Ask Mr. Smarty Plants

Redbuds and many other trees normally drop some leaves early when drought-stressed.  This is an adaptation to reduce the need to take up moisture from the dry soil.  Don't worry too much if the branches are still alive (scrape off the bark on a defoliated twig to see if it has green and moist tissue) and if 50 % of the leaves remain green.  New leaves will appear next spring.

If a great number of leaves are turning yellow, the trees may still be drought-stressed.  Drought stress can show up even a year or two after a drought because part of the root system has not yet recovered.  If that is the case, you should make sure that the soil around the trees remains moist.  Deep watering throughout the root zone (out to the drip line) once a week will assure that the trees are not short of water.  Mulching the area under the trees will help conserve soil moisture.

Plant nurseries around the country sell several varieties of redbud.  Cercis canadensis var. texensis (Texas redbud) is more drought-resistant than Cercis canadensis (Eastern redbud) and less drought-resistant than Cercis canadensis var. mexicana (Mexican redbud).  The seed pods of Texas redbud tend to be reddish in color while the Eastern redbud seed pods remain green.  That may help you identify yours.

Drought stress would make the trees more susceptible to disease.  It is possible that you trees are not drought-stressed but suffering from verticillium wilt.  Here is a web site on this subject. 

Good luck in keeping these attractive trees alive!