Shares of Green Tree Financial Corp. tumbled almost 9% Monday after its debt was downgraded by Standard & Poor's Corp., the credit rating agency.

Standard & Poor's revised its outlook on the company from "stable" to "negative," citing a highly competitive operating environment.

The manufactured housing lender tumbled $2 a share, to close at $20.50, on the news.

Green Tree shares have declined more than 49% since the company announced Nov. 13 that it would take a $150 million writedown to cover faster-than-anticipated prepayment speeds.

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