In the 1950s, farmers primarily saved banana crops from Panama disease by discovering that one species of banana tree, specifically the Cavendish banana, was resistant to the disease and then bred that species commercially. While there were other methods being researched, such as treatments and pest control, the widespread shift to cultivating the Cavendish banana was the most significant approach taken at that time to mitigate the impact of Panama disease. Therefore, the most accurate answer would be:
They discovered that one species of banana tree was resistant to Panama disease and bred that species commercially.