Such symptoms point to one of 3 things: dry soil conditions, soggy wet (poorly drained) soil conditions, or some type of root rot. The best way to diagnose root rot is to either take a living by declining plant to your County Extension Office or to pull it and check to see if the roots are brown and dead more so than the top parts. Root rots are sometimes due to wet soil conditions but not always. There is often nothing that can be done but with some rot organisms you can drench with an appropriate labeled fungicide if you catch the problem early enough.
The only way to know for sure which root rot organism is involved and thus to know whether a fungicide will work, or which one to use is to have the problem diagnosed by sending a sample to the state plant pathology lab at Texas A&M. There is a $30 charge for the service. For more information on sending in a sample contact your County Extension Office or see the following web site.
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