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Common Crabapple Diseases That Cause Summer Defoliation

crabapples hanging on a branch

Crabapples are beautiful trees that grow well in the Dayton, OH area. However, many are susceptible to a number of crabapple diseases that cause them to drop leaves and generally look unhealthy by the end of summer.

If your crabapple has wilted and blackened leaves, dead or orange spots on the leaves, or strange orange growths, it’s likely caused by one of the three most common crabapple diseases in southwest Ohio. Fireblight, apple scab, and cedar-apple rust infect crabapple trees throughout our region every summer. And while the symptoms aren’t necessarily fatal, they’re pretty unsightly.

Keep reading for details on each of these crabapple problems, including the symptoms, causes, and treatment options.

APPLE SCAB

If there's one crabapple problem that most people have seen or experienced with their own trees, it's apple scab.

What to look for

Symptoms often first appear in late spring, when you’ll see dark blotches (called lesions) on leaves, as well as a dark, velvety growth over the leaf surface. In summer, leaves turn yellow and fall prematurely, often leaving the tree mostly, or even completely, defoliated. Infected fruit are usually deformed and develop circular, rough spots on the skin. Fruit rot quickly and fall before ripening.

early stages of apple scab on crabapple leaf
Early stages of apple scab. Image courtesty of Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org.
apple scab on leaf
Apple scab lesions continuing to spread. Image courtesy of Joseph OBrien, USDA Forest Service, Bugwood.org.
apple scab
At this point in the disease, leaves will start to drop. Image courtesy of James Chatfield, Ohio State University, Bugwood.org.

Causes

Apple scab is caused by a fungus called Venturia inaequalis that’s found on most types of apple trees, especially in the Midwest. As with most fungal diseases, it first develops when conditions are humid and not too hot (spring and early summer).

Treatment Options

Do nothing – A healthy crabapple can withstand annual defoliation for many years, although it will eventually weaken the tree and make it more vulnerable to other diseases and pest infestations. If the tree retains most of its leaves and you don’t mind the sparse leaf coverage in summer, doing nothing is a reasonable option.

Spring fungicide applications – For trees that are highly susceptible to the apple scab fungus, annual fungicide treatments are the best option. These treatments are applied as leaves emerge throughout spring and early summer, and can protect them from infection by fungal spores. However, it does not “cure” the disease.

Dispose of all debris from infected trees – Carefully clean up all fallen branches, leaves, fruit and twigs, bag them, and put them in the trash. Good sanitation is essential to minimize the likelihood of fungal spores re-infecting the tree next spring.

Ensure the tree isn’t crowded – Late winter pruning can help open up the canopy and improve air circulation, as can pruning back any nearby shrubs or trees that are crowding the crabapple.

Plant scab-resistant crabapple cultivars – It may be best to remove a tree that succumbs to apple scab each summer and replace it with a scab-resistant one. The following crabapple varieties have been found to develop only slight to moderate scab infections: Adams, Baskatong, Brandywine, Callaway, David, Dolgo, Donald Wyman, Malus floribunda, Henry Kohankie, Henningi, Jewelberry, Ormiston Roy, Professor Sprenger, Malus seiboldi var. zum cultivar Calocarpa, Silver Moon, Sugartyme, Malus tschonoski, Weeping Candy Apple, White Angel, and White Cascade.

Fireblight

This disease is more damaging to crabapples than apple scab as it can kill multiple branches and even the entire tree during a severe infection. Severe fireblight infections may not occur every year, but when they do the results can be devastating.

What to look for

The first symptoms usually appear in late spring when flower petals start to fall. Twig and branch ends suddenly wilt, leaves and flowers turn black (or dark brown) and die. Leaves and flowers killed by fireblight have a very distinctive look – instead of falling off, they hang down from the twigs and branches. Branch tips often curl into a shepherd’s crook shape. Overall, the tree looks like it was scorched by fire.

