Consumers, troubled by inflation, have turned markedly pessimistic about the economic outlook. The Conference Board and Citibank reported in separate surveys released yesterday.

The Conference Board's monthly survey of consumer attitudes showed a sharp drop in confidence in May. The confidence index dropped 8.5 points from April to 88.5 in May, the lowest level since last October.

The index that measures consumer buying plans dropped to 95.3, off a remarkable 15 points from April. The Conference Board, a private business research organization, said plans to buy homes and automobiles are off the most.

"The rapid pace of inflation, with further increases expected, is clearly deflating the consumer's spirits," commented Fabian Linden, director of consumer economics at The Conference Board.

"The heating up of inflation is viewed by consumers as a sign of trouble ahead," Linden said. "Recent history suggests their instincts are sound" he added, and noted that the last sharp bout of inflation was followed by the 1974-75 recession, the steepest business slump since World War II.

The latest Citibank survey of consumers showed that only 19 percent of the 1,403 respondents felt the economy would get better during the next six months, while 60 percent thought it would worsen.

Citibank said the degree of pessimism has increased markedly since January 1977 when only 22.7 percent indicated doubts about where the economy was headed.

Consumer sentiment is now following a pattern similar to 1973-74 when confidence hit its lowest ebb of the decade, the nation's second largest bank said.

Most of the pessimism was laid directly to inflation, with higher food prices cited most frequently, followed by energy costs.

The Conference Board survey, which is conducted for it monthly by National Family Opinion, Inc., by Toledo, Ohio, and which polls 5,000 families, found only 25 percent of its respondents labeled business conditions as "good," down from about 29 percent in April, and 33 percent at the beginning of the year.

Looking ahead, more than 13 percent of the families said they expect business conditions to worsen during the next six months, up from 9 percent in April and 5 percent at the beginning of 1978.