State Farm Mutual, the nations's largest automobile insurance company has become the major independent stockholder of the B. F. Saul Real Estate Investment Trust, a Washington-based enterprise with property interests across the country.

In another development, Saul Chairman B. Francis Saul II told stockholders of the trust yesterday that he expects agreement soon on a new book credit line. And that should be followed shortly by a resumption of dividends to stockholders -- last paid in fiscal 1974, before recession led most real estate trusts into trouble, he said.

A proxy statement for yesterday's annual meeting revealed the State Farm investment -- which would consist of more than 1 million shares, or about 16 percent, if outstanding debentures were converted into regular shares. State Farm actually owns about 550,000 debentures and 468,000 shares outright.

This total of potential shares upon conversion of debt investment actually surpasses the total for the B. F. Saul Co., which helped to organize the real estate trust and which itself has significantly increased its holdings of the trust in recent months to about 16 percent. Saul purchased 305,500 shares in November.

The Saul Co. owned 954,310 shares as of Dec. 1 -- virtually all of which are actual shares with only 130 shares that would be acquired through conversion of debentures.

Saul stock has been trading on the New York Stock Exchange in a range of $4 to $7.50 a share in the past year while the debentures owned by State Farm are convertible to shares of Saul at $23 a share and $15 a share. Saul closed yesterday at $7, up 1/8.

In material filed with the Securities and Exchange Commission, State Farm has stated that its interest in the Saul Real Estate Trust is for investment purposes only.

The actual disclosure in the proxy statement was mandated for the first time by new SEC regulations, requiring that any convertible debts owned by an investor must be translated into potential stock shares if the debentures were converted -- something State Farm apparently does not plan. The Saul debentures carry a good income yield for the auto insurer, which has similar debt investments in other companies.

One source said State Farm was upset by the new SEC rules because it is forcing proxy statements at some two dozen corporations to list the Bloomington, Ill., company as a major owner -- and there was concern that state regulators might not understand that the investment listed was not actually an equity share or representative of a bid for control.

State Farm investment officials were unavailable yesterday to comment on their Saul holdings but trust chairman Saul said, in response to a question at the meeting, that the insurer has no business relationship with the trust.

After several years during which the Saul REIT accumulated net losses of some $40 million, most stockholders yesterday were interested in the outlook for 1979 and beyond. And Saul was decidedly optimistic. He said:

The "most important goal" this year will be winning a new bank loan agreement. Currently, Saul has a credit agreement with 37 banks that carries an interest charge higher than the prime rate and which expires April 30.

Borrowings under this plan have been cut sharply to less than $48 million from $116 million at the start of fiscal year 1978. Saul said a new agreement is being negotiated with 11 banks for a $50 million credit line with "standard terms" and with "reason to expect" that there will be no restriction on resuming cash dividends.

To soften the impact of high interest rates, Saul will continue to pay down the bank loans from a positive cash flow that is expected to continue for the year and by financing properties with mortgages.

"Now we can go forward... the repairing of the dam is over," with plans to convert Saul's apartment units to condominiums on a gradual basis over four or five years, possibly bringing in enough profits to take advantage of the $40 million in tax-loss benefits Saul has accumulated -- upon which no income taxes would be paid.

The trust also is drawing up plans to expand some existing motel, shopping center and office development properties or to sell and lease some land parcels that have been accumulated. Saul has 64 income properties representing an investment of $212 million -- 44 percent of which is in shopping centers, 32 percent in apartments, 13 percent in motels and 11 percent in office or industrial parks.