Bayer's shares have plunged about a quarter in value since August 2018, when the company lost a U.S. lawsuit claiming that weedkiller Roundup - acquired via its $63 billion takeover of Monsanto earlier in the year - causes cancer.

Wenning, who strongly supported Baumann's pursuit of Monsanto, said earlier on Wednesday he would step down at the company's annual shareholder meeting in April.

He will be replaced by Norbert Winkeljohann, the former head of Europe at audit and consulting firm PricewaterhouseCoopers, who joined Bayer's non-executive supervisory board in 2018 and did not play a role in the Monsanto deal.

"What speaks in favor of Winkeljohann is the fact that he can look at the Monsanto issue with fresh eyes," said Ingo Speich, a fund manager at Deka Investment.

"That can be a challenge for Baumann, particularly in the difficult phase that the company is in," he said, adding he did not think Baumann was in immediate danger of losing his job.

Janne Werning, in charge of environmental, social and governance issues at Union Investment, said Wenning's departure came at the right time.

"Mr Wenning was closely associated with the current difficult situation. It's now essential that Bayer's further strategic positioning is subject to independent oversight," he told Reuters in a written comment.

Both Speich and Werning were disappointed, however, that Winkeljohann has no expertise in running international healthcare or agriculture businesses.

That drawback is compounded by the fact another board nominee, the former finance chief of travel group TUI, Horst Baier, also brings no specific industry experience, said Union's Werning.

Deka and Union are among Bayer's top 20 investors, according to Refinitiv data. They declined to give the size of their holdings.

With the number of Roundup lawsuits rising, Bayer's top management faced an unprecedented show of shareholder disapproval at last April's annual general meeting.

The company is now in talks to settle the claims, with some analysts saying that could cost it up to $12 billion.

"As Mr Baumann has lost one of his most important supporters, his future in Bayer might be ... uncertain and linked to the share price development of Bayer after the settlement announcement," said Baader Helvea analyst Markus Mayer in a research note.

(Reporting by Ludwig Burger; Editing by Mark Potter)

By Ludwig Burger and Patricia Weiss

Stocks treated in this article : Bayer AG, TUI AG, Mayer Holdings Limited, Siemens AG