Why Are the Leaves of My Flowering Crabapple Not Green?

Aphids and Sooty Mold

• Aphids are well-known pests of crabapples. They are small, slow insects that feed on leaves and other soft plant tissue by sucking out the sap. While aphids can cause stunted growth and deformed leaves and buds, the color changes to leaves are due to the secondary fungal infection commonly known as sooty mold. When aphids feed, they leave behind a sweet, gooey substance called honeydew. This honeydew provides food for the fungus. Though the fungus is harmless, it looks like soot and can cover leaves, giving the leaves a black appearance. To treat the root problem -- the aphids -- eliminate nearby weeds and release populations of lady bugs or lacewings, usually available at gardening stores. Aphids can live on nearby weeds; lady bugs and lacewings prey on aphids.

Powdery Mildew

• Most trees are vulnerable to powdery mildew and crabapples are among those that are the most susceptible. Powdery mildew is a fungal infection producing millions of tiny white spores. These spores can cover the leaves, making them appear white. If you look closely, you might also see tiny black specks, which help some powdery mildew fungi survive the winter. Treat with a wettable sulfur spray at a rate of 2 to 4 lbs. per 100 gallons. Be aware this spray can damage tender leaves, particularly in hot weather.

Scab

• Apple scab is a common fungal infection in both apple and crabapple species. It gets it name from the scablike, brown or black lesions that develop on leaves. If the infection is left untreated, eventually the leaves will yellow and drop off prematurely. Several consecutive years of severe defoliation from scab can weaken crabapples, leaving them more vulnerable to other, possibly fatal, infections. Crabapple scab is primarily treated by destroying fallen fruits and leaves -- as the fungus overwinters there -- and spraying with the fungicide chlorothalonil according to package instructions.

Natural Fall Color Changes

• Some crabapple cultivars produce fall displays of orange, yellow, red or purple. Whether or not a crabapple tree's leaves change in the fall, and to what colors, depends on its cultivar, its sodium, iron, magnesium and phosphorus levels, and the acidity of the chemicals in the leaves. These factors are partially dependent on soil nutrients and pH.