Shares of Apple bled more than 17 percent of their value in Monday morning trading on the Nasdaq stock market after analysts for investment banks Morgan Stanley and RBC Capital both downgraded their outlook on the company, citing a worsening consumer environment.

Morgan Stanley

Morgan Stanley analyst Katy Huberty cited three primary concerns while downgrading shares from Overweight to Equal-weight, and cutting her fiscal 2009 per-share earnings growth estimate to 6 percent — 9 percent below the Street's 15.5 percent consensus target.

First, she said, PC unit growth is decelerating to the point where the remaining growth opportunities largely exist in the sub-$1,000 market, a segment where Apple doesn't yet compete.

The analyst also outlined a new series of performance scenarios where the best case would have the company's per-share earnings growth decelerating "meaningfully" from levels reported at the end of the June quarter. She now expects December quarter per-share earnings to decline 8 percent year-over-year, compared to the 29 percent growth witnessed during the June quarter.

"Lastly, we believe multiples for high growth stocks will continue to compress in the current environment and, in the context of our 6% fiscal 2009 per-share earnings growth assumption, we donât believe Apple is immune to this trend," Huberty wrote.

She cut her price target on the Mac maker's shares from $178 to $115.

RBC Capital Markets

Over at the Royal Bank of Canada, analyst Mike Abramsky downgraded Apple shares to Sector Perform from Outperform, citing a "worsening consumer spending environment" that has led to reduced visibility to growth and margins, as well as elevated risk to valuation.

In particular, he pointed to his firm's most recent Changewave study that shows Mac purchase intentions suddenly moderating, with 29% intending to purchase a Mac laptop next 90 days (down from 34% in August) and 26% intending to purchase a Mac desktop (down from 30% in August).

"These are the biggest declines in 2-1/2 years," he wrote, adding that a separate Changewave study of 4,100 respondents revealed that 40% of consumers plan on spending less on electronics next 90 days, which is "the weakest outlook ever seen."

While Abramsky is a firm believer that Mac momentum remains strong, he modeled his 3 million unit September quarter estimate down to 2.9 million units, and said he sees elevated risk for disappointing guidance from the company for its holiday quarter.

Still, the analyst expects the company to report sales of 14 million iPhones in 2008, 24 million in 2009, and see its shares of the PC market rise to 4.1 percent by the end of 2009.

Abramsky, who cut his price target on Apple shares to $140 from $200, suggested the company could use some of its $21 billion in cash to repurchase about 5 percent of its outstanding shares "without significantly impacting its financial position and return to shareholders some of its expected strong fiscal 2009/2010 cashflow."