Issue 4, May 27, 2014

Crabapple Scab

Crabapples are showing early symptoms of scab. The lengthy cool, wet conditions have resulted in an ideal environment for the disease. These conditions and high disease pressure will likely result in infections occurring to disease susceptible cultivars as well as resistant cultivars normally unaffected by scab. The highly scab resistant Prairifire crabapple is one example that I observed on the U of I campus.

Early symptoms of crabapple scab on Prairifire crabapple.

Apple Scab is a common fungal disease caused by the pathogen Venturia inequalis. The disease initially causes olive green spots on the foliage of apples and crabapples. The spots often form along or near the leaf veins, eventually developing a dark, velvety appearance.  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.

Scab infected crabapple leaves changing to a yellow-orange color before falling from the tree.

The pathogen overwinters on previous year’s leaves as well as on buds. Mild, rainy weather induces spore dispersal from fallen leaves which then cause primary infections.  Control strategies target these spores. Raking and destroying fallen leaves may provide some help by reducing the amount of inoculum present. Fungicide sprays can effectively protect developing leaves on susceptible cultivars. It’s too late to protect this year’s foliage from infection.  The first spray should be applied when leaves just begin to emerge from buds (about 1/4 inch green). Sprays must be continued according to label intervals until 2 weeks after petal fall to give maximum protection.

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. Look for it on the Web at (Travis Cleveland)

Travis Cleveland

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