Can we fix the title? It is not the original headline, and it implies that the weed killer is giving US workers in 50 countries Parkinson's.
Corrected: U.S. workers say weed killer banned in 50 countries is giving them Parkinson's.
Corrected: U.S. workers say weed killer banned in 50 countries is giving them Parkinson's.
I think that generally the accepted parse is the greedy one where modifiers are as close to their objects as possible. So "Banned weed killer" gets compressed to an object X. That leaves us with "US Workers say X in 50 countries is giving them Parkinson's". How would you interpret that?