THANKS TO THE generosity and determination of people who have known and enjoyed Wolf Trap Farm Park, the summer season of popular events is proceeding remarkably well--as is the staggering challenge of rebuilding the fire-ruined Filene Center. Now the White House pledge of help is official: from the West Coast came word that President Reagan has asked Congress to appropriate $9 million as a grant to help rebuild the center in the fiscal year that starts Oct. 1.

In addition, the administration is requesting a $9 million loan for the center--which should provide would-be contributing businesses with an even stronger incentive to respond with contributions now rather than later. The recovery of Filene has to be a joint venture of individuals, private businesses and the government, because no one group can shoulder the entire cost.

And as friends of Wolf Trap from all around the country know, this project is not a government giveaway of money for some wealthy arts patrons; on the contrary, the project is a federal building on federal land, owned and operated by the government in a park given to all Americans by Catherine Filene Shouse. That is why the administration is helping, and why Congress should respond quickly.