Here we go again.

On Friday, September 11, regulators seized Corus Bank of Chicago.

It’s the biggest bank failure to date with its roots in the commercial real estate and construction loan market. And it marks the start of the long-feared next stage of the banking crisis.

The first stage was fueld by bad loans in the residential real estate market–mortgages to individual home buyers. The next stage will take down bank that made bad loans to real estate developers.

This time most of the damage will be done to small and medium-size banks with big exposure to local commercial real estate markets.

Such as Corus Bank.

Corus, owned by holding company Corus Bankshares (CORS), had made $3.9 billion in condominium construction loans in overbuilt markets such as South Florida. More than half of those loans had stopped making payments or were in foreclosure, the company disclosed in April. The portfolio had continued to deteriorate since then.

Corus is/was an extreme case: construction loans accounted for more than 85% of the bank’s outstanding loans at the end of the first quarter. That’s a higher percentage than at any other U.S. bank with more than $100 million in loans outstanding.

But while the degree of its exposure to the sector pushed Corus over the brink early, the bank’s failure is just the first of many to come fueled by the problems in the commercial real estate sector.

Banking analyst Meredith Whitney of Meredith Whitney Advisory Group has projected that 300 banks will fail in 2009. Corus is the 91st failure of the year so far. (For more on Whitney’s projections see my post, “Home prices could fall by another 25% this year”  from September 11.)

The deteriorating commercial real estate sector could trigger an additional $100 billion in losses at more than 900 small and midsize banks if the economy continues to struggle, according to data complied by the Wall Street Journal.