Idaho's economic boom up until last year is giving way to more moderate and sustainable growth because the state diversified its industry, KeyCorp.'s chief economist says.

That diversification makes it less vulnerable to economic swings, Jeff Thredgold said Thursday at Key Bank of Idaho's annual economic outlook conference. Key Bank is the Idaho subsidiary of Albany, N.Y.-based KeyCorp.Thredgold said more personal income is being derived from high technology, value-added manufacturing and services. Meanwhile, traditional natural resources, which suffered during the 1980s, have restructured and now are healthier.

Agriculture has enjoyed three strong years, which helped to restore the balance sheets of many farmers. Several commodity prices are lower this year, but they are high enough to generate profits, Thredgold said.

"The Idaho economy has such strong underpinnings that we will continue to see solid growth in 1991," said Robert O'Connor, chairman of The Idaho Co. and former chief of Idaho Power Co.

He said he based his optimism on commodity prices, loan activity and "the fundamental strength of the Idaho people. They are all doers."

C.E. "Gene" Hill, chief executive officer of US WEST Communications in southern Idaho, said his company has noticed a slowdown in growth.

Last year, the telephone company hooked up 14,000 new lines, with about one-third of the demand coming from California immigrants. This year, the number may fall to 10,000.

"I think it's probably a reflection of the timber industry and a little softening of the agricultural economy, and people know we are in a recession," Hill said.