Crab Apple Tree Fungus

The crab apple tree has beautiful pink flowers in the spring.

Crab apple trees are ornamental flowering trees that produce fruit in the late summer and early fall. The flowers of the crab apple are generally pink and appear before the leaves in the spring. In general, this tree will grow from 15 to 25 feet tall in full sun and well-drained soil. According to Landscape America, the crab apple is susceptible to several types of fungus that can damage your tree.

Life Cycle

Most fungus that infects your crab apple tree winters nearby and begins to grow in the spring. Apple scab is caused by a fungus that lies dormant throughout the winter in the leaves and then releases spores in early spring when the leaves are wet. The spores are carried to new apple leaves, spreading the fungus. Cedar-apple rust winters in the galls on cedar trees. Wet weather and wind carry spores to nearby crab apple trees in the spring. According to Kearneysville, symptoms will occur 10 to 14 days later. Powdery mildew winters on infected leaves and generally appears late in the season when the days are warm but nights cool. According to the University of Connecticut, it does not need water to grow.

Symptoms

Apple scab symptoms include brown or olive green fuzzy circles that appear on your leaves. Eventually these spots will become black, and the crab apple leaves will turn yellow and fall from the tree. Cedar-apple rust is characterized by bright orange lesions on the leaves and the fruit of your crab apple. Powdery mildew will leave a white, powdery coating on the flowers, leaves and fruit of your crab apple tree in humid weather.

Chemical Treatment

Spray your crab apple tree with a fungicide every spring after the flower buds emerge but before they bloom. Repeat the application every seven to 10 days until the wet season is over and the danger of fungus growth is greatly reduced. In the case of cedar-apple rust remove any cedar trees in a two-mile radius of your crab apple tree if possible. Neem oil has been shown to be effective against powdery mildew.

Prevention

Keeping your tree healthy is the best way to prevent any fungus from damaging your tree. As soon as possible rake up and destroy any leaves that have fallen from your crab apple tree. Don't wait until the leaves age and become brittle; these leaves are harder to rake up and still may harbor fungus. Prune your crab apple tree in the late winter to improve circulation among the branches. This will help your tree's leaves dry more quickly in wet weather and prevent the growth of fungus. Prune away any suckers or water spouts growing near the base of the tree as these will drain your tree's energy and weaken it, making it susceptible to disease.

Resistant Cultivars

Several cultivars of crab apple tree have been developed with resistance to fungus. Ann E., Bob White, redbud and Sinai fire are all resistant to apple scab. Adams, Bob White and Donald Wyman are resistant to powdery mildew. Adams, Mary Potter and snowdrift are cultivars of crab apple resistant to cedar-apple rust.

References

• Purdue University: Apple Scab of Flowering Crabapples
• University of Connecticut: Horticulture Integrated Pest Management: Apple Rust Disease
• University of Connecticut: Horticulture Integrated Pest Management: Powdery Mildew on Ornamentals
• Kearneysville: Plant Disease Fact Sheet: Cedar-Apple Rust
• Clemson University: Cooperate Extension: Apple and Crabapple Diseases