Leaf-drop diseases are varied and cause the leaves to fall from your tree when they should still be on the branch. From mild dropping of leaves to total defoliation, problems occur due to poor maintenance as well as infectious diseases or pests. Identify ways to prevent leaf drop, different pathogens that cause it and how to manage a problem once it has begun.
When to Worry
Leaf drop is a normal part of nearly every tree, even those that are completely healthy. Deciduous trees usually drop most or all of their leaves during the fall season. Though evergreens are known for keeping their leaves, even these trees lose a small amount of older leaves closer in toward the trunk of the tree, so a small amount of leaf litter is not a red flag. However, if your trees are exhibiting leaf drop during the incorrect season, or if the amount of leaves falling from the tree seems excessive, your tree may be suffering from disease, according to Clemson University Extension.
Sometimes disease is not caused by a particular fungal or bacterial pathogen, but a tree ends up in an ill state due to improper care or culture. Identify key needs for healthy trees including the required sun exposure like full sun or partial shade, the type of soil your tree thrives in, whether alkaline or acid and the level of necessary moisture. Trees are often tolerant to extremes, but "tolerant" is not the same as immune. A tree tolerant to drought may fare well for a week, but extended periods of extreme cultural conditions can cause illness. Grow your trees on appropriate sites within the USDA hardiness zones where they grow best to prevent leaf drop and to prevent attack of infectious diseases and pests that target weakened plants.
Pathogens and Culprits
There are many infectious diseases that contribute to leaf drop. For example, tar spots of maple trees are caused by the pathogen Rhytisma acerinum. Leaves often wither and drop due to this fungal infection. Frogeye leaf spot of crabapple trees, caused by the pathogen called Botryosphaeria obtusa, also causes leaves to yellow and subsequent leaf drop, according to Ohio State University Extension. Different strains of infections exist in a wide variety and affect a vast quantity of tree species. Determine the species of your tree and research diseases or contact a local county extension agent when in need of assistance. Additionally, infestation by insect pests can cause leaf drop.
Leaf drop is often preceded by other symptoms that will alert you to an underlying disease that may cause defoliation. Look for spots or blotchy areas on leaves, yellowing of leaves, growths on leaf surfaces or fungus on any part of the tree, cankers or areas of dead plant tissue, leaves that appear larger or smaller than normal or the presence of what appears to be mold or mildew, particularly on leaf surfaces. Leaf drop is often part of the damage that occurs after other symptoms make themselves present.
Fungicides like mancozeb, used to treat frogeye leaf spot, are formulated for use on specific trees to treat particular diseases, according to Ohio State University Extension. When you notice leaf drop, determine which disease is affecting your tree and from there, whether or not there is an appropriate fungicidal treatment. If your tree experiences an insect infestation, act in the same way by determining which pest is present and finding an appropriate insecticide. For certain problems, chemical controls may not be available; in this instance, either allow your plant to heal while employing cultural management or seek professional assistance to determine whether or not removal is your only solution. For all plants, collect and destroy affected plant parts to prevent the spread of disease.