Tree owners and those responsible for maintaining trees often observe leaf drop occurring in mid-summer. Sometimes the number of leaves falling appears excessive, but in reality, the number of leaves lost is small in comparison the overall number of leaves in the tree. Some suggestions for the cause or causes of mid-summer leaf drop include:
- Drought: Trees may lose as many as 10% of their leaves during a drought without being irreversibly affected. Typically, once drought conditions are in place, leaves begin to drop and continue to drop for the extent of the drought period. While a tree uses leaves to make food, this amount of leaf loss does little or no harm, and may actually be helpful to the tree. By shedding these leaves the tree loses less water through transpiration.
- Adjustment to summer conditions: Sometimes trees just make too many leaves! When cool moist spring weather turns to hotter, dryer summer conditions a number of leaves may drop suddenly. In Ohio, this may happen sometime in mid-June to mid-July. This is called "physiological leaf drop" and does not harm the plant's health.
- Inner leaf drop: The observer should look carefully at the tree. If the falling leaves are from the inside of the tree they may have been "shaded out". Inner leaf drop occurs when the leaves on the outside and top of the tree are so thick that the leaves inside the tree do not receive enough sunlight. After such leaf drop, the larger branches inside the tree and close to the trunk look bare. Inner leaf drop is normal and not harmful.
- Shading: If leaves can be lost due to shading from within a single tree, then it follows that leaves can be lost when an entire tree is shaded. Trees are living things and a tree may grow so large that it begins to cast shade on another tree which was once in sunlight. The smaller tree no longer receives enough light to support its leaves which begin to drop. Another form of shading occurs when closely planted trees grow so large that they begin to shade each other on the sides that face each other. Often, this leads to leaf loss on the sides of the trees where they are in close contact or are intermingling branches.
- Insect damage: Leaves may fall when they have been damaged by insects. Most types of trees can be affected by one type of insect or another, few insects affect many types of trees. In Ohio, a common example of an insect which causes leaves to drop is the maple petiole borer which appears most years in May and June. Although leaf drop from an affected maple may seem dramatic, the leaf loss lasts only a week or so and then is over for the year and the number of leaves lost is not significant to the tree. On oak, there are various leaf mining insects that cause small amounts of damage. Aphids that attack tree leaves during the summer can also cause early yellowing and leaf drop. This is most common on tuliptrees, oaks, birches and some maple species.
- Sooty molds: Some insects excrete honeydew, a sugary substance in which various molds (some of them quite dark) may grow; these are known as sooty molds. In Ohio, these insects are usually aphids or scale insects, but leafhoppers, planthoppers and spittlebugs are also common culprits. If the honeydew excretions and the sooty mold coating are heavy, leaves may drop because light cannot penetrate the coating of mold to reach the leaves. In essence, a coating of sooty mold is another form of shading. The sooty mold does no direct harm to the tree, but the observer may want to determine the source of the honeydew. The insect excreting the honeydew may be so plentiful that damage is also occurring from the feeding activity of the insect.
- Leaf diseases: Leaves may fall after they've been infected by various leaf spot diseases. In Ohio, several anthracnose diseases may cause leaves of maple, black walnut, oak or ash to fall in early to mid-summer. Leaf drop usually appears sudden and dramatic. On crabapple, apple scab may cause leaves to drop all season, leaving a sparse-looking tree by the end of the season. On sycamore, powdery mildew may cause leaves to fall from late summer onward. Usually, the number of leaves lost to these types of disease is not significant. If ALL the leaves are lost, the observer might consider treatment of the tree the following year.
- Fatal diseases or injuries: Leaves may fall if a tree has been seriously injured or is in the process of dying. A lightning strike may cause leaves to fall and may lead to death of the tree. Dutch elm disease (of elm) or Verticillium wilt (of maple) may lead to leaf drop. In the case of fatal diseases, the observer will note that the entire tree is affected, that healthy leaves do not remain on the tree, and that the branches and leaf buds are dead or dying.