Answer: Based on your description, I would suspect either a root problem or possibly a fungal infection but it is very difficult to make this type of diagnosis long distance. I would suggest you consult with your county extension and/or local professional nursery staff to obtain a specific diagnosis of what is happening and then based on that, treat accordingly. If a chemical control is needed, your county extension will have the most up to date information on what to use and how/when is best to apply it. Unfortunately, without knowing exactly what is affecting the plants I can't tell you how to proceed.
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