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This has two main causes. This can happen when there is too much nitrogen in the soil, or the pH is too low. You can offset this by adding a bloom formula, which will have potassium and phosphorus, but little or no nitrogen. If you believe your soil is very fertile already, and just a low pH, then you can add dolomitic lime to raise the pH. If that does not help within a year, then you will have to add fertilizers.
This article will help you on the care of these: https://www.gardeningknowhow.com/ornamental/vines/trumpet-vine/growing-trumpet-vines.htm