Fig. 4. Fire blight canker.
Photo: G. Sundin.
Fig. 5. Leaf and shoot dieback on a tree infected with fire blight.
Photo: G. Sundin.
Fig. 6. Shepherds crook caused by fire blight.
Photo: G. Sundin.
Fire blight is a bacterial disease of apple trees that can kill blossoms, new green shoots, branches, and sometimes entire trees. In Minnesota, fire blight is rarely a problem on blossoms due to cool temperatures during bloom. Infection of young green shoots occurs occasionally when warm wet weather coincides with new growth on susceptible varieties. If left untreated, infections can move into the main trunk and roots of the tree, resulting in death of the tree. However, several management strategies are available eradicate fire blight from infected trees before it becomes deadly.
Infected flower blossoms first appear green gray, but quickly turn black and appear scorched. Blighted blossoms remain attached to the tree through the season and often into winter. Blossom blight is rare in Minnesota due to cool temperatures at bloom time.
If fruit are infected through the flower or twig, they remain small and are often dark colored and shriveled. Fruit infected through insect or hail damage may have reddish brown to black spots.
Young growing shoots that are infected turn green gray and bend over forming a ‘shepherd’s crook’. Infected leaves start out green gray, then turn brown around the edges and eventually turn completely brown and wilted. Blighted leaves remain attached to the tree throughout the season and into winter. If many branches of the tree have been infected, it may appear to have been scorched by fire, giving the disease its name.
Infected branches and twigs are often darker in color and the bark may appear sunken and cracked. If the bark is cut away from the infected branch reddish brown discoloration can be seen on the inner wood. Branch cankers can be avoided by pruning out infections that occur on young shoots.
On a warm wet day, cream or honey colored sticky droplets of liquid, known as bacterial ooze, arise from all infected plant parts, including fruit, flowers, leaves, and branches. Severely infected branches may release so much bacterial ooze, that liquid can be seen running down the tree.
Fire blight differs from other diseases of apple in that it is caused by bacteria, not fungi.
The fire blight bacteria infect plants within the Rosaceae family. In addition to apples and crab apples, this family includes mountain ash (Sorbus), Hawthorn (Crataegus), Serviceberry (Amelanchier), and brambles like raspberry and blackberry (Rubus). The fire blight bacteria survive Minnesota’s winters inside infected branches. In the spring when the weather is wet and warm (above 60F), bacteria begin to multiply and ooze out of infected branches in a yellow white sticky liquid known as bacterial ooze.
Insects are attracted to this sweet, sticky ooze from infected tissue, either in your planting or nearby. Since the ooze contains bacterial cells, even if a microscopic amount sticks to an insect, the bacteria can be transferred to apple flowers that the insect later visits. Infection of flowers is called "blossom blight.”
The bacteria can also be spread on moist air currents. Airborne fire blight bacteria can infect young green tissue, particularly after a storm has whipped the branches against each other, or after hail has injured soft tissue. This type of infection is called "shoot blight.”
Regardless of if the infection starts in a flower or green shoot, it moves from these tissues into the branch. The infection will continue down the branch, killing attached leaves as it moves closer to the main trunk. If the infection travels to the main trunk and the roots of the tree, the tree will die. Some cultivars of apple are able to wall off the infection and prevent the spread of the bacteria into the rest of the tree; others are susceptible to the infection and can be killed once the bacteria infect the trunk all the way around or enter the roots.
In the yard and garden, fire blight can easily be controlled with several cultural control practices if they are used in a timely manner.
Plant resistant varieties
Start out with a resistant variety of apple tree. It is important to know that most apple trees are two different plants grafted together. The scion is the top part of the tree. The variety name you see on the plant label typically describes the scion. The rootstock is the underground part of the tree. Ideally, both scion and rootstock would have resistance to fire blight.
Among rootstocks typically available to home growers, M7 is somewhat tolerant to fire blight, while M26 and M9 rootstocks are more susceptible. Newer rootstock cultivars have been selected specifically for fire blight resistance, but are not as widely available to home growers yet. Bud 9 is one example that has moderate to good fire blight tolerance. If you are planning a new planting, it may be worthwhile to seek out nurseries that offer trees on fire blight-resistant rootstocks.
