BEFORE THE ashes had ceased smoking, the Wolf Trap Foundation was working to recoup its summer season from the loss of the Filene Center in a fire last week. Wolf Trap Associates, the 3,000-member volunteer arm of the Wolf Trap Foundation, immediately went to work, not only to carry out those plans but to aid the rebuilding effort as well. So while the show does go on, the volunteers will be working behind the scenes to raise money for a new Filene Center, and the "Wolf Trap Picnic Cookbook" will be part of that effort.

Cookbook sales would normally supplement this year's $50,000 pledge to support the Wolf Trap Opera Company. But Betty Spear, executive director of the