(By Jason Simpkins) The earnings season beginning today (Tuesday) is shaping up to be an important one, as it could have a significant impact on a struggling stock market rally.

Since the stock market rally reached a pinnacle nearly two weeks ago, the Dow Jones Industrial Average has lost about 3.3% while the Standard & Poor's 500 Index has fallen about 3.7%. And if this week's earnings report come in below expectations, the rally that helped stock prices surge more than 50% could come to an abrupt end.

Fortunately, many of the companies set to report earnings this week are traditionally strong performers and for the most part, companies that have weathered the financial crisis. But not all of them have met Wall Street's expectations.

The quarterly results for five companies in particular – Yum! Brands Inc. (NYSE: YUM), Alcoa Inc. (NYSE: AA), Costco Wholesale Corp. (Nasdaq: COST), Monsanto Corp. (NYSE: MON) and PepsiCo Inc. (NYSE: PEP) – will of particular interest to investors.

Yum! Brands Inc.

Scheduled to report today (Tuesday), the Louisville, Ky.-based Yum! will be one of the first companies to report its quarterly take.

As owner of the Taco Bell, Kentucky Fried Chicken (KFC) and Pizza Hut brands, Yum! is the world's largest restaurant company. Even more impressive, the company has beaten the market's consensus forecast in the last four quarterly reporting periods.

Analysts' estimates for the quarter ending September 2009 range from a low of 52 cents a share to a high of 63 cents a share, with a consensus of $0.59 a share.