For nearly two decades Evan Dooley quietly made a living trading commodities like wheat in his home state, Tennessee, far from the hurly-burly of Wall Street.

But on Thursday Mr. Dooley, 40, became the talk of the financial markets when MF Global, the giant commodities brokerage firm, accused him of making unauthorized trades that led to $141.5 million in losses for the firm. Mr. Dooley, the firm said, wagered on wheat futures with money he did not have.

Mr. Dooley, who worked in the firm’s Memphis office, had bought as many as 15,000 wheat futures, the equivalent of about 10 percent of the market for these contracts for any given month, company officials said. MF Global discovered the trades early Wednesday morning and ran up the losses as it desperately unwound the positions in a volatile market.

“This is a very disappointing situation for us,” said Kevin Davis, the chief executive of MF Global, in a conference call with investors and reporters on Thursday. The company’s stock plunged nearly 28 percent.