In the Garden:
Northern & Central Midwest
June, 2004
Regional Report

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This old crab apple is quite susceptible to disfiguring apple scab.

Apple Scab, the Scourge of Midwest Apples!

Have you noticed your crab apples dropping an unusual number of yellow leaves? Are the leaves that remain on the tree covered with olive green or brown spots with a nondescript outline? This is apple scab. Eventually those spots will turn into a smoky blotch on both sides of the leaves, followed by the entire leaf yellowing and falling in July and August.

Apple scab is one of the Midwest's most prevalent diseases of apples and their relatives. During a favorable season it can affect one quarter of the world's apple crop, and it infects nearly all cultivars of apple and crab apple.

This disfiguring fungal disease makes corky scabs on the fruit and leaves and can eventually completely defoliate trees. A prolonged cool, moist spring is ideal for the fungal spores to thrive, and it's certainly disheartening to see a crab with no leaves by early August.

The Disease Cycle
Apple scab's life cycle is fairly simple. The spores spend the winter on infected leaves on the ground or in lesions on twigs. In spring, spores are released and blown by the wind, landing on and infecting leaves, buds, and succulent twigs. The infected tissue produces spores that infect other leaves, fruits, and twigs, causing further decay and creating a problem most of the summer.

The first line of defense is to purchase resistant varieties. If you are planning to plant an apple or crab apple, this is a great chance to check out some of the hundreds of resistant cultivars. If you have an older tree that is prone to the disease, it is possible to control it, but you must be vigilant and continue your controls all season.

Reducing Scab
First, be sure to clean up all leaves and fruit in the fall to reduce the number of overwintering spores. At the end of the season, remove and discard any fruit remaining on a tree that is infected. Then prune the tree to create an open crown so there is plenty of air circulation -- an unfavorable condition for the spores. Be sure to prune off any infected twigs as well. If your tree is fairly old, this pruning may need to be drastic to be effective.

If you know your crab apple or apple tree has scab every year, it can be almost completely controlled by preventive spraying with lime-sulfur. This spray must be applied before blooming, just as the infection starts, and must be reapplied after every rain.

In the most extreme cases, it may be best to remove the tree. I have eating apples planted in my yard, and even though they are resistant varieties, there is no reason to introduce scab to them. So I made the decision last year to remove my old crab apples. As much as I hate to remove trees, I can imagine their spots filled with beautiful river birches and corneliancherry dogwoods. And, scab-free apples in the fall!


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