Fire Blight Disease and Treatment

Fire blight is a bacterial disease that gives the plants a scorched appearance. Although the disease mainly affects plants in the rose family, it can also destroy other plant species. Apples and pears are among the common fruits affected by this disease.

Fire blight prevention requires plant caregivers and growers to be knowledgeable about fire blight disease and treatment. What does fire blight look like? How to prevent it? Below, we answer all these questions.

Plant pathogens are organisms that cause diseases in plants. Bacterial, fungi, insects, and viruses are some of the most common plant pathogens. Bacteria are responsible for causing diseases like bacterial leaf spot, crown gall, bacterial wilt, blackleg, bacterial canker, and fire blight.

Fire blight is a bacterial plant disease commonly affecting apple and pear trees. The bacterium Erwinia amylovora causes this disease by infecting the plant through open flowers.

It produces a toxin that causes the leaves and stems to blacken and die, giving them a scorched appearance. Fire blight disease kills branches, twigs, shoots, fruits, and blossoms on most trees. In some cases, it might kill the whole infected tree.

Young trees can generally die in a season. Meanwhile, older trees have a better survival rate. They can live for several years despite continuous dieback.

The fire blight bacteria first makes its appearance in spring. When the temperature exceeds 65 degrees F and heavy dews or rains start, fire blight bacteria infects.

Since fire blight bacteria need specific environmental conditions to infect and spread, their incidence can vary from year to year, depending on that year's weather situation.

Fire blight bacteria overwinter in the living tissues on the tree trunk as cankers. They also form cankers on mummified fruit and the main branches.

The primary infection starts during the bloom period. During this period, the bacteria travel from fire blight cankers to open flowers through pollinating insects and splashing rain.

Only a few cankers that overwinter become active during spring to produce bacteria. But even if fire blight canker activates, it can create millions of bacteria. Together, these bacteria can infect a whole apple orchard.

These bacteria use the blossom nectar to multiply at a fast rate. Then, they spread to twigs bearing blossoms, also called spurs. The bacteria also travel to new shoots and branches, causing secondary infections.

Sucking insects, like leafhoppers and aphids, may also cause plant wounds through which shoot infections occur. Besides, frost damage, wind-driven rain, hail, and wind whipping can also aid the spread of the bacteria.

After a shoot gets infected, the bacteria spread rapidly and show symptoms in as little as three days.

Appearance

Fire blight makes it seem like the plant is burnt by fire and is about to die. But apart from this, cankers also develop.



These are slightly sunken lesions in the plant trunk that have a cracked and sharp margin. The lesions can range in color from purple to dark brown.

Causes

Although the bacteria, Erwinia amylovora, is the causative agent of fire blight disease, other factors, such as weather conditions and management practices, also contribute to its occurrence. Here are some causes of this plant pathology:

• Humid Conditions: The bacteria causing fire blight thrive in humid and wet conditions. Erwinia amylovora prefers warm and humid weather as it facilitates the bacteria's growth and reproduction. Since these conditions usually arise during the trees' bloom periods, plants become more susceptible to fire blight when they flower.
• Plant Varieties: Some plant varieties are more susceptible to fire blight than others. For example, some varieties of apples, like Honeycrisp and Gala, are more likely to be affected by fire blight than others.
• Insects: Insects like wasps and grasshoppers can assist in the spread of disease. They can either carry the bacteria from infected branches to healthy trees. Or, they may create wound openings in the tree for the bacteria to enter through.
• Pruning Practices: Using improper pruning techniques is one of the main causes of fire blight diseases. For example, if you cut too close to the trunk or leave stubs behind, you're inviting the fire blight bacterium. These cutting practices induce entry points in trees, which become passageways for bacteria. Pruning a tree during wet conditions or overfertilizing it with nitrogen can also facilitate fire blight disease occurrence.
• Contaminated Tools: Pruning shears, knives, saws, and other garden tools can also spread fire blight bacteria. If you use these tools on infected branches and use them on a healthy tree without disinfecting it, you risk transmitting the bacteria. The same goes for using clippers on healthy and diseased trees.
• Infected Plant Material: Fire blight may also spread to healthy trees via infected plant material. These include grafts and cuttings from infected trees.

