I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
You can prune dogwoods to a single trunk but wait until late winter to do so. Your description doesn't bring clarity to which specific species you have. I suggest you contact your local County Extension Office to see if they can view a sample or a digital photo and identify them more specifically. Then any pruning and other treatments will be easier to prescribe.
The ivory discoloration may be early signs of iron chlorosis due to the typically high pH soils in the Plano area. I suggest using Miracid and other such fertilizers for acid loving plants to help remedy this condition.
Thanks for the question. Please stop in again soon!
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