Answer: Monarda is often attacked by mildew, but in my experience the symptoms of that infection primarily affect the foliage rather than the stems as you described so I do not think that is the problem based on your description. (It can be controlled with applications of horticultural oil every two to three weeks according to the label instructions. ) It is possible that you are describing "yellows" a particularly nasty problem in that it is not easily treatable. Here is an illustrated description of that http://www.agric.gov.ab.ca/agdex/600/30000001.html for you to compare with your plants.
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