Fire blight symptoms differ, depending on the part of the plant. Here are the symptoms of fire blight for different tree parts.

Flowers

• Flowers or their clusters initially appear as if they are soaked in water. They then droop and shrivel with time, turning black or brown.
• Black and brown flowers remain attached to the tree throughout the growing season.

Leaves

• Young shoots and leaves turn gray-green. They bend forward and create a shepherd's crook shape.
• Brown lesions may also appear on the leaves, which subsequently curl and turn black or brown in color.
• If multiple shoots are infected on a tree, the tree might look burnt by fire.
• Brown leaves stay attached to three even during winter and hang downward.

Fruits

• Fruits shrivel to form a mummy. It is a hard, dried fruit with a blackish-brown outer layer.
• The shriveled and dark fruit stays attached to the tree for a few months.

Bark

• The bark appears dark and sunken. It might seem like it is peeling or cracking.
• When the bark peels, it shows brain sapwood stains.
• Light-yellow colored bacterial ooze droplets cover the infected shoots and branches. The bacterial ooze may also be found on the fruits after it rains or during humid weather.

Trees Affected

Fire blight affects the following trees:

• Apple
• Blackberry
• Cotoneaster
• Crabapple
• Hawthorn
• Mountain Ash
• Pyracantha
• Quince
• Raspberry
• Spirea



In the Rosaceae family, the disease affects the following plants:

• Amelanchier (serviceberry)
• Chaenomeles (flowering quince)
• Cotoneaster (cotoneaster)
• Crataegus (hawthorn)
• Eriobotrya (loquat)
• Malus (apple and crabapple)
• Prunus (flowering almond, plum, and cherry)
• Pyracantha (pyracantha)
• Pyrus (pear)
• Rosa (rose)
• Photinia (photinia)
• Spirea (spirea)

Fire blight treatment can be quite difficult since the infected tissue starts dying quickly. So, it's better to know how to prevent fire blight since that is the only way to save young trees. Here are some methods to prevent or manage disease development.

Tolerant Varieties

The simplest way of fire blight prevention is to plant resistant varieties. These trees have a natural resistance to the bacteria, reducing their chance of infection.

The following apple and crabapple varieties are most resistant to fire blight:

• Haralson
• Liberty
• SnowSweet
• Chestnut crab
• Dolgo crab
• Red Baron

These varieties are somewhat resistant:

• Honeycrisp 
• Zestar

The following varieties are less resistant but can be planted if you follow proper pruning techniques to prevent fire blight infections.

• Cortland
• Keepsake
• McIntosh
• Sweet Sixteen
• Centennial crab
• Whitney crab
• State Fair

If you live in an area with a history of fire blight infections, you should avoid planting the following apple varieties. These are the least resistant to fire blight infection.

• Beacon
• Fireside
• Honeygold
• Paulared
• Wealthy

Here are some other plant species and their tolerant varieties.

Ornamental Crabapple

Ornamental crabapple belongs to the Malus genus and is more resistant to fire blight than apples. The following ornamental varieties are the most resistant:

• Adams
• Adirondack
• Tina
• Dolgo
• David
• Camelot
• Lancelot

Meanwhile, the following are somewhat resistant:

• Candymint
• Pink Princess
• Red Splendor
• Silverdrift

Avoid planting the following varieties as they are the least resistant:

• Madonna
• Sinai Fire
• Snowdrift

Some varieties, such as Red Jade, Spring Snow, and Purple Prince, are less resistant than Candymint and Silverdrift. But they can survive in cool climates with a lower risk of fire blight infection.

Hawthorn

Downy Hawthorn (C. mollis) is the most resistant hawthorn variety. But most other varieties of the Crataegus genus, such as Washington hawthorn (C. phaenopyrum), are also relatively resistant to fire blight infection.

Mountain Ash

There's insufficient research on mountain ashes to understand the genetic variations within this species. 

Some varieties may be more susceptible to fire blight in some conditions than others. But European mountain ash (S. aucuparia) is moderately resistant in most instances.

Prevention

One of the best ways of fire blight control is pruning the infected branches. You should prune the infected shoots in late winter when the bacteria and the tree are dormant. 

It helps reduce the sucker growth from wounds resulting from pruning. So, there's a lower risk of disease development through bacterial spread between pruning cuts.

Here's how to prune the infected branches to limit fire blight damage:

• Prune a healthy branch at 8 inches below the canker's discolored bark.
• If fire blight has caused significant damage to a cotoneaster hedge, cut up to six inches off the hedge above the ground. Make sure you do it in winter. But if the disease has only spread to a few stems, remove them as mentioned in the step above.
• If you must prune during the growing season, sterilize your pruning tools before every cut. Use a disinfectant to sterilize and spray it on the cutting blade.
• Another way to disinfect pruning tools is to submerge the cutting blade in a disinfectant solution for 60 seconds between cuts.
• 10% bleach solution is a good disinfectant. Make this solution by mixing nine parts of water and one part of bleach in a spray bottle. Or you can make one with undiluted Lyssol.
• After cutting the infected shoots, bury or burn them.

If the disease has progressed to the tree's main trunk, it can no longer be cured. The tree will now die. You should remove the whole tree, including the stump when it dies. Doing this will remove the source of bacteria from the area, protecting other plants from a similar fate.

Avoid Overfertilizing 

Overfertilizing a tree with a nitrogen-rich fertilizer increases the risk of fire blight. Excessive nitrogen promotes the growth of new succulent shoots that can be highly vulnerable to infection. If you're unsure about the right type or amount of fertilizer to use, consult an arborist.

You can prevent overfertilization by doing the following:

• Soil Testing: Before you apply fertilizer, test the soil to determine the tree's needs. Soil testing will help you measure the right amount of fertilizer for each tree.
• Timing: Trees are most susceptible to fire blight infection in early summer and spring. Do not apply fertilizer during this period.
• Balanced Fertilizer: Buy a balanced fertilizer that only provides the nutrients the plant needs. It should not overstimulate the plant's growth.

Chemicals

If you're wondering how to get rid of fire blight chemically, you should know that pesticides are not always the fix. Only use them if fire blight is a continuous menace in your garden.

You can use three types of pesticides against this destructive bacterial disease.

• Copper-based pesticide
• Streptomycin sulfate 
• Serenade Garden Defense (Bacillus subtillis)

Copper blossom sprays are usually applied during the first signs of flowering in the plant. But they can damage the tree's fruits and have limited effectiveness.

Streptomycin sprays are also helpful in preventing new fire blight infections. But they can result in antibiotic-resistant outbreaks. Basically, if you keep applying Sprep sprays, the fire blight bacterium will become resistant over time.

The next time it attacks the tree, the spray will no longer be effective. Overusing these sprays results in creating resistant bacterial varieties that can be even harder to manage.

Here are a few things to keep in mind about pesticides for fire blight:

• They will not help protect the tree against branch cankers.
• Apply these pesticides below or in the early bloom season to protect new shoots.
• If there has been a hail storm and the area is known to be prone to fire blight, apply these pesticides to susceptible trees. Do note that they will only help against wound infections if used right after the hail storm.

It's best to hire professional arborists to apply pesticides on susceptible trees. These professionals can identify the exact site of infection and recommend the right course of action.

Sweet New Earth's Final Word

Thanks for stopping by! If you enjoyed this article, be sure to check out some of our others on tree diseases.

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