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There are a number of conditions in gardens that may affect the appearance of a plant, but not pose a serious threat to its overall health. These conditions have a variety of causes, from environment and climate, to insects and bacteria or fungi. Usually control measures are not necessary or even possible. However, an understanding of the origins of some of the more common nuisances can eliminate much of the anxiety when a plant appears to be in distress.
The occasional leaf that falls from a tree during periods of active growth is not a cause for concern. However, when the ground beneath a tree becomes covered with fallen leaves, then there is probably an insect at work. In fact, there are two insects whose eating habits cause leaf drop.
Maple Petiole Borer
The preferred host of the maple petiole borer is the sugar maple and occasionally the Norway maple. The larva of this small wasp bores into the leaf stems (petioles) in mid-spring, and lays its eggs near the leaf blades. The white grubs that hatch will tunnel inside the leaf stem for 2-3 weeks, weaken the stem, and cause the leaf to drop to the ground. Infestations of petiole borers are not predictable and spray application control methods are difficult to time. However, since the health of the tree is not affected by the leaf drop, chemical controls are not necessary.
When leaves fall while still attached to twigs, you may suspect an insect called, appropriately enough, the twig pruner. Most ornamental trees can be hosts for this insect, a beetle whose life cycle will, at various stages, damage a twig to the point of separating it from the tree. Adults chew a hole in the bark at the tip of a twig as a site to deposit an egg. Emerging larvae bore into the twig and feed on the wood as they tunnel toward the base of the twig. Larvae are full-grown by late summer, at which time they cut through the twig wood and completely sever it from the tree.
There are a number of bacteria and fungi that cause leaf spots on the foliage of a wide variety of trees, shrubs, and herbaceous plants. However, they generally do little or no harm to the leaf. Leaf spot diseases often appear initially as small round or oval dead areas, scattered over the surface of the leaf. Color may vary from yellow, to purple, to brown or black. Texture can be either smooth or rough, and the spots can appear as flat on the surface or slightly raised. Some leaf spots drop out of the leaf altogether, leaving a ragged hole behind. Because an exact identification of the causal agent is difficult, control methods are not usually recommended. While affecting the cosmetic appearance of the plant, leaf spot does not generally harm healthy plants.
As a tender, young leaf emerges on a tree in early spring, it is highly susceptible to enviromental stresses until it has fully expanded and developed its natural waxy protective coating. In the spring, late frosts and high winds may cause damage to patches of cells in this soft tissue. As the leaves continue to expand, the young growth shows ragged edges, tears, and holes, which fortunately become less noticeable as the tree matures. Leaf tatter is unattractive but not harmful to the overall health of the affected tree.
Leaf miners are the larval stages of large number of different insect species. What they have in common is that they live in and consume the inner layers of leaves on a variety of plants, which can be easily identified by the conspicuous white or brown “trails” that appear on the leaf surface. With over 300 species of leaf-mining insects, precise identification is challenging, if not impossible. Vegetables, annuals, fruits, perennials, and woody ornamentals are all potential hosts. Leaf miner damage can be seen in all but the coldest winter months. In herbaceous perennials and woody ornamentals, an infestation is unsightly but not lethal. Young vegetables, however, can be severely damaged. Removal of any affected leaves is the best form of control.
As its name implies, sooty mold appears as a charcoal-gray/black coating on the leaves, needles, fruits, and branches of host plants, and can even appear on landscape structures and furniture. A fungus, this mold lives on the clear, sticky substance known as “honeydew,” which is produced by a number of insects, most commonly aphids, immature scale, or other sap-sucking insects. While unsightly, the coating will not kill the host plant, although a thick enough covering can block out enough light to prohibit or interfere with photosynthesis. If the infesting insect is identified and control measures undertaken to remove the pest, sooty mold can be reduced or eliminated. Otherwise, molds can be removed by rubbing the leaf surface to expose the underlying green tissue, and the base “honeydew” itself can be washed away with a strong stream of water.
Earwigs are most active at night and will feed on a wide variety of materials, leaving a ragged appearance on the foliage it consumes. Trees and shrubs are rarely affected by earwigs. Flowers and vegetables, particularly sweet corn, are most often affected, but the damage is not fatal to the host plant. Indoors, earwigs can typically be found hiding in any dark, confined space such as stacks of newspapers, under potted plants, or in cabinet corners. Good sanitation to avoid providing earwigs with nesting or hiding places is the best control method.