As recently as December, Magna, a unit of Interpublic Group of Cos IPG -6.19%., previously forecast U.S. ad spending to grow 6.6%.

Magna said the current crisis is unparalleled.

“The closest historical equivalent would be a combination of the Great Recession and 9/11,” said Vincent Letang, Magna’s executive vice president of global market research. “This is not a normal economic crisis, it is far more complex. It’s a demand crisis, a supply crisis and a psychological crisis.”

Advertising is often among some of the first things cut by companies looking to reserve cash in times of crisis, because it is seen as discretionary spending within many corporations, some marketers said.

“Many CEOs and [chief financial officers] see ad spending as the honey pot that can be raided by the hungry bear,” said Joe Tripodi, the former chief marketing officer of the Subway sandwich chain and Coca-Cola Co. “I expect most brands will pull back somewhat on advertising.”

Magna is also cutting its TV ad forecast, due in part to the lack of sports programming. The National Basketball Association, National Hockey League and Major League Baseball suspended operations because of the coronavirus, while the NCAA canceled its men’s and women’s basketball tournaments.

Sports has won over a bigger share of ad budgets in recent years as it has been somewhat immune to the rating declines that have plagued entertainment programming.

Magna now expected spending on national TV ads in the U.S. to decline 13% this year, far greater than the 0.4% dip it had been anticipating in December. U.S. spending on digital ads is expected to grow a paltry 3.9% this year, a far cry from its earlier forecast of 11%.

Spending on U.S. print ads is expected to plummet 25%, while radio can expect a 14% drop, Magna said.

There is plenty of evidence that ad cuts are already hurting media and tech companies.

Earlier this week, Facebook Inc. said that its business was being adversely affected by the ad pullback, while Twitter Inc. said its financial performance would fall short this quarter as a result of the pandemic.

The New York Times cut its first-quarter advertising-revenue forecast at the beginning of March, citing a slowdown in international and domestic ad bookings because of uncertainty and anxiety about the virus.

Many ad agencies have initiated hiring freezes. Magna’s parent company Interpublic Group withdrew its full-year financial performance targets on Thursday due to significant macroeconomic uncertainty resulting from the coronavirus pandemic.

Magna said it expects the impact on marketing activities to be severe for sectors such as travel, restaurants and movie studios and significant for retailers, financial services firms and the automotive industry.

Any major pullback from those sectors is worrisome since those industries are among the top 10 ad spending industries in the U.S.

Last week, Darden Restaurants Inc., DRI -7.18%the owner of Olive Garden and other chains, said that it had “dramatically reduced” its advertising spending.

Retailers, the top spending sector, shelled out $16.9 billion on ads last year while the automotive industry spent $12.3 billion, according to ad-tracker Kantar. Travel and tourism was the sixth-largest spending sector, with $7.7 billion in outlays, while restaurants, the ninth-largest, spent $6.2 billion on ads.

Despite the downbeat forecast, Magna does see the ad market stabilizing and recovering in the back half of 2020. That prediction is based on the assumption that the U.S. will see a stabilization in coronavirus cases in April and a gradual return to business in May and June, the company said.

The firm also upgraded its 2021 forecast and now expects spending to increase 2.5% to $222.5 billion because the Tokyo Olympics are being moved to 2021. It had been anticipating a 1.4% increase for next year.

Mr. Letang wouldn’t speculate on what will happen if the coronavirus crisis is protracted.

“I am cautiously optimistic,” he said.