The decision follows three rejected takeover offers of the Swiss-based company by Monsanto, the latest of which was valid as of Tuesday 25 August.
On Tuesday Monsanto increased its bid to Syngenta from €415 per share to €435 per share, an offer which valued the comany at more than €40bn.
According to Reuters, Syngenta shares fell more than 18% on news that Monsanto dropped the higher offer. Monsanto shares jumped more than 8%.
Monsanto, the world's largest seed company, said it still believes in the value of a combination, but will focus on building its core business and meeting long-term growth objectives.
"Without a basis for constructive engagement from Syngenta, Monsanto will continue to focus on its growth opportunities built on its existing core business..." the company said in a statement. The company also said it was resuming a share buyback program.
Commenting on Monstanto's decision, Syngenta said it unanimously rejected Monsanto's latest proposal this week, as it significantly undervalued the company and was fraught with execution risk.
It also said certain key issues were not addressed by Monsanto in sufficient detail to allow Syngenta to make a proper assessment of the proposed new entity, which would have been 30% owned by Syngenta shareholders.
"We engaged with Monsanto in good faith and highlighted those key issues which required more concrete information in order to continue a dialogue," Michel Demaré, Chairman of Syngenta said in a statement.
Any merger of Monsanto and Syngenta would create by far the largest seed and chemical company in the world, with a turnover close to €30bn. Combined they would control about 40% of both the seed and chemical markets.