Ask Mr. Smarty Plants

Tecoma stans (yellow trumpetbush) (Esperanza) is native to North America and to Texas, but most of the areas where it is growing are in far south and southwest Texas. It thrives in rocky, alkaline soils and needs watering only about every one or two weeks. We are not sure what kind of soil you have in your area of the state, but if  you have a clay soil, and have not amended the soil for better drainage, that could be the problem. While Texas has been uncommonly dry this year, you have probably been watering your garden, and water may simply be standing on the roots of the bush. If your Yellow Bells has been planted recently, continuing to water is probably important, but drainage is even more important. Work some compost into the soil around the roots and avoid watering with an automatic sprinkler system. Overhead watering (except rain) can cause problems for some desert plants. It also does not need fertilizer, since it is native.

We frequently find that chlorosis can cause yellowing leaves. In the case of the Esperanza, the overwatering of the roots may be preventing the roots from accessing some of the vital trace elements in the soil, especially iron, and this can cause chlorosis. In the case of Lagerstroemia indica, Crape myrtle) there may be the same problem, but in this case we have no information on it in our Native Plant Database because it is non-native. At the Lady Bird Johnson Wildflower Center we are committed to the growth, care and propagation of plants native not only to North america, but to the spot in which they are being grown. 

We checked on the possibility of leaf spot diseases, and found none that fit the description you gave. This Floridata site can give you more information on Lagerstroemia indica but, again, good drainage is about the only recommendation.  Both plants would likely benefit from composting and mulching, and both should be in full sun, 6 or more hours of sun a day.

Tecoma stans

Tecoma stans

Tecoma stans

Tecoma stans