Virginia Corrections Director Robert M. Landon resigned after a series of escapes and inmate disturbances had eroded his influence with Gov. Charles S. Robb and a newly assertive state Board of Corrections, a key legislator said today.

"He was finding it increasingly difficult to run the department," said Del. Franklin M. Slayton (D-Halifax), chairman of a legislative subcommittee that oversees the corrections budget.

Robb announced Landon's resignation Friday and said the major reason was Landon's admitted inability to overcome problems in the department following the widely publicized escape May 31 of six death-row inmates from the Mecklenburg maximum-security prison in Southside Virginia.

The resignation announcement, which was to have been made public Monday, was advanced after the escape Thanksgiving Day of five inmates from the Nottoway Correctional Center, about 45 miles southwest of Richmond. All five inmates, including a Falls Church man who was serving life plus 235 years for rape, sodomy and abduction, have been captured.

"The bottom line is, over a period of time, I just couldn't extricate myself from the death-row escape at Mecklenburg," Landon was quoted as telling United Press International today.

Slayton said Landon decided to offer his resignation earlier this month after a corrections board report not only sharply criticized management practices that led to the Mecklenburg escape, but also suggested the board needs more staff and control over the department.

Landon believed those changes "would create an intolerable situation for him or anybody else" attempting to run the department, Slayton said. As a result the director "found himself in an interminable position" of increasing isolation within the Robb administration and new demands from the corrections board, the legislator said.

"Bob may have found after talking to[Secretary of Transportation and Public Safety Frank]White that the administration was going to accept the findings of the board of corrections," Slayton said.

For example, Landon was not present at a briefing for Robb of correction problems by the corrections board. Landon was briefed later, after members of the news media had been given copies of the report.

Slayton noted that the corrections board, despite new powers given to it several years ago by the legislature, "had been relatively inactive until after Mecklenburg."

The latest escape, which raised questions about staffing and design at the four-month-old, $25 million Nottoway facility, was the most recent in a months-long series of inmate problems that have preoccupied Robb and top officials of his government.

The prison issue, including an August inmate uprising at Mecklenburg that ended peacefully only after the intervention of outside mediators and the threat of force by Robb, has drawn increasing fire from Republicans and some Democrats.

Robb has appointed Allyn R. Sielaff, currently the deputy secretary of transportation and public safety, to replace Landon for the remainder of Robb's term as governor, which ends in January 1986.

Robb acknowledged that appointing a new director was not the end to what he said were institutional problems that had been years in the making and probably will take years to resolve. Slayton praised Sielaff today but said that "it's been a long time since he was on the front line" as a corrections official in Wisconsin and Ilinois.

Slayton said the move would give the administration time to address corrections issues without being distracted by questions about Landon's future.

Slayton noted that Senate and House committees have scheduled a joint hearing in December to address issues raised in the corrections board report. "I think they're going to find these institutions[the prisons]are not being staffed as they should be," Slayton said.

Slayton said he talked to Nottoway Warden Dave Garraghty last July before the medium security, 500-bed facility opened, and that Garraghty said it would be understaffed.

"They're not getting the kind of resources they need. [The wardens]also feel they are being muffled in their administrative requests to the legislature," Slayton said.