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Gail Ruhl, Plant Disease Diagnostician
Each year we receive phone calls and samples inquiring about cupping and curling of tulip poplar leaves on well-established trees. Similar symptoms are absent on neighboring trees and shrubs. This foliar symptom is most often attributed to root stress brought on by a variety of noninfectious, environmental and site related factors. Damage to tree roots from moisture and temperature extremes, construction, grade changes, underground pipe work, compaction (as when a tree is near a driveway), establishment of flower beds in the root zone, or poor soil conditions all contribute to cupping of foliage. Damage to cambial tissue from temperature extremes, as well as stress brought on by hot, dry winds may also contribute to leaf cupping and curling. It may be possible to diagnose the causal factor(s) through the process of elimination and begin corrective treatment. Make sure the tree receives at least one inch of water per week, and fertilize according to the instructions given in the bulletin HO-140, Fertilizing Woody Plants (PDF 60K).
Be aware that although no other plants in the vicinity of the tulip poplar trees may be observed to exhibit similar foliar symptoms it is important not to automatically rule out growth regulator-type herbicides. If the neighboring vegetation consists of plants that are less sensitive to growth regulator-type herbicides than tulip poplar, leaves on those other plants may not exhibit the cupping symptoms. Another possible causal factor of leaf cupping might be aphid feeding. Upon close inspection of the underside of symptomatic leaves, if cupping is due to aphid feeding, the aphids or cast skins should be visible.
Plant and Pest Digital Library and Digitally Assisted Diagnosis, Purdue University, West Lafayette, Indiana.