Skip to main content

Flowering Crabapple and Apple Tree Diseases

Our weather continues to put a lot of disease pressure on our flowering crabapples and apple trees due to the cool and wet conditions. The disease that is easily seen right now is Cedar Apple Rust (CAR). CAR is a two-host rust, and right now, it can be seen on the cedars and junipers as a strange looking gall about the size of a golf ball covered in a bright yellow to orange jelly-like fingers. Those orange structures are sending out fungal spores that will land on newly developing and expanding crabapple and apple leaves, which we will infect them and we will notice later on in the summer.

Home orchardists know that to protect their apple trees, sprays are needed from the time buds begin to swell until a bit after flower petals begin to fall. If you have an ornamental flowering crabapple in the landscape, the same protection period applies. While CAR will not kill either the apple or crabapple, it can, over years, weaken the trees and will allow other problems to show up. For the home orchard, CAR-infected apple trees will not be able to produce all the energy needed to fulfill both the apple fruit and create next year's potential flowers and vegetative buds. One simple trick to control potential disease is to remove the galls found on cedars and junipers. That can be done now while easily seen and again in the fall when the spores have returned to the evergreen hosts where they look like a small gray brain on the evergreen branch.

The second and more damaging fungal disease known as Apple scab can both be aesthetically damaging and economically damaging for the home orchard. Apple scab only needs one deciduous host, the apple or crabapple. While CAR causes leaf damage, Apple scab actually causes the foliage to fall off the tree by early- to mid-summer and can disfigure apples too, so the damage to the apple tree is two-fold. There is a loss of leaves so the tree looks as bad as the crabapple, but the bigger part is without leaves the tree cannot produce a quality apple. Apple scab fungus overwinters on fallen leaves and returns to the crabapple or apple tree during the same period as CAR. If trees are being treated for one disease, the other also is treated. Sanitation can help control the disease by removing infected fallen leaves in the fall.

Both diseases are airborne, so sanitation is helpful (removing the CAR galls and raking up fallen AS infected leaves), but since both are spread by spores in the wind, treatments are likely to still be needed. Flowering crabapples can be treated with a fungicide only product. Apple trees are typically treated with a combination product containing a fungicide for leaf diseases and an insecticide for apple tree insects because no one wants to find a worm in an apple. As long as the current weather pattern continues, preventative sprays are needed. As always, read and follow label instructions.

Richard Hentschel is a Horticulture Extension Educator with University of Illinois Extension, serving DuPage, Kane and Kendall counties. Stay tuned to more garden and yard updates with "This Week in the Garden" on Facebook at

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.