Fire blight susceptibility of apple varieties grown in Minnesota
|Susceptible||Moderately susceptible||Moderately resistant||Resistant|
|Wealthy||Sweet Sixteen||Chestnut Crab|
|Centennial Crab||Dolgo Crab|
Trees that are particularly lush and vigorous are very susceptible to fire blight because soft, succulent shoots are easily wounded and thus easily infected. Trees that exhibit more modest growth (12 inches or less per year) are not as susceptible. If you will be planting apple trees in rich soil, in a part of Minnesota that gets plenty of rain, or if your tree will be irrigated and fertilized as part of a lawn, choose a less vigorous rootstock. On sandy or poor soils, or in sites that are dry, a more vigorous rootstock will be necessary, but probably will not lead to the kind of lush green growth that invites fire blight. Consult the University of Minnesota Extension publication Apples and pears in Minnesota home gardens to choose a rootstock right for your area.
Maintain tree health through proper pruning and fertilizer use
Application of excessive nitrogen fertilizer can result in a flush of new growth that will be very susceptible to fire blight. To calculate the amount of nitrogen needed by the tree, submit a soil sample to the University of Minnesota soil testing lab or follow the calculations described in the University of Minnesota Extension publication Apples and pears in Minnesota home gardens. Remember tree roots extend into the soil up to 2 times beyond the tree canopy. Lawn fertilizer applied to this area will also be taken up by the apple tree.
Bacteria thrive and multiply best in warm humid environments. Healthy trees should be pruned to maintain an open canopy that allows air to reach all leave so that they dry quickly after rain or dew. For proper pruning of apples see Apples and pears in Minnesota home gardens. In addition to proper pruning in the main canopy, all water sprouts, or suckers (small twigs growing directly from the main trunk) should be pruned off.
Pruning out infected branches
If fire blight is present in an apple tree, all infected branches must be removed and destroyed. This will stop the infection from moving further into the tree, and will prevent the bacteria from spreading to other trees. The best time of year to do this is in February or March, when the tree and the bacteria are dormant. Infected branches should be pruned back eight inches closer to the trunk than the apparent infection. Between cuts, disinfect pruning tools with a 10% bleach solution or an anti-bacterial cleaner such as Lysol or Listerine, to avoid spreading the disease. Burn cuttings or include them in your trash.
New shoots infected during the growing season can be removed following the same procedure. These cuts are best made during cool dry weather. Always remember to sterilize pruning tools between cuts!
In cases of serious infection, when the bacteria have entered the trunk of the tree, it’s best to remove the tree entirely, including the stump. Infected tissue allowed to remain in the planting is a source of bacteria to infect the rest of your trees. Once the main trunk is infected, trees cannot be cured of the disease.
Pesticides are typically not necessary to control fire blight in Minnesota. In trees where fire blight has been a problem in past years, a dormant spray of a copper-containing product can be applied to reduce new infections. Copper should be sprayed in spring between silver tip (when buds are just beginning to swell) and ½ inch green tip (when buds have opened and ½ inch of green leaf tissue is visible). Spray coverage should be more than just a mist, leaving a visible residue on the branches. This coating will kill the fire blight bacteria as they emerge from branch cankers and stop them from starting new infections. Copper sprayed later in the spring can be very harmful to foliage and fruit, so do not spray copper after ½ inch green tip.
Although several sprays exist that can be used to stop the fire blight bacteria from starting a new infection, these sprays are unnecessary in trees that do not have a history of fire blight infection. In Minnesota, sprays are only necessary where fire blight has been a problem in the past, susceptible cultivars are being grown and weather is wet and warm (above 60° F) during bloom. Sprays are only applied during blossom.
Serenade Garden Defense (Bacillus subtillis) is a biological control product containing an antibiotic producing bacteria. This product has been show to reduce infections on apple blossoms if applied to healthy but at risk blossoms.
Streptomycin sulfate is an antibiotic that can prevent fire blight infections if it is sprayed on flowers or shoots before the bacteria infect. Although this product is available to gardeners, it is not recommended due to problems with the bacteria becoming resistant to the effect of the antibiotic if it is improperly used.
The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Minnesota Extension is implied.