Answer: I am so sorry about your trees. There are many possible reasons for this. Evergreens are subject to a wide variety of pests and diseases as well as environmental stresses such as drought and air pollution. Incorrect planting technique, poor maintenance, and inappropriate plant selection for the growing conditions can also result in dieback.
Unfortunately, based on the information in your description, I am not able to make a diagnosis -- this type of problem is very difficult to diagnose long distance. I would strongly suggest you work with your local county extension and also possibly with a professionally and academically trained and certified arborist with experience working with ornamental trees to obtain a specific diagnosis and determine how to proceed.
It is important to have a specific identification of the problem before deciding how to treat the trees or dec iding to remove them and try something else. 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 for the maximum results. For example, some disease or pest problems need to be sprayed for at a certain time of year and applying at the wrong time is completely ineffective. And, if it is an enviromental problem they should be able to suggest more sutiable replacements.
Good luck with your trees.
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