Grapes struggle in the Houston area. The problems with your vine sound like a foliar fungal disease like black spot and possibly either root damage from drought or excess soil moisture. I suggest you take a sample of the foliage to your Harris County Extension Office so they can identify the type of grape (bunch type, muscadine, etc.) and try to diagnose the problem better. They should have some free grape growing info on hand as well. In the meantime, keep the soil moist but not soggy wet.
The Extension office's contact info is on the following web site:
Thanks and hope this helps.
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