Purdue Extension Service

Vanderburgh County, Indiana

Apple Scab on Apple, Crab

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, July 4, 2008

A lot of older crabapples are appearing very sick right about now. The trees are dropping leaves and getting very thin. A close examination reveals the cause: the fungal disease apple scab.

Apple scab first appears as brown to olive-green round spots that form on the leaves of crabapples and apples. The spots eventually become black and velvety. The leaves turn yellow, and then drop. Highly susceptible cultivars of crabapple can become nearly completely defoliated by the end of June.

Apple scab is a big problem during wet springs. The spores are easily splashed around, the presence of water on the leaves makes it easy for the spores to germinate and infect leaf tissue. As you may remember, we had an exceptionally wet spring this year.

As I mentioned in this column back in February, apple scab is one of the few tree diseases that can be easily controlled by buying resistant cultivars. Just as some people don't seem to get winter colds while others have drippy noses all winter long, some varieties of crabapple can pretty much shrug off the fungus attack. They may drop a few leaves, but you won't see any serious defoliation.

If you are in the market for a new crabapple tree, some of the cultivars you might want to look for include: Ann E., Bob White, Prairie Maid, Prairifire, Red Jewel, and Sargent. There are other varieties that have excellent to moderate resistance to scab; contact the Extension office for a list.

But what if you already have a mature crabapple that is badly attacked by scab each year? In this situation, you are faced with three choices: 1) Do nothing, and wait for the tree to weaken and die; 2) cut the tree down and plant one of the resistant crabapples; or 3) spray the tree with a fungicide every 7 to 10 days from the time of bud break until the weather turns hot and dry. Since we are now in the "hot and dry" stage of the year, it's already too late to spray.

If you choose to spray your crabapple next year, the fungicides you may want to consider using include captan (an ingredient in most home orchard sprays) or chlorothalonil (sold as Daconil). Be sure to read and follow all label directions when using these or any other pesticide. And remember: one or two sprays will not protect your trees. You must keep up a coat of fungicide protection throughout the cool, wet spring weather, or you will still get scab.

This same procedure should be followed for other similar diseases, such as black spot on roses, blotch on peonies, or black rot on grapes. Use a suitable fungicide with that crop on the label, and spray throughout the infection period.

For more information on fungal diseases, contact the Purdue Extension Service at (812) 435-5287.