Prince George's County Council Chairman Francis W. White and five other incumbent Democratic officials were purged yesterday from the slate of candidates the powerful party organization will field for the Sept. 12 primary election.

White, a veteran councilman who ran on the party ticket in the last two elections, was dumped this year because of his use of funds raised at testimonials for personal expenses, according to sources on the organization's slate making panel.

Also purged from the Democrats '78 slate were Sheriff Don Edward Ansell, County Council members Darlene Z. White and Samuel W. Bogley; 23rd District Del. Perry O. Wilkinson Jr. and 25th District Del. Decatur Trotter.

All other incumbent Democrats except Del. Leo Green, who has never been part of the party organization, were endorsed by the panel.

The rejection of the six incumbents did not come as a surprise. All had been aware for several weeks that their positions were in jeopardy because of past disputes with the party leadership.

Although the nine-member selection panel was said to be acting on its own, most county Democrats acknowledge that the two party leaders - County Executive Winfield M. Kelly Jr. and Maryland Senate President Steny H. Hoyer - have a major say on who gets on the slate.

Darlene White had been at odds with Kelly for two years because of her repeated absences from Council meetings and her open promotion of a controversial hospital in southern Prince George's. Bogley, Wilkinson and Trotter were out of favor with the state senators from their district. Selection committee sources said Wilkinson also was hurt by an article in the Washington Post that detailed what one panel member called "his embarrassing antics" as a delegate in Annapolis.

Of the six rejected incumbents, all but Bogley have indicated they will run against the organization slate in the primary. Darlene White said she was thinking of running for a delegate seat in the 26th district.

The selection committee, which included one of Kelly's top political aides, John McDonough, chose two of the county executive's assistants for slate positions in the council race. They are John A. Lally, Kelly's press secretary, and Deborah Marshall, executive director of the county's Commission on Women. Lally and Marshall will run for the at-large seats held by Francis White and Bogley.

Ann Lombardi, a public health activist and wife of Orphan's Court Judge James Lombardi, was selected to fill Dariene White's position in the 4th Council district.

Wilkinson, the 39-year-old son of a former Maryland Speaker of the House, was passed over in favor of Marion Hoffman, a 44-year-old Bladensburg resident who heads the United Democratic Women's Club of Maryland. Syivania Woods Jr., a 24-year-old Glenarden councilman, was named to fill the delegate slate position held by Trotter, who plans to challenge incumbent Tommie Broadwater for the 25th district senate seat.

Ansell, the controversial sheriff who was recently acquitted on charges of misapporpriating funds from a Deputy of the Year testimonial, did not appear before the selection panel, which chose James V. Aluisi as the sheriff candidate. Ansell is expected to run as an independent or republican this year.

The selection panel also neamed political newcomers to fill positions that have opened up because of retirements and resignations.

In the 29th district, Del. R.W. Dovan was selected for the senate post abandoned by Hoyer, who is running forlieutenant governor. To fill the delegate seats held by Donovan and Craig Knoll, who is not seeking reelection, the panel selected Dennis Donaldson, a recreation director in District Heights, and John J. Williams, a teacher at Fairmont Heights High School.

The slate endorsement for the 22nd district delegate seat held by Ann Hull, who accounced her retirement last month, was given to Richard Palumbo, a member of the Democratic Central Committee.

For the Orphan's Court judgeship held by James Lombardi, who is returning to private law practice, the panel selected Steven Platt, an Oxon Hill attorney who has served as president of the county Human Relations Commisstion.

In the Bowie-bound 24th district, former assistant statee's attorney Anthony McCarthy was picked for a delegate seat now held by Leo Green, who is challenging incumbent Edward Conroy for the state senate post.

In Prince George's County, where the Democrats control every elected office, the Democratic primary is considered as important as the general election. And, since 1970, when Hoyer and attorney Peter O'Malley assumed control of the organization, getting a position on the party slate has been considered the single most important step toward gaining public office.

The last time around, in 1974, the organization slate carried all but three contests in the primary. Ironically, two of the three maverick Democrats who beat the slate that year - Del. Kay Bienen andstate Sen. Thomas O'Reilly - sought and gained the organization endorsement this year. Only Green decided to challenge the dominant slate again.

The leaders of the county Democratic party say that their pre-primary slate-making is a major reason why they have been so successful in recent general elections. "Our own history has shown that when we fight among ourselves in the primary, the Republicans beat some of us to November," said party chairman Lance Billingsley in the recent interview. "It is a matter of survival."

Those candidates who are embraced by the slate enjoy a common pool of funds, campaign workers, bumper stickers and brochures, all of which are particularly important to candidates who are trying to make themselves known to the 250,000 registered voters in the sprawling suburban county.

"Without a slate," said John McDonough, the organization's campaign orgainzer, "most of these candidates would be lost in the crowd. A slate works to all our advantage."

There have been indications in recent weeks that the Prince George's Democrats are looking at the 1978 general election with more concern than they did four years ago. The expected entrance of Republican former U.S. Rep. Lawrence J. Hogan into the county executive race apparently has shaken some Democrats.

In the District Heights area, for example, Del. Lorraine Sheehan spent several months preparing to fight Del. B.W. Donovan for Hoyer's old Senate seat. "I decided not to do it when I heard Hogan was running," she said. "With that guy in it, we've can't fight among ourselves. We've got to pull all our energies into keeping him out of the county executive's post."