potato bean vs yam bean

potato bean

yam bean

noun
  • A tropical vine, Pachyrhizus erosus, grown for its crisp, starchy edible root, the jicama. 

  • Any of the plants in the genus Pachyrhizus, including several similar cultivated species. 

How often have the words potato bean and yam bean occurred in a corpus of books? (source: Google Ngram Viewer )