The Jeffco Clinic, Arapahoe County Extension and a number of tree care companies have been fielding questions about failing catalpa. Leaves turn chlorotic, then wilt, turn brown, shrivel up and drop. In some cases, internal streaking is found in small twigs. Often, only one portion of the tree is damaged, while the rest looks normal (a healthy Catalpa is shown in the photo).
In consultation with Dr. Ned Tisserat, extension plant pathologist and Dr. Bill Jacobi, professor of plant pathology at CSU, it appears the problem is verticillium wilt. This fungal disease infects the conductive tissue, causing the symptoms described. The cool and moist weather this year was conducive to the development of verticillium wilt. One of the most confounding things about this disease is that the internal streaking caused by the fungus does not always have to be present. It is also difficult to culture. There is no chemical control for verticillium wilt. Branches that are dead should be removed and disposed of. Branches that show slight wilting should be left on to see if they recover. If a plant dies from verticillium, replace with resistant varieties such as conifers.
For more information about verticillium wilt, see pg 142 of Insects and Diseases of Woody Plants of the Rockies.