If you're wondering why your crepe myrtle shrubs have developed brown leaves, the first thing to look at is the calendar. Reasons for crepe myrtle leaves turning brown are related to the time of year when the problem occurs.
Brown Leaves vs. Brown Spots
Crepe myrtle leaves turning brown—with the whole leaf turning brown or browning along the edges, and sometimes accompanied by curling—is different from the emergence of brown spots in the middle of the leaf. Such spots can indicate Cercospora leaf spot, which is caused by a type of fungus (Cercospora lythracearum) and is an entirely different problem. In the case of brown spotting, give your bushes plenty of space (to promote good air circulation) when you plant them, and prune off (and properly dispose of) affected branches (to prevent transmission of the fungus) to ward off Cercospora leaf spot.
Crepe Myrtle Leaves Turning Brown in Summer
If you are seeing brown leaves (or even partially brown leaves) on crepe myrtle shrubs at the end of a dry summer, the problem could be due to drought. These bushes flourish in the southeastern United States, where they are very popular due to their ability to withstand the summer heat. Nonetheless, they do need water, and they may dry up during especially hot summers with little rainfall. Northerners who grow the bush (it is not nearly as popular in the North) are far less likely to experience a problem with their crepe myrtles drying out. This is more of an issue for Southern growers, who garden under more extreme conditions in summer. If you suspect that drought is the problem, water the plants enough to keep the soil evenly moist but not wet.
Crepe Myrtle Leaves Turning Brown in Spring
If you are seeing brown leaves on your crepe myrtle plant in the spring, it could be due to your area having experienced a cold snap. Crepe myrtle is not as tolerant of cold weather as many bushes are. An untimely drop in temperature after the tender leaves have made their appearance can spell death for them. Short of growing your bush in a sheltered area (which requires foresight), there is not much that you can do to prevent the problem of browning in this case.
Will My Crepe Myrtle Die?
Here is the good news: Neither case of brown leaves (that is, either in summer or in spring) needs be fatal to your crepe myrtle. Browning due to a moderate cold snap ruins the appearance of the plant, but it probably will not kill your shrub. And while this bush is more likely to die from drought, it is usually easy enough to remedy the situation with artificial irrigation once you detect the first hint of browning.
Obviously, in the latter case, vigilance is of the utmost importance. Keep tabs on your plants. Inspect them on a daily basis. Stay ahead of the game. If your crepe myrtle shrubs do not seem to be performing as they usually do, start asking questions. Many plant problems can be solved fairly easily if you learn about them soon enough and take appropriate action in a timely fashion.