You may also notice slightly sunken cankers on the bark, some of which can ooze a cloudy liquid during wet spring weather. These cankers will spread, killing branches and even the main tree trunk.

fireblight
The classic "shepherd's crook" and dead, hanging leaves indicative of fireblight.
Image by William Jacobi, Colorado State University, Bugwood.org.
fireblight canker on crabapple stem
Sunken cankers appear on stems. Image courtesy of William Jacobi, Colorado State University, Bugwood.org.
fireblight on a crabapple
Fireblight can spread quickly throughout the tree

Causes

Fireblight is caused by the bacterium Erwinia amylovora and is spread by insects and splashing water from rain or irrigation. Bacteria overwinter around the edges of cankers on infected trees, making it likely that the tree will be hit with an even more severe case of fireblight the next spring if the cankers aren’t removed.

Fireblight needs specific conditions to develop; new growth and/or open flowers on the tree, warm temperatures (about 65o), and humid or rainy weather.

Treatment options

Apply a bacteriacide or copper-based spray – These treatments are only available to professionals, such as a Certified Arborist. They act as an antibiotic, killing the bacteria after it comes into contact with the spray residue, or suppressing its ability to spread.

Prune out affected branches – Cut off affected twigs and branches at least 3 to 4 inches below the base of the canker. This should be done while the tree is dormant (usually between late November and early March) and the weather is dry. Disinfect pruning tools between each cut.

Fertilize carefully – The fireblight-causing bacteria attack succulent new growth so avoid high nitrogen fertilizers or heavy fertilizer use that could encourage growth later in the season.

Plant fireblight-resistant crabapple cultivars – Several excellent varieties of crabapple have shown good resistance to fireblight, including Adams, Callaway, David, Dolgo, Harvest Gold, Indian Summer, Jewelberry, Liset, Profusion, Red Baron, Selkirk, and Sentinel. Some of these are also resistant to apple scab.

CEDAR-APPLE RUST

This fungal disease needs two different tree species to grow – an apple or crabapple, and nearby (within about a mile) trees or shrubs of the Juniperus species, such as Eastern red cedar and common juniper. It’s most common in areas with a large number of red cedars and/or junipers nearby (many grow wild in our region).

What to look for

Shortly after flowers have bloomed in spring, the leaves of crabapples infected with cedar-apple rust develop bright yellow spots with a darker yellow-orange center. As the disease progresses, you may see small black raised areas on the upper surface of the spots. Later, those will develop into orange structures with fringed edges on the underside of the infected leaves. In cases of severe infection, leaves may drop prematurely. Often, crabapple fruits develop similar spots and rotten areas.

Although it doesn’t kill Juniperus spp., it does cause galls to form on small branches, followed by bright orange tentacle-like spore horns during wet conditions in late spring.

cedar-apple rust spots on apple leaves
Orange spots with bright yellow rims are a classic sign of cedar-apple rust.
crabapple fruit infected with cedar-apple rust
Crabapple fruit infected with cedar-apple rust
fruiting body of cedar-apple rust on a cedar
Bright orange spore horns on an Eastern red cedar.
Image by James Chatfield, Ohio State University, Bugwood.org.

Causes

Cedar-apple rust in crabapples is caused by the fungus Gymnosporangium juniperi-virginianae. It appears during cool, wet springs and mostly infects our native crabapples, such as ‘Brandywine’ and ‘Charlottae’. Crabapples originating in Asia seem to generally be unaffected by cedar-apple rust.

Treatment options

Apply a fungicide - Control can be achieved by spraying a fungicide at bud break (usually in mid- to late-April in our area) and then weekly until flower petals fall.

Plant resistant varieties - Consider planting trees that are immune or resistant to rust diseases and avoid planting junipers nearby (or, if possible, remove unwanted junipers).

Remove galls from junipers – During the winter, gently remove galls from nearby juniper branches to minimize spreading the fungus in spring.

 

Other Crabapple Problems

As with most other trees, crabapples can also be affected by a number of pests that are common to the Dayton area. Spider mites, aphids, and scale insects can cause leaf yellowing, wilting, spotted and distorted leaves, and even tree death. Environmental conditions, such as drought, can make crabapples vulnerable to Botryosphaeria canker, while wet or humid summers predispose them to fungal attacks.

While the three diseases described above are most common (and pretty distinctive), it’s always a good idea to consult with a Certified Arborist to rule out any other pest or disease problems. Since the treatment options (as well as the timing of applications) vary based on the cause, you want to be sure you’re doing the right thing for your crabapple tree(s).

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