Answer: The cause of suddenly excessive thorn production is rose rosette disease, which is caused by a virus. Unfortunately, there is no cure and the plants will die within a year or two. Infected plants should be destroyed. Cut the plant down to soil level (or make a fresh cut if it is already down to soil level) then paint the fresh cut with full strength vegetation killer (Round-Up or Triox). The herbicide will travel down to the roots of the plant and kill it off. If there are many large roots, and if you can get to them to cut and paint them, by all means do so!
Best wishes with your project!
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