I would take a cutting from the rosebush and take it to a local university extension office or seek out a consulting rosarian in your area, they can look at the cutting firsthand to see what might be going on. This could be a bacteria problem that is getting started or it could be normal as some thorns are a dark maroon color and may appear brown. Better to take a cutting in and have it looked at than to try and treat the unknown.
Answered on April 12, 2015
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