

Post harvest

Apply copper sprays if fire blight is active
Continue to remove new infections

References & Additional Resources

Prepared by the Plant Health Unit, Plant and Animal Health Branch, B.C. Ministry of Agriculture and Food.

Updated March 2025