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Apple Scab: What Causes It and What Can Be Done About It?

Updated on January 2, 2018
Casey White profile image

Dorothy McKenney is a former newspaper reporter turned researcher. Her husband, Mike, is a professional landscape/nature photographer.

Apples are my favorite fruit so it breaks my heart to see any of them ruined.  This "WAS" going to be a healthy snack but now it's garbage.
Apples are my favorite fruit so it breaks my heart to see any of them ruined. This "WAS" going to be a healthy snack but now it's garbage.

Here in Washington State We Know About Apples!

  • Living in Washington state, you learn a lot about apples, so here I am with another article on a fungus that can attack your apple trees. This time it is apple scab and it affects apple and crabapple trees. It is, in fact, the most prevalent apple disease in the world and coastal states are particularly vulnerable to it because the fungus grows quickly in wet climates.
  • Moist, cool weather causes this fungus to thrive, and guess what the conditions are in Washington state most of the time! Moist and cool is the name of the game here in this state.
  • Apple scab disease is caused by a fungus called Venturia inaequalis, and it overwinters on the dead apple leaves on the ground. According to sources here in the state, the fungus will infect fruit around the bloom after only five to six hours of wetness. The spores are carried by winds and quickly infect young flowers, fruit and foliage.

This is a Great Video Relating to Apple Scab Management

The Life Cycle of the Fungus That Causes Apple Scab

How to Recognize Apple Scab Disease

  • A few weeks after those spores are blown about in the wind, the signs of the fungus will begin with dark green to black patches on apple tree leaves. The patches will look like scabs or blisters on the upper leaf surface, and as the infection continues to grow, the spots will enlarge and often join, causing the entire leaf undersurface to be covered. Then, the leaves will become twisted with a "puckered" look and turn yellow. The infected leaves will most likely fall early and trees that are severely infected may lose all their leaves.
  • On the fruit, look for dark scab-like patches to appear that will ultimately develop a white rim, which will later disappear from around the velvety-like centers. After it disappears, the centers will sometimes become raised.
  • The result of apple scab is usually fruit that is smaller than normal and distorted. Often, the fruit will be cracked and will drop early. Those cracks in the fruit allow organisms into the apple that cause it to rot.

Apple scab disease on leaves.
Apple scab disease on leaves.

How to Manage Apple Scab Fungus

  • It is important to apply fungicides to your apple trees early during the most critical period from the breaking of cluster buds until the leaves are fully expanded. THOROUGHLY apply fungicide containing triforine, sulfur, lime sulfur or captan and follow the directions on the label completely.
  • What you are trying to do is keep the scabs from appearing, because once they do, there is no cure. If the scabs do appear, be sure to remove the scabby shoots and fruits and discard all fallen leaves. Although infected fruits are U G L Y, they sometimes can be used unless they are infected very early.

Infected crabapple leaves.
Infected crabapple leaves.

How to Prevent Apple Scab Fungus

  • Find out from your local cooperative extension service in your area which varieties of apple trees are resistant to this disease in your growing zone. If you live in a coastal area where it is cool and damp, you might want to try heirloom varieties, which can be scab resistant.
  • Always prune your trees to allow proper air circulation so that the amount of wet foliage is reduced. Always rake up and discard fallen leaves and fruit to keep the fungus from further spreading.

© 2012 Mike and Dorothy McKenney


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    • Casey White profile image

      Mike and Dorothy McKenney 5 years ago from United States

      Thanks for the comments. Although the apples that are infected may be unsightly, they are perfectly safe to eat. I would just peel or cut away the scabs and enjoy what is left!

    • Sheepsquatch profile image

      Sheepsquatch 5 years ago from Springfield, MO

      Are the apples safe to eat if the tree has been infected?

    • mwilliams66 profile image

      mwilliams66 5 years ago from Left Coast, USA

      I landed on your hub through a hop. What a pleasant surprise. We have an apple tree in our yard that is clearly suffering from this apple ailment. I had no idea what it was and had planned to begin researching. I found your article to be very informative. Thank you for posting.