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Crabapple Tree Fungus

June 20, 2018

While the ornamental crabapple tree is a common landscaping choice for homeowners, it is not unusual for trees to fall prey to an array of diseases and pests. Fungus problems are common. Iowa State University Extension and Outreach horticulturists offer tips on caring for crabapple trees. For more information, contact the ISU Hortline at 515-294-3108

What is responsible for the yellow spots on my crabapple leaves?
Cedar-apple rust is likely responsible for the yellow spots on the crabapple leaves. Cedar-apple rust is a fungal disease. The fungus requires both a crabapple or apple and a cedar (juniper) to complete its life cycle. Crabapple and apple cultivars differ in their susceptibility to cedar-apple rust. Some cultivars are susceptible, others are resistant.

On junipers, the fungus produces reddish brown growths (galls) that are up to golf ball size on young twigs. During rainy periods in spring, these galls swell and push out orange, gelatinous, tentacle-like structures. Winds carry fungal spores from these spore-producing structures to susceptible crabapple and apple cultivars. On crabapples and apples, cedar-apple rust produces yellow spots on the foliage. These spots often have orange-red borders. On the lower leaf surfaces, small, brown, spiky projections (spore producing structures) develop in the spots. Cedar-apple rust on crabapples and apples is most severe when there are frequent rains in spring.

Fortunately, cedar-apple rust does not cause serious harm to crabapples. No control measures are necessary.

My crabapple is dropping some of its leaves. Why?
The leaf drop is probably due to apple scab. Apple scab is a fungal disease caused by the fungus Venturia inaequalis. Cool, wet weather in spring favors apple scab development. Crabapple cultivars differ in their susceptibility to apple scab. Some cultivars are very susceptible to the disease, while others are resistant to apple scab.

Scab appears on leaves as circular, velvety, olive-green spots on both the upper and lower leaf surfaces. The spots eventually turn dark brown to black. The margins of these spots are feathery rather than distinct. Heavily infected leaves curl up, become distorted in shape, turn yellow and fall from the tree. Highly susceptible crabapple cultivars may lose a majority of their leaves by mid-summer. The premature leaf drop weakens trees, but usually doesn’t kill them. The damage is mainly aesthetic. Heavily defoliated trees are unattractive.

Apple scab may be prevented by the application of fungicides, such as chlorothalonil, from just prior to bloom until the middle of June. Infections are less likely to occur with the arrival of warmer, drier weather in early summer. For most home gardeners, however, controlling apple scab with fungicides isn’t practical. The best control for apple scab is to plant scab resistant cultivars. When purchasing a crabapple at a garden center or nursery, select a scab resistant cultivar.

Source: Iowa State University