Apple Scab

What is Apple Scab?

Apple Scab is a widespread fungal disease caused by Venturia inaequalis that primarily affects apple and crabapple trees throughout Missouri, including the St. Louis region. The fungus overwinters in fallen leaves and becomes active in the spring, especially during wet, cool weather. Once spores are released, they infect new leaves and fruit, causing distinctive olive-green to black spots that gradually darken and become velvety or cracked. Left untreated,

Apple Scab can lead to premature leaf drop, severely weakening the tree and diminishing its ornamental and fruit-bearing value. In highly infected trees, the disease can recur annually, leading to long-term stress and decline. While not usually fatal, it significantly reduces aesthetic appeal and can make trees more vulnerable to other pests and problems.

What Does Apple Scab Look Like: The Signs You Might Have It

Apple Scab often begins subtly but can quickly worsen if not addressed. The fungus primarily affects leaves and fruit, though it can also target young shoots. In Missouri’s wet spring climate, symptoms usually appear in late spring to early summer and become more visible as the season progresses.

Here are the most common signs that your tree might have Apple Scab:

• Dark, velvety spots on leaves
  
  These spots are usually olive-green to black and may have a fuzzy or cracked texture as they mature.
  
  
• Premature leaf drop
  
  Severely infected leaves will yellow and fall off by early to mid-summer, making the tree look sparse or stressed.
  
  
• Scabby lesions on fruit
  
  Apples and crabapples develop corky, black spots or misshapen areas. The fruit may crack or become deformed.
  
  
• Twisting or curling leaves
  
  New growth may appear distorted due to early infection in developing leaf tissue.
  
  
• Reduced flowering or fruit yield
  
  Chronic infections weaken the tree over time, reducing overall vigor and productivity.
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• Recurrent seasonal symptoms
  
  Apple Scab tends to return every year if not managed, as spores overwinter in fallen leaves and infect new growth in spring.
  

How Apple Scab Spreads to Other Trees

Apple Scab spreads through fungal spores that thrive in moist, cool spring conditions, making St. Louis yards especially vulnerable after wet winters. The disease overwinters on fallen leaves and fruit, then reactivates when temperatures rise and rainfall increases.

Here’s how it spreads from tree to tree:

• Overwintering spores in leaf litter
  
  The fungus (Venturia inaequalis) survives the winter in infected leaves and fallen fruit on the ground. As temperatures warm in spring, it releases spores into the air.
  
  
• Windborne and rain-splashed spores
  
  Spores travel easily through spring rains or light wind, landing on new leaves and blossoms of nearby trees. Close tree spacing increases the chance of infection.
  
  
• High humidity and leaf wetness
  
  Extended moisture on leaf surfaces—especially during cool spring mornings—creates the perfect environment for spore germination and infection.
  
  
• Tree-to-tree proximity
  
  Trees planted too close together or in shaded areas with poor airflow are more susceptible, as the damp, crowded conditions favor fungal growth and spread.
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• Repeated seasonal cycles
  Apple Scab recurs yearly if fallen leaves aren’t removed or if fungicides aren’t applied. Spores build up each season, increasing disease severity over time.
  

Common Trees in Missouri at Risk from Apple Scab

While apple and crabapple trees are the most well-known victims of Apple Scab, the disease can also affect other related trees in the rose family (Rosaceae). Some ornamental species popular in Missouri landscapes are also at risk, especially in moist, shaded areas where the fungus thrives. Because crabapple and ornamental apple varieties are so popular in local landscaping, Apple Scab is one of the most frequently diagnosed fungal issues in St. Louis-area yards.

Here are the most commonly affected trees:

• Apple Trees (Malus domestica)
  
  The primary host. Apple Scab affects both leaves and fruit, making trees look unsightly and reducing fruit yield.
  
  
• Crabapple Trees (Malus spp.)
  
  Extremely susceptible, especially older or non-resistant cultivars common in St. Louis landscaping. Defoliation and scabby fruit are major issues.
  
  
• Hawthorn Trees (Crataegus spp.)
  
  Can develop similar leaf spots, though usually less severe than on crabapples. Common in both urban and rural Missouri.
  
  
• Mountain Ash (Sorbus spp.)
  
  May occasionally show symptoms if conditions are favorable for fungal growth, especially in wetter parts of the state.
  
  
• Pear Trees (Pyrus spp.)
  
  Generally less susceptible, but certain ornamental varieties (like Bradford Pears) can show minor leaf spotting in high-pressure years.
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• Other ornamental Rosaceae shrubs and trees
  Some varieties of quince and serviceberry may also show mild symptoms under persistent wet conditions.
  

Treatment and Prevention for Appe Scab

Managing Apple Scab takes a combination of proactive maintenance, smart tree selection, and when needed targeted treatment. Because this disease returns each spring if untreated, prevention is just as important as curing an active outbreak.

Prevention Strategies

• Plant scab-resistant varieties
  
  Modern cultivars of apple and crabapple trees bred for resistance are the most effective long-term solution. Ask a local nursery or arborist about disease-resistant options for Missouri.
  
  
• Clean up fallen leaves and fruit
  
  Infected leaves and fruit on the ground are a major source of spores. Raking and disposing of leaf litter in fall greatly reduces next spring’s infection risk.
  
  
• Prune for better airflow
  
  Thinning out crowded branches improves sunlight penetration and air movement, helping foliage dry out faster and making conditions less favorable for fungal growth.
  
  
• Avoid overhead watering
  
  Sprinklers that wet the foliage can increase leaf moisture, creating ideal conditions for spores to germinate. Use drip irrigation or water early in the morning if needed.
  
  

Treatment Options

• Fungicide applications
  
  Timed preventive fungicide sprays—starting in early spring when new leaves are just emerging—can protect vulnerable trees. Repeat applications may be necessary during wet seasons. A certified arborist can create a schedule based on local weather patterns and your tree’s condition.
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• Professional monitoring
  If Apple Scab returns each year or your tree is defoliating by mid-summer, it’s time to consult a professional. Arborists can assess tree health, apply advanced treatments, and help reduce disease pressure year over year.