Answer: It’s so difficult to diagnose without seeing your tree in person. Make sure to contact a local arborist for a full evaluation.
Note, but do not overreact to, any yellowed, withered, eaten or mildewed leaves. All result from treatable pests and rarely prove fatal.
Munched leaves might indicate that the catalpa sphinx moth has found your tree. Treat the tree with Orthene during the larva stage of this infection, when you first spot a caterpillar. If left unchecked, however, nature usually takes care of the problem; nectar dripping from wounded leaves attracts other pests that feed on the moth larva, eliminating the infestation.
Powdery mildew appears on leaves as a white powder; treat the leaves with appropriate fungicides. Perhaps the most dangerous pest attack is that launched by rabbits, which will completely devour the bark of a young tree, girdling the stem. Protect your tree with tree tubes or fencing.”
More general info about catalpa trees: Catalpa trees can thrive in altitudes up to 7000 ft, making it an exotic tree for your Denver yard. Catalpas (also known as Catawba) are native to warm areas such as Atlanta and Houston, but are found throughout North America.
Catalpa are hampered by catalpa sphinx moth. The larva stage is the best time to treat this infestation, before it eats the leaves and robs the catalpa of energy reserves. Treat when you first see catepillars using Orthene.
Catalpa also is prone to fungus which might cause withered leaves. A trunk injection with ArborJet which contains an anti-fungal agent can help stop the tree from going into decline.
Powdery mildew is a common catalpa problem. It appears as a powdery substance on the leaf surface.