Answer: The behavior is odd for a trumpet vine so I suspect there's some stress related to the affected plant. They typically are not bothered by pests or diseases, but there's an off chance that a fungal infection is causing the early dormancy. You might want to collect some of the leaves as they begin to yellow and die and take them to your local cooperative extension for microscopic examination. Or, the problem might be associated with root stress. Even though you treat both your plants the same, and they are growing close to one another, the soil may be slightly different for the affected plant. It might hold too much moisture, or is compacted and does not allow good moisture penetration. I'd first rule out a fungal disease by having the plant leaves examined. Then I'd dig down into the root area to see just how moist the soil remains a few days after watering. Between checking the soil and ruling out (or treating for fungal disease) you should be able to correct the growing conditions so your trumpet vine flourishes. Best wishes with your landscape!
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