A Homeowner’s Guide to Apple Scab

Apples and crabapples may be wonderful landscape trees for many Michigan properties, but they do come with some downsides. For example, they are unfortunately susceptible to a number of pest and disease problems, including apple scab. 

Likely the most common disease afflicting apple trees in our area, apple scab rarely kills otherwise healthy individuals. However, it can make infected trees more susceptible to secondary pathogens and abiotic (non-living) stressors, such as drought or poor soil conditions. 

We’ll try to help you give your trees the best chance of thriving by sharing some of the symptoms the disease causes, the conditions that foster its development, and the treatment options available.  

What Is Apple Scab?

Apple scab is a common disease caused by the fungus Venturia inaequalis. Many species within the rose family (Rosaceae) can be infected by similar fungi, but the disease is most commonly associated with apples (Malus spp.). 

The disease has been known for some time, having first been identified in the early 19^th century. It likely originated in Central Asia before spreading to Europe and North America via domestic apple trees. Today, it is found virtually everywhere that commercial apple production exists on a large scale. 

What Are the Symptoms of Apple Scab?

Although the disease is best known for damaging the fruit of apple trees, it usually causes symptoms to first appear on the leaves. 

You may see circular “scabs” forming on the undersides of leaves or velvety spots on the upper leaf surfaces. In some cases, you may also notice that the leaves appear twisted, curled, or distorted. They will often turn yellow over time and be shed before the end of the summer. 

Shortly thereafter, you may start noticing tan to black spots forming on the surface of the fruit. Like the lesions found on the undersides of the leaves, these may have a scabby appearance. These hardened areas can prevent the fruit from developing properly, leading to cracks or splits. 

Elongated bark lesions may occur on twigs or branches at times, but this isn’t as common as the symptoms affecting the leaves and fruit.   

Apple Scab Treatments 

There are a number of ways homeowners can try to treat apple scab, though some may require the assistance of an ISA-certified arborist. 

However, it’s important to note that treatment is not always required — mature, healthy trees will often tolerate apple scab, though you’ll likely lose a significant amount of the current year’s crop. Many homeowners will simply allow the disease to run its course and hope their trees remain healthy next year. 

• Supportive Care: The healthier an apple tree is, the more likely it will be able to tolerate the fungus. Ensure that the tree is watered during dry periods, protected from mechanical damage, and provided with a 2- to 4-inch-thick layer of mulch over the root zone. 
• Good Hygiene: Apple scab overwinters on fallen leaves, meaning that the source of next year’s infection is likely found right below the tree. So, rake up and dispose of apple leaves as quickly as possible, while trying to prevent the spores from being blown around by the wind. 
• Proper Pruning: It’s important to have your apple trees pruned regularly to impart good form, manage fruit production, and — most importantly — allow air to flow through the canopy to prevent the interior foliage and branches from remaining wet for long periods of time.
• Fungicide Application: A number of fungicides can help protect apple trees, including copper, triflumizole, and propiconazole, among others. However, these must be applied before the spores germinate; they’re not effective at treating existing infections. 

Susceptible & Resistant Apple Varieties in Michigan

Apple trees vary in their susceptibility to apple scab, and some are largely resistant to the disease. So, one of the best ways to address apple scab is by selecting and installing resistant varieties and cultivars. 

A few of the most resistant options include: 

• Prairifire Crabapple (Malus ‘Prairifire’): One of the most widely recommended ornamental crabapples, Prairifire crabapple offers excellent resistance to apple scab, as well as fire blight and cedar-apple rust. 
• Sugar Tyme Crabapple (Malus ‘Sutyzam’): A highly disease-resistant cultivar prized for its attractive white flowers and abundant red fruit, Sugar Tyme crabapple performs exceptionally well in Michigan landscapes. 
• Spring Snow Crabapple (Malus ‘Spring Snow’): This fruitless cultivar provides excellent apple scab resistance and produces abundant white spring blooms that provide beauty without forcing homeowners to deal with fruit litter. 
• Adirondack Crabapple (Malus ‘Adirondack’): A compact, upright cultivar with excellent resistance to apple scab and good overall disease resistance, Adirondack crabapple is an ideal choice for small landscapes. 
• Liberty Apple (Malus domestica ‘Liberty’): One of the most disease-resistant eating apples available, the Liberty apple exhibits excellent resistance to apple scab and good resistance to several other common apple diseases. 
• Enterprise Apple (Malus domestica ‘Enterprise’): Bred specifically for disease resistance, the Enterprise apple provides excellent resistance to apple scab (and fire blight) while producing high-quality fruit.

Meanwhile, the following varieties are quite susceptible to apple scab and may be worth avoiding:

• Hopa Crabapple (Malus ‘Hopa’): A beautiful older cultivar, Hopa crabapple is highly susceptible to apple scab and often suffers significant defoliation during wet springs. 
• Radiant Crabapple (Malus ‘Radiant’): Another attractive but notoriously disease-prone ornamental, the Radiant crabapple is commonly affected by apple scab. 
• Royalty Crabapple (Malus ‘Royalty’): While valued for its deep purple foliage, the Royalty crabapple can be quite susceptible to apple scab in Michigan’s climate. 
• Cortland Apple (Malus domestica ‘Cortland’): A popular heirloom eating apple, the Cortland apple is highly susceptible to apple scab and often requires regular fungicide applications.
• McIntosh Apple (Malus domestica ‘McIntosh’): One of Michigan’s most beloved apples, the McIntosh apple is unfortunately quite susceptible to apple scab and several other diseases. 
• Jonathan Apple (Malus domestica ‘Jonathan’): This classic cultivar is highly susceptible to apple scab and benefits from preventative disease management. 

Apple Scab in Michigan: FAQ

Below are answers to some of the questions we hear most often from homeowners about identifying, preventing, and managing apple scab in Michigan.

Can apple scab kill an apple tree?

Apple scab rarely kills healthy, established apple trees outright. 

However, severe or repeated infections can cause significant defoliation and stress, leaving trees more vulnerable to other pests, diseases, and environmental stressors. Over time, chronic infections may contribute to reduced vigor and decline.

What does apple scab look like?

Apple scab typically begins as olive-green or dark brown spots on leaves, which gradually become darker and velvety in appearance. Infected leaves may become distorted or fall prematurely, while fruit can develop dark, corky lesions that affect both appearance and quality.

Is apple scab contagious?

Yes. Apple scab spreads through fungal spores that are carried by wind and rain. The disease can move readily between susceptible apple and crabapple trees, particularly during cool, wet spring weather.

Can apple scab be treated?

In many cases, yes. Treatment options may include fungicide applications, improved sanitation practices, and pruning to improve airflow. You can also select resistant varieties when planting new trees.

Ultimately, the most effective management strategy will depend on the severity of the infection and the tree’s overall health.

Are crabapple trees susceptible to apple scab?

Many crabapple cultivars are susceptible to apple scab, though resistance varies considerably between varieties. Fortunately, numerous modern cultivars have been bred specifically for excellent disease resistance and are well suited for Michigan landscapes.

When is apple scab most active in Michigan?

Apple scab infections typically occur during cool, wet periods in spring as new leaves emerge. Prolonged rainfall and high humidity create ideal conditions for fungal spores to infect susceptible foliage and developing fruit.

Should I remove an apple tree with apple scab?

Not necessarily. Most apple trees affected by apple scab can be successfully managed through proper cultural practices and, when appropriate, professional treatment. 

Removal is generally only recommended when a tree is experiencing severe decline or suffers from multiple significant health or structural problems in addition to apple scab.

Protect Your Apple Trees from Apple Scab

Apple scab is certainly a threat that Michigan homeowners with apple trees will want to be aware of. Fortunately, by selecting resistant varieties and giving them proactive care, you have a great chance of keeping your apple trees healthy.   

If you suspect that your trees are suffering from apple scab, contact the professionals at Longtree Tree Service. We’ll have an ISA-certified arborist evaluate your trees and recommend an appropriate treatment.

Sources and References

• MSU Extension – “Apple Scab”
  Integrated pest management factsheet on apple scab (Venturia inaequalis), covering symptoms, the overwintering and infection cycle, fungicide timing, and the relative susceptibility of common apple varieties.
  https://www.canr.msu.edu/ipm/diseases/apple_scab
• Michigan Gardener – “How to Identify and Control Apple Scab”
  Regional gardening-magazine guide to recognizing apple scab and managing it through cultural practices, resistant cultivars, and properly timed fungicide applications.
  https://www.michigangardener.com/how-to-identify-and-control-apple-scab/
• MSU Extension – “Scab-Immune Apple Varieties Are Worth a Try”
  Homeowner-focused article explaining how planting scab-immune apple varieties, combined with sanitation and fungicides, can reduce pesticide use in backyard trees.
  https://www.canr.msu.edu/news/scab_immune_apple_varieties_can_reduce_pesticide_use_for_the_backyard
• Missouri Botanical Garden – “Apple Scab”
  Home-gardener factsheet from the Garden’s plant-problem library describing apple scab symptoms on apple and crabapple and outlining cultural and chemical control measures.
  https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/scabs/apple-scab
• University of Minnesota Extension – “Apple Scab”
  Extension resource on identifying and managing apple scab on apples and crabapples, including scab-resistant variety recommendations suited to cold climates.
  https://extension.umn.edu/plant-diseases/apple-scab
• Michigan Apples – “Michigan Apple Varieties”
  Industry commission guide to the apple varieties grown in Michigan, with harvest timing, flavor profiles, and use recommendations for each.
  https://www.michiganapples.com/about/varieties/