Leaf curl can be a problem with a number of fruit trees grown in home orchards, including apple (Malus spp.). While cultural conditions can be to blame, a number of pests and diseases can also cause apple tree leaf curl. Eradicating pests that find refuge inside the curled leaves can pose a challenge.
Apple Tree Pests
A number of insects can cause apple tree leaves to curl, including the larvae of the apple leafcurling midge, which feeds on the edges of developing leaves. As a result, new leaves curl inward and eventually roll into tubes, harboring the midge inside. This makes it very hard to eradicate the larvae using insecticides.
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Curled leaves can also be the work aphids, which are soft-bodied insects that extract sap from plant leaves using special needlelike mouth organs. There are three species of aphids that affect apple trees: the green apple aphid, the rosy apple aphid and the wooly apple aphid. The feeding of these insects can cause the leaves to curl, which also protects the aphids. Young apple trees are more likely to be damaged by aphids than established trees.
You can try to dislodge aphids from apple trees using a stream of water from a hose. Broad-spectrum pesticides are not recommended for battling aphids, as these chemicals kill the beneficial insects that feed on aphids and can potentially make the infestation worse.
Types of Apple Tree Diseases
If you notice that the leaves on your apple tree are curled and have black lesions on the underside, the tree may be infected by the fungus Venturia inaequalis, which causes a disease known as apple scab. This disease can eventually cause premature leaf drop. You can avoid it by choosing cultivars that are resistant to apple scab. Powdery mildew, a fungal disease that causes powdery white patches to develop on the leaves of many trees and plants, can also cause new leaves to become curled.
Many fungi, including those that cause apple scab and powdery mildew, are capable of overwintering on plant debris, so removing fallen leaves can help prevent these diseases. Pruning trees allows air to circulate, which also helps keep fungal pathogens in check. The good news is that oftentimes, these diseases are a nuisance but are not fatal to the tree.
Other Apple Tree Problems
Curled leaves on an apple tree are not always caused by pests or diseases. If an apple tree is exposed to temperatures below freezing after the buds have set, the first leaves to emerge may be curled and distorted. Using herbicides – chemicals to kill unwanted plants, namely weeds – in close proximity to fruit trees can also result in curled leaves.
Other cultural conditions that could potentially cause leaf curl on apple trees include excess water, lack of water and excess fertilizer. Transplant shock, which is most likely to happen if you move an apple tree on a hot summer day, can also result in leaf curl, as can planting the tree too deeply or not deeply enough.
- Utah State University Cooperative Extension: Apple Aphids
- Better Homes & Gardens: This is Why the Leaves Are Curling on Your Apple Tree
- Washington State University: Apple Leafcurling Midge
- Clemson Cooperative Extension: Apple & Crabapple Diseases
- Missouri Botanical Garden: Apple Scab
- University of Maryland Extension: Apple and Pear Problems Not Caused by Diseases or Insects
- The University of Maine Cooperative Extension: Why Are the Leaves Curling on My Honeycrisp Apple Tree?