​Question: ​Weeping willow, six years old and 9″ diameter. Wet, clay ground and thriving. Suddenly late last summer, I noticed the leaves were dead, and the bark on the trunk was completely loose and falling off. No other trees or shrubs within 50 feet (black walnut, sycamore, maple, rose of sharon) were affected. No obvious sign of insects, boring, trails, worms, etc.

Answer: When a tree starts to lose leaves, especially in the spring when they should be expanding for the new growing season, it can be puzzling. Often, this is a sign of troubles which can be caused by biotic or abiotic issues but not always a major cause for alarm. The dropping leaves can be a symptom of foliar diseases which weakens the tree, or it could be the result of an insect pest feeding on the petioles of the leaves. There are many pests which can cause leaf drop.

Another possible cause is the response to abiotic disorders which is typically a complex of issues. Leaf drop can occur on trees that have been exposed to prolonged wetness in heavy, clay soils. Some tree species like wet soils but not prolonged wetness without drainage. If trees are exposed to continual wetness, their roots can become diseased and cause the leaf drop. Additionally, it has been noted that trees which were planted improperly, especially if planted too deep, can result in several physiological issues such as decline and dieback.

If trees reveal symptoms of premature fall color, yellowing of leaves or unusual leaf drop, it may be necessary to send in a sample for diagnosis. The Purdue Plant and Pest Diagnostic Laboratory (PPDL) provides this service by a very capable team of pathologists, entomologists and extension specialists to analyze plant issues in the landscape. This is an inexpensive approach to investigating the issues and leading to the best possible curative measures. Protocol for submitting plant samples can be found on the PPDL website.

Resources
Hardwood Tree Improvement and Regeneration Center, Paula Pijut, Research Plant Physiologist, Purdue University
Relationships Between Advance Oak Regeneration and Biotic and Abiotic Factors, Songlin Fei, Associate Professor of Measurements and Quantitative Analysis, Purdue University
Diseases of Landscape Plants: Leaf Diseases, The Education Store, Purdue Extension Resource Center

Lindsey Purcell, Urban Forestry Specialist
Department of Forestry and Natural Resources, Purdue University