San Jose, Calif. ? Silicon Valley, get out your checkbooks.

The valley’s nonprofits and arts groups are facing the death of a generous corporate donor when Knight Ridder is acquired this summer, taking with it about $2 million in annual giving.

The San Jose media company, which owns the San Jose Mercury News, has been one of Silicon Valley’s leaders in corporate giving, and has been an especially important donor to hard-pressed arts groups, which are spreading the word that they need to find new donors fast.

“It’s a bitter pill, and I am sad,” said Irene Dalis, founder and executive director of Opera San Jose. “It’s a bad, bad blow for the arts development. We’re all struggling.”

Dalis said that since Knight Ridder moved here from Miami in 1998, it has given Opera San Jose $25,000 a year – $50,000 one year. Its 2007 donation was already in the organization’s budget, and now must somehow be replaced.

“It will be a significant loss to the nonprofit community,” said Debra Jones, development director of Downtown College Prep, which Knight Ridder has given $175,000 since 2000. “We’re hoping MediaNews will be as good corporate citizens as Knight Ridder was.”

MediaNews, which has agreed to buy the Mercury News, leaves philanthropic decisions up to its individual newspapers, according to MediaNews Vice Chairman and chief executive officer Dean Singleton. But the loss of Knight Ridder’s corporate headquarters will be a blow to San Jose, he acknowledged.

“Our newspapers are pretty generous,” Singleton said. “We’ll be generous because we believe in that, but it won’t match a corporate headquarters.”