Leaves Yellow & Drop in summer due to not enough Water
Tulip Trees have relatively shallow roots, and are found in the wild in moist areas near rivers or streams. Consequently, they feel drought more readily than many other trees. They react to the stress by dropping their leaves. First the leaves turn yellow, sometimes showing brownish speckles on their surface between the veins, and then they turn brown and drop. Be sure the tree has sufficient water during periods of sparse rainfall.
Leaves Puckered, Sticky
Aphids - A small green aphid typically attacks Tulip Trees. Aphids are tiny sucking insects that cluster on leaves and stems to suck cell juices. Their feeding retards and distorts tree development, especially on the tender new growth that they prefer. As they feed, aphids exude sticky "honeydew" which coats foliage and sometimes fosters the growth of a sooty fungus mold on the leaves. Infested leaves curl, wilt and turn brown. Spray light, or \"superior\" horticultural oil on affected foliage in the spring before beneficial insects are in sufficient numbers to control aphids. Later in the season if major infestations develop, spray the aphids with neem insecticide 2 or 3 times at 10-day intervals. Be sure to spray the undersides of the leaves.
For more information see the file Controlling Aphids
Yellowed Leaves, Bumps on Twigs
Scale - The presence of scale insects is signaled by large oval orange or wrinkled brown bumps encrusting twigs, leaves and smaller branches. These waxy shells, about 1/3 inch in diameter, shelter small insects as they suck plant juices. Scale over winters on plants as partially grown insects, and new young "crawlers" appear in August just as the adults die. They attack lower branches first, moving upward on the afflicted tree. Heavy scale infestation causes leaves to yellow and the foliage canopy to die back. The insects secrete copious amounts of sticky honeydew that attracts mold. Before the buds open in early spring, select a warm day and spray dormant, or heavy, horticultural oil over bare bark and branches. This will smother over wintering scale. If scale appears after leaves are open, spray them and stems and branches with light, or superior, horticultural oil as directed on the product label.
For more information see the file Controlling Scale
Purplish Spots on Leaves
Gall Fly - Tulip Trees sometimes develop purplish spots about 1/8 inch in diameter on their leaves. They look like a possible fungal infection, but they are not. Although they are unattractive, they cause no major harm to the tree. There is no need to take any action other than to maintain the health of tree.
Gray Coating on Foliage
Powdery Mildew - Tulip Trees are susceptible to powdery mildew that is caused by a fungus that coats the leaves. Usually the gray blotches appear well into the growing season, causing the foliage to look unsightly. This disease is not truly harmful and can be ignored. If you wish to treat small trees for cosmetic reasons, spray the foliage with wettable sulfur once or twice at weekly intervals. Read and follow label instructions carefully. For more information see the file Dealing With Fungal Disease
Rotting Visible on Branches, Bark
Canker - Six different kinds of fungi are known to attack Tulip Trees and cause them to develop canker, or wood decay. Their airborne spores land at the site of an injury on a tree and germinate there. The only treatment is to prune away infected branches promptly and throw them in the trash so that the infection does not spread. To prevent recurrence spray valuable trees with copper fungicide. Do not spray it on new growth or during cool, damp weather. Take pains to keep trees watered and fertilized so that they enjoy optimum vigor and summon their own defenses against disease. Avoid injuring trees with yard care equipment.
Leaves Wilt Suddenly
Verticillium Wilt - Tulip Trees are sometimes victims of verticillium wilt. A common soil borne fungus disease of plants, it attacks tree roots below ground causing infection even before symptoms are outwardly visible. If roots on one side are infected, that one side of the tree dies. It is capable of badly damaging Tulip Trees in a residential landscape. Stress from drought makes them more vulnerable, intensifies foliage wilting, and hastens death. Sometimes verticillium wilt, not winter injury, causes tree death during the winter and the tree simply fails to produce leaves in spring. If a tree shows a mild infection, having only one or two wilted branches, try spraying its foliage with a powdered nitrogen fertilizer diluted in water or in liquid form every couple of weeks over the spring. This may not succeed in halting the progress of the disease, but it is worth a try. If a tree is severely infected, it cannot be saved. Cut it down and destroy the wood, do not compost or recycle it. Remove as many roots as possible and discard them too. Since the soil is infected, plant only trees that are not susceptible to this wilt disease such as broad or narrow leaved evergreens.