Chinese officials have issued warnings to seed dealers and farmers not to use unapproved genetically modified seeds in the country’s main crop belt, shortly after Greenpeace said it had found widespread GM contamination in corn. The unprecedented action by rural authorities in the past two weeks also comes as state-owned ChemChina agreed a $43 billion deal for seed and agrichemicals giant Syngenta AG (SYNN.VX), a move seen as bringing leading technology and know-how to China’s fragmented seed industry as it grapples with a divisive GM policy.