Answer: Based on your description of the white substance, it may well be a cytospora canker infection possibly combined with something else such as spider mites or a fungal infection. I would strongly suggest you work with a professionally trained and certified nurseryman, or a professionally trained and certified arborist to get a specific diagnosis, once you know what the problem/s is/are then you can figure out how to proceed. Unfortunately it is very difficult to make this type of diagnosis long distance. But you need to find out what it is specifically in order to treat it, many pests can only be controlled successfully if treated with the appropriate chemical applied at the right time of year. I'm sorry I can't be more specific for you.
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