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What Is Apple Scab and How Is It Treated?

Raising apple trees in your yard can bring you boundless joy year-round, but what happens when your tree gets sick? Unfortunately, tree disease is something that you will need to look out for. You might have read “5 Tree Diseases to Watch Out for in the Pacific Northwest,” but there is another disease to be aware of when you have apple trees, and that is apple scab. This disease occurs everywhere in the world where apples can be grown and causes more tree losses than any other apple tree disease.

What Is Apple Scab?


Caused by a fungus called Venturia inaequalis, apple scab is a serious disease. It’s quite contagious, spreading very fast and easily between trees. The disease can infect both the fruit and the leaves of a tree. If there’s an infection present, the fruit is inedible. It primarily affects apple trees and crabapple trees, but can also be hosted by loquat trees, firethorn trees, and mountain ash trees. Apple scab is the most common disease of apple and crabapple trees.

How Can You Identify Apple Scab?

Leaves of infected trees will have dark spots on them, often of an olive green or brown color. These spots are round and can be as big as a half-inch across. The texture of these spots is almost velvet-like, but the borders are fringed. The spots are typically formed along the veins of the leaf. Additionally, the spots can be simple circular spots or sunken scabs. Leaves that are infected can also become twisted and deformed.

Fruit that is infected by the disease will also have olive-green spots on them. These spots will then turn brown as time goes on and the disease progresses. If the fruit is still young when infected, the fruit will become deformed. It may even crack the skin of the fruit as it attempts to grow. Young fruit will fall off the tree earlier than it’s supposed to. Meanwhile, fruit that is infected when mature can potentially develop apple scab in storage.

Why Did My Tree Become Infected with Apple Scab?

Venturia inaequalis grows and lives through the winter season, meaning that leaves and fruit that were infected and left on the ground keep the disease alive. It can then easily spread to any nearby trees come springtime. It spreads especially easily during a cooler and wetter spring. This type of weather allows the fungus to grow to maturity and release its spores. The rainy and windy weather will then carry these spores through the air, and they land on apple leaves and fruit.

If these leaves and fruit are wet from such weather, the spores will germinate, leading to growth into the leaf or fruit tissue. Within just a few weeks, the scab or circular spot will begin to form. The more the scab can mature, the more spores it’s able to produce. These are then released into the air causing the process all over again and infecting more nearby trees.

How to Prevent Apple Scab?

There are some tactics you can use to help prevent apple scab from infecting your trees. For starters, there are scab-resistant varieties of apple and crabapple trees that you can choose to plant in your yard. If you’re unable to choose one of these scab-resistant varieties, there are fungicides you can spray on your trees as a preventative measure.

Additionally, be sure to clean up any fallen fruit or leaves off your trees, especially in the winter months. Pruning your trees to keep them at their healthiest is also good maintenance to help prevent disease. Trimming your trees is also recommended, as it will increase the airflow around the remaining branches and therefore reduce the likelihood of fungus growth.
When you’re planting your apple or crabapple trees, it’s important to plant them in areas of full sun and with plenty of space around them, as trees that are closer together are more susceptible to disease. Watering in the early morning or evening hours will help avoid heat irritation and gives the fruit and leaves time to dry out.

How Is Apple Scab Treated?

One of the most important things you can do to treat a tree already infected with apple scab is proper clearing. Any leaves or apples that fall from the tree should be removed and destroyed immediately. This will prevent the fungus from overwintering and decrease the spread of the disease. When the springtime does arrive, it’s critical that you monitor the leaf and flower blooms on the trees that were previously infected. Spraying these with fungicides will help to break the cycle of infection.

Additionally, spreading a three- to six-inch layer of compost under these trees, ensuring that the soil is covered, will help prevent any sort of splash dispersal of the spores of the fungus. Make sure that this compost doesn’t touch the trunk of the tree.

One of the most proactive measures you can take if you notice that your tree has been infected with apple scab is to call a professional arborist to come in and look at your tree. A professional has dealt with apple scab before and knows what to do. Additionally, they will be able to tell if the apple scab is in its early stages or if it has progressed to a more mature level. They can also advise you of what next steps should be taken and handle the treatment for you, ensuring that it’s done correctly and helping to preserve the health of any nearby trees in your yard.

If you think you have seen signs of apple scab on your trees, you can contact Mr. Tree. We’ll come take a look, confirm whether it is apple scab disease, and provide you with a personalized treatment recommendation. We want to make sure the apple scab is contained before damaging any other surrounding trees. We always want your yard to be at its most beautiful and at its healthiest.