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Over the Garden Fence

Crabapple Scab and Cedar Apple Rust

Down the Garden Path

Richard Hentschel, Extension Educator

Our beautiful ornamental flowering crabapples that grace so many yards have a couple of foliage diseases that can really impact how they look once the bloom show is gone. Both diseases readily infect the crabapple leaf.

Apple Scab (crabapple scab) will really detract from the look of your crabapples, as it will cause the leaves to actually fall from the tree by mid-summer, leaving just a few uninfected leaves at the very ends of the branches. Cedar Apple Rust can be found on the leaves yet does not cause them to drop.

These are both fungal diseases that use the cooler, wetter periods of spring to infect the leaves. How they get there is the same, by floating through the air. How they overwinter is very different. Scab makes it through the winter on last year's affected leaves that had fallen off and became part of the leaf litter last fall along with the rest of the leaves in your yard. Cedar Apple Rust (CAR) as the name implies, uses an evergreen host and a deciduous host to complete its life cycle. The evergreen hosts are cedars in nature and Junipers in the home landscape.

Both scab and CAR infect crabapples in the spring of the year. Scab will continue to cycle with additional generations right on the crabapple while CAR will send spores off through the air to infect cedars or junipers for its overwintering stage during the summer months.

Cedar Apple Rust shows up as a yellow or yellow orange spot or spots on the leaf and later as protrusions from the lower side of the leaf that look like stubby hairs. These contain the spores that will be sent through the air to the evergreen host. Apple Scab develops as an olive green spot and it only takes one or two on a leaf to cause it to drop.

If you have an older cultivar of crab, you are more likely to experience either or both of these fungal leaf diseases. New cultivars have resistance to these. In a high-pressure year, you will still see these diseases show up, but the severity is less and the trees can recover easier.

If you also have apple fruit trees in the landscape, then you are already familiar with both of these diseases and know how to manage the problem. Both scab and CAR require fungicide treatments to prevent infection over a period of several weeks during our cooler, wetter spring weather. Once the weather warms and dries, the infection period for CAR really drops. Because scab remains on the tree, the disease can continue to spread if the weather conditions allow.

Both of these foliar diseases are beginning to show up so to prevent any further infections, begin your management program now. Be sure to read the label before you buy to be sure it will control both diseases and many will contain an insecticide like the home orchard products, so there are additional precautions to prevent bees from being killed.

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.