Apple tree leaves curl for various reasons, including pests, diseases, and environmental conditions. The Apple Curl Aphid is a prime culprit when it comes to apple tree leaf curling, and they feed on the sap of the tree. They are small, green insects that consume the sap of apple trees, causing the leaves to cup and curl. Other pests and diseases, such as the Apple Leaf Curling Midge, can also cause leaf curling. Apple Leaf Scab, Powdery Mildew, Fire Blight and Bacterial Canker are all examples of diseases that can also lead to apple tree leaves curling. These diseases are caused by too much water or too little water, dry windy spells, temperature extremes, and nutrient deficiencies.
Drought Stress
Apple trees are particularly sensitive to drought stress, as they require an abundance of water to produce healthy foliage and abundant fruit. If the tree does not get enough water, it will take measures to try and conserve its resources, such as curling its leaves. The leaves will curl inward, conserving moisture like a living cauldron. The tree might also lose leaves and flowers to save on the strain of transpiring, or losing water through evaporation.
Winter Cold
Winter cold can also lead to apple tree leaf curling. Apple trees are cold hardy and can survive temperatures as cold as -20°C (or -4°F). But when the temperature goes below this threshold, it can cause the leaves to curl as a way of preventing any water vapour to escape. The curled leaves work as an insulator, protecting the tree from the cold.
Sunscald
Too much sun can also cause apple tree leaves to curl. Apple trees require lots of sunlight, but too much of a good thing can also be a problem. When apple trees are exposed to too much direct sunlight, the leaves on one side of the tree can dry out, curl and eventually fall off. To prevent this from happening, apple trees should be planted in well-shaded areas and given enough irrigation to remain healthy and vigorous.
Nutrient Deficiency
Nutrient deficiencies, such as a lack of nitrogen or magnesium, can also cause apple tree leaves to curl. When the apple tree lacks these essential nutrients, it takes measures to protect itself, such as curling its leaves. The leaves will curl inward, conserving moisture and energy. If the tree is not provided with the necessary nutrients, it might end up looking sparse and bare.
Tissue Injuries
Tissue injuries, such as frost damage, can also lead to leaf curling in apple trees. Certain varieties of apple trees are more susceptible to frost damage than others, and their leaves can suffer as a result. The leaves curl to protect the tree from further injury.
Improper Pruning
Improper pruning can also lead to apple tree leaf curling. Pruning apple trees should be done in the late winter or early spring, but if the tree is pruned during the summer or fall months it can cause the leaves to curl. Pruning during the wrong time of year can also make the tree more vulnerable to drought or winter cold.
Insect Infestation
Insect infestations such as thrips, mites, aphids and leafhoppers can lead to apple tree leaf curling. These insects feed on the sap of the tree, which can lead to the leaves curling and becoming distorted. If the infestation is severe, the tree may not be able to produce enough sap to support the foliage, leading to diminished yields.
Pesticides
Pesticides can also cause apple tree leaves to curl. Some pesticides are designed to kill pests, but may end up attacking the foliage of an apple tree as well. This can lead to leaf distortion and curling, as well as discolouration of the leaves. It is important to be sure that any pesticides used are specifically designed for apple trees, as some generic pesticides can harm the tree.
Fungal Infections
Fungal infections can cause apple tree leaves to curl and shrivel. Common fungal infections include powdery mildew, apple scab, and fire blight. These infections are easily treated with fungicidal sprays or powders, but if left untreated they can cause serious damage to the tree.
Excessive Watering
Finally, excessive watering can also lead to apple tree leaf curling. Too much water can saturate the soil and prevent oxygen from reaching the roots. The tree will take measures to protect itself, such as shedding leaves or curling them inward to conserve moisture. If the soil is too wet, the tree may eventually die.
