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.
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.
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.