Issue 5, May 22, 2009

Crabapple Scab Note

Have you looked at your crabapple tree leaves lately? The lengthy cool, wet conditions have resulted in more than the normal amount of crabapple scab this year. Many trees in the Champaign/Urbana area that usually have minor scab infection are already heavily infected and dropping some leaves.

Commercial applicators who treat trees annually for this fungal pathogen will look very good this year. It is, of course, too late to spray now. Fungicides are used as preventives and must begin as buds first begin to open. University of Illinois recommendations state that the first spray should be applied when leaves just begin to emerge from buds (about one-quarter inch green). This is to protect new leaves. Sprays must be continued according to label intervals until two weeks after petal fall to give maximum protection against ascospore infection. For now, make sure you can identify this disease and mark your calendar to take fungicide action next spring or consider a resistant cultivar.

Many crabapple cultivars have resistance to scab, and resistance is definitely the long-term solution to infection. If you are planting new crabapples this year, look for varieties with resistance to scab, rust, fire blight, and powdery mildew. A publication that may help is this reference by U of I professors Dave Williams and Gary Kling: Recommended Crabapples for Illinois Landscapes (Adobe PDF). When looking at resistance options, look at ratings that have been made for your geographical area.

Crabapple scab causes velvety growth on the foliage as you can see in the picture. It starts out most commonly along veins but eventually appears as black spots on the foliage, much like black spot of rose. Susceptible leaves turn yellow, except for these black areas. Foliage drops in mid-summer, giving infected crabapples thin canopies. Diagnostic labs can view the causal spores with the help of a microscope. Often incubation of infected leaves is not even necessary.

Although the disease does not kill crabapples, it certainly weakens growth and may predispose the tree to other disease and insect problems. Many websites are available on scab diseases. For University of Illinois details on this disease, consult Report on Plant Disease #803, Apple and Crabapple Scab.--Nancy Pataky

Nancy Pataky

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