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10 Apple Tree Diseases: How To Identify and Treat Them

Keeping apple trees free of apple tree diseases is a basic part of proper care. There are many diseases that can affect apple trees. These vary from fungal to bacterial infections. Some only cause superficial damage, while others can be devastating, killing off your apple trees and leading to a 90 percent or more loss of fruit.

In this article, we’ll talk about the most common apple tree diseases and detail how to recognize and treat them, ensuring that you have all the information you need to keep your garden healthy!

Apple Scab 

Closeup of two small apples with black pits caused by apple scab, a common apple tree disease.
Apple scab, a common apple tree disease.

Apple scab is one of the most common diseases affecting apple trees. It’s vital to inspect your tree regularly and learn to recognize the signs of apple scab. 

What Is Apple Scab?

Apple scab is caused by fungus that leads to infected leaves and fruit. This bacteria usually thrives in cold, moist climates, so you’re more likely to see it if you live in the north. 

How To Identify Apple Scab

Apple scab causes black lesions on both the leaves and the fruit of the apple tree. If you notice odd, dark splotches on your tree, it may be infected with apple scab. The lesions might sometimes look olive green. In bad cases, the fruit can eventually turn brown, and the leaves may turn yellow and fall off prematurely.

How To Treat Apple Scab

Once you have determined that your apple tree is infected with scab, you need to begin a rigorous treatment regimen right away. This includes a few important steps:

  • First, clear all the fallen leaves from around your apple tree. This reduces the risk of infected fallen leaves spreading spores to the tree when spring arrives again. 
  • Make sure you keep your apple tree pruned. This reduces the foliage available to spread the fungus and encourages healthy circulation through the branches. 
  • When spring arrives, apply a commercial fungicide to your apple tree every two weeks as soon as new shoots appear.

Sooty Blotch and Flyspeck

Closeup of apples showing brown blotches caused by sooty blotch and flyspeck,  an apple tree disease.
Apples affected by sooty blotch and flyspeck.

Sooty blotch and flyspeck tend to attack apple trees in the late summer and early autumn. The good news is that sooty blotch and flyspeck are superficial, only affecting the appearance of the apple tree. However, they may also shorten the apples’ shelf life. 

What Is Sooty Blotch And Flyspeck? 

These two outbreaks usually occur together as a combined disease thanks to multiple types of bacteria working together: peltaster fructicola, geastrumia polystigmatis, and leptodontium elatius. 

As the name implies, these tree diseases look like matte black or gray blotches and tiny black specks. 

How To Identify Sooty Blotch And Flyspeck 

You can spot sooty blotch and flyspeck by the characteristic small black blotches or speckles on the fruit. This makes apples look much less palatable, though it doesn’t affect the taste or safety of the fruit. 

How To Treat Sooty Blotch And Flyspeck 

Treating these types of fungus comes down to two simple steps: pruning and sanitizing. 

You can reduce the risk of developing sooty blotch and flyspeck by keeping your apple tree pruned. This increases air circulation to healthy fruit and branches and reduces the risk of bacteria forming. Make sure not to let your apple tree get too wet, as this fungus thrives in high humidity. 

If you notice signs of sooty blotch or flyspeck, mix one ounce of bleach to one gallon of water. Rub the fruit gently with the solution and a wet cloth to remove.

Cedar Apple Rust

Closeup of apple tree leaves with yellowy brown spots that indicate cedar apple rust, a very common apple tree disease.
The telltale signs of cedar apple rust.

Cedar apple rust is common in many parts of the United States. It gets its name from the fact that it needs two hosts to reproduce: a cedar tree and an apple tree. Other forms of cedar rust also exist.

What Is Cedar Apple Rust? 

Cedar apple rust is a fungus that uses two hosts to reproduce. It gets its name due to the fact that it usually infects cedar trees, though it can also affect juniper and other related trees. It is caused by the pathogen gymnosporangium juniperi-vinginianae. 

How To Identify Cedar Apple Rust

You can identify cedar apple rust by the lesions it causes on branches and fruit. These usually affect young branches and enlarge every growing season.

How To Treat Cedar Apple Rust

Unfortunately, there is not much you can do to treat cedar apple rust other than extensive pruning. Remove all dead branches and affected fruit and clear the debris to prevent reinfection from remaining spores. 

If you spot any lesions on cedar trees in the area, remove them as well. This will disrupt the fungal life cycle and reduce the risk of infection recurring. 

Powdery Mildew

Powdery mildew is one of the most common types of apple diseases in home orchards. It is estimated that powdery mildew, apple scab, and moniliasis are responsible for as much 50 to 60 percent of fruit losses every year.

What Is Powdery Mildew? 

Closeup of apple tree leaves that are infected with silvery gray-colored powdery mildew, an apple tree disease fungus.
Leaves infected with powdery mildew.

Powdery mildew is a fungus that can affect almost every part of a tree. It can be a serious issue for apples, reducing fruit yields and recurring annually when the weather becomes dry and warm. 

How To Identify Powdery Mildew

Growers will notice powdery mildew developing throughout the growing season. Buds become infected before they open, causing a delay in blooming by a few days. Once they open, the leaves and blossoms will be covered in white or gray powder. 

Flowers may grow poorly and be green or white in color. The tree will not produce fruit. Eventually, the leaves will curl and turn brown.

How To Treat Powdery Mildew 

To treat powdery mildew, you will need to act quickly, when the fungus is still in the buds. Spray regularly with mildew spray, then repeatedly will fungicide until the mildew is no longer apparent on new shoots (usually by the fourth application). For severe infections, you may need to use a sterol inhibitor fungicide. 

Fire Blight

An apple tree branch with brown shriveled leaves caused by the apple tree disease fire blight.
Fire blight, an apple disease caused by bacteria.

Fire blight is a bacterial disease rather than a fungus. It can be very serious for apple trees, particularly in certain regions. It heavily affects apple cultivars such as Jonathan and Granny Smith

What Is Fire Blight?

Fire blight is a bacterial illness that affects fruit trees include apples and pears. It thrives in hot, humid climates, usually appearing strongly in the spring and reducing as dryer summer temperatures occur.

The bacteria infects trees undetectably in the fall or winter, hiding in branches and unopened buds. Then in the spring, it begins to emerge through openings in the branches and foliage and becomes apparent. The infection can be spread through gardening tools. 

How To Identify Fire Blight 

Fire blight manifests as lesions on the branches and leaves of the tree, known as “shepherd’s crooks”. Eventually, the affected parts of the tree will begin to die. 

How To Treat Fire Blight

Unfortunately, fire blight is very difficult to treat effectively, and overusing certain bacterial sprays can cause the tree to develop resistance to treatment. 

The best ways to fight fire blight is prompt pruning and sanitizing tools. As soon as you notice symptoms of infection, prune away the affected branches 8-12 inches from the lesion. Sanitize all gardening tools in a 10 percent bleach solution mixed with a few drops of dish liquid to avoid spreading the bacteria between trees and branches. 

Keep the sanitizing solution nearby as you prune and dip the tools in between each cut to avoid spreading. 

Bitter Rot

Closeup of a yellow apple with a sunken brown lesion closely resembling bitter rot.

Bitter rot is a fungus that affects apple and pear trees and can cause lost fruit. Other types of bitter rot can also affect peaches, strawberries, blueberries, and chestnuts. 

What Is Bitter Rot?

Bitter rot is a fungus that targets the fruit of the tree rather than branches or foliage. Unlike most fungus, it can eat through the skin of the fruit. Then it lies dormant until the temperature and humidity rise before breaking out and becoming apparent. 

How To Identify Bitter Rot

Bitter rot becomes apparent in mid to late summer when the weather becomes warm and humid. Then the dormant fungus breaks out. It begins as a small brown spot on the skin of the fruit, which grows and darkens quickly. As they grow, they will sink into depressions. 

Eventually the fruit will develop black spots, which are the fruiting spores of the fungus. When the rot becomes severe, the fruit may ooze pink matter through open lesions. Eventually it will rot and shrivel. 

How To Treat Bitter Rot

Treating bitter rot is an exhaustive process that requires treatment through the whole season. This is because infection happens long before it becomes apparent, overwintering in invisible spores. 

The best treatment is prevention, with application of fungicide before winter sets in. Once the infection becomes obvious, the best way to manage it is through pruning of infected bodies and thorough tool sanitation. While some products exist for treating bitter rot, they are much less effective than prevention.

Black Rot

Closeup of an apple with a sunken black lesion caused by the apple tree disease black rot.
The apple tree disease, black rot, is easy to identify in infected apples.

Black rot is a fairly common apple tree disease, especially in certain regions. It may go hand-in-hand with fire blight and often affects trees that have other environmental stressors such as drought.

What Is Black Rot?

Black rot is a fungal infection that can affect fruit, branches, and foliage of affected trees. While the damage can be extensive, it can usually be managed through proper care and rarely needs to be treated with fungicide. 

How To Identify Black Rot

Black rot causes distinctive cankers on leaves, branches, and fruit. These are usually dark brown and may have concentric rings in them. On fruit, the lesions look like large rotten spots that are still hard to the touch. 

Black rot causes circular brown spots on leaves and similar cankers on branches. You will notice leaves and branches becoming brown and looking shriveled where affected. 

How To Treat Black Rot 

Black rot is most effectively treated by pruning the affected parts of the tree. Make sure to prune 8 to 12 inches away from the lesions and sanitize your tools between each cut to avoid spreading the infection. 

What Apple Varieties Are Resistant To Black Rot?

Unfortunately, there are no apple varieties that are resistant to black rot.

White Rot

Closeup of a yellow apple infected with a light brown lesion that has a reddish colored edge -- an indication of the apple tree disease white rot.
A light brown lesion with a reddish colored edge is an indication of white rot in apples.

White rot is a fungus that affects not only apple trees, but also pear, crabapple, and chestnut trees as well as grapevines. 

What Is White Rot?

White rot is a fungal infection that is usually brought on by other environmental stressors such as drought or fire blight. An infection may lead to a 50 percent loss of fruit. 

How To Identify White Rot

White rot is characterized by light brown cankers on fruit, leaves, and branches. On fruit it causes large brown rotten spots that may have concentric rings inside them. 

How To Treat White Rot 

The best way to reduce the risk of white rot is to reduce as many stressors to your tree as possible, including drought and other infections. 

Treatment rarely requires fungicide and instead relies on extensive pruning and sanitation. 

Cork Spot

Closeup of two wild apples with pitting closely resembling the pits caused by cork spot.
Wild apples with pitting similar to cork spot.

Cork spot is not technically an apple disease. Rather, researchers think it may be related to a vitamin deficiency in the tree. Although it causes superficial damage that looks unpalatable, the apples are still edible. 

What Is Cork Spot?

Cork spot is a condition that causes pits or depressions in apples. Though it is often confused with apple diseases, it is actually caused by a reduced level of calcium in the tree.

How To Identify Cork Spot

Cork spot appears as “dimples”, pits, or depressions on the surface of the apple. They are not significantly different in texture from the rest of the fruit but often look unappealing. 

How To Treat Cork Spot

There is no way to treat cork spot once it appears, since it seems to be related to vitamin levels in developing fruit. However, there seems to be a correlation between low soil pH and fruit overgrowth and higher incidence of cork spot. 

Therefore, you may reduce your chances of developing cork spot by keeping your soil at a healthy pH and pruning your apple trees properly.

Phytophthora Rot

Phytophthora rot affects both apple and pear trees. It can be dormant in soil for several years before attacking root systems. 

What Is Phytophthora Rot?

Phytophthora rot is a fungal infection that comes from the soil around fruit trees. Therefore, it starts by infecting root systems before moving on to the rest of the tree. 

How To Identify Phytophthora Rot

Phytophthora rot can affect almost every part of an apple tree. The most distinctive characteristic is a “girdling” around the lower part of the trunk, which appears as a dark brown collar. It may be wet or slimy to the touch. 

How To Treat Phytophthora Rot

Phytophthora rot is unfortunately an extremely destructive disease and cannot be treated very effectively. The best way to avoid it is through prevention, which is by choosing a moderately resistant rootstock. 

The best way to treat it is to catch it in the early stages. If you notice early girdling, you may be able to stop the infection by removing the infected soil, letting the rootstock dry out, and transplanting the tree. 

Preventing Apple Tree Diseases In The First Place

While it is important to inspect your apple trees regularly for disease and infestation, ideally you’ll never have to deal with these in the first place! 

Water your trees properly — Apple trees should not be watered too frequently. If the roots stay wet for more than 24 hours, the likelihood of developing disease drastically increases. 

Instead, water your apple trees about once every 10 days that it doesn’t rain. 

Treat your soil — Soil with a pH that is too low may be more susceptible to developing infection. To reduce the risk of this, use a soil test kit and treat the garden with lime if your soil pH is below 6.0.

Clean your tools — Keeping your tools clean is vital to reducing the spread of bacteria from tree to tree or even branch to branch. When pruning, dip your tools in a 10 percent bleach and water solution or a 70 percent alcohol solution

Prune regularly — Learn how to prune your apple trees properly and do so annually. This increases circulation to healthy branches and fruit while cutting away any dead or diseased parts. 

Apple Tree Disease FAQ

What Should I Use To Spray My Apple Trees? 

One of the best ways to prevent disease is to treat your apple tree with fungicide every fall after harvest. There are many effective commercial fungicides available. Some gardeners also use copper spray to fight off pests and disease. 

How Do I Protect My Apple Trees From Bugs? 

Protecting apple trees from bugs is a multi-step process. Keep your trees pruned and free of standing water. If needed, spray them with a commercial insecticide. 

How Do You Save A Dying Apple Tree?

Unfortunately, once an apple tree’s infection is advanced enough, you may not be able to save it. Some diseases can be treated effectively with fungal sprays and extensive pruning, while others completely destroy a tree.

When Should I Call A Professional?

If an infection is widespread around your garden or isn’t responding to treatment, it’s time to contact a professional. If you want to jump on the problem when it first appears, that is a fine option, too! 

Conclusion

The most effective way to protect your apple trees from disease is through prevention, which you can do through proper watering, pruning, soil treatment, and sanitation. To learn more about how to care for your apple trees, check out our post here.