The 13-year legal battle over portable toilets and clean water for farm laborers has taken a new twist, with a federal appeals court's decision to dismiss the suit filed in 1973 by a migrant workers' advocacy group.

The ruling last week by a three-judge panel of the U.S. Court of Appeals for the District of Columbia in effect nullified an Aug. 10 deadline facing Labor Secretary William E. Brock, who had until then to decide whether he would issue standards requiring farmers to provide toilets and clean water for more than 500,000 farm workers.

An aide to the secretary said yesterday that Brock, who had been expected to announce his decision this week, now will not deal with the issue until he returns from a vacation in mid-August. At his Senate confirmation hearings last spring, Brock said that he favored a field sanitation standard and that he would reconsider the department's earlier rejection of the standard.

The lengthy battle over the standard, which includes demands for a rule by more than 100 members of Congress and heavy lobbying against it by agribusiness and farmer groups, appeared to have reached a climax in April when the department announced it would not issue a standard.

Robert A. Rowland, then head of the Occupational Safety and Health Administration, said the department would not issue a rule because 13 states that employ two-thirds of the country's farm workers already had field sanitation standards of one kind or another.

"We're back to where we were in 1972, when we petitioned the secretary of labor to require sanitary facilities," said Charles Horwitz, attorney for the Migrant Legal Action Program. "But we're still hopeful Secretary Brock will hold firm and not back down to pressures."

Horwitz said that his group will petition for a rehearing before the full appeals court in an effort to overturn the panel's ruling. "Our position is that the secretary still is obligated to make a decision by Aug. 10."

Brock had been under a district court order to rule on the sanitation standards by July 10, but the deadline was extended a month after he said he needed more time.

Under a settlement reached in July 1982, the secretary was committed to draw up standards and to hold hearings to determine the extent of health problems suffered by farm laborers as a result of inadequate field sanitation.

Although medical specialists described serious health problems among farm workers, Rowland said that the various state regulations were adequate. The former OSHA chief also said his agency was so short-handed that it probably would be unable to enforce federal rules even if they were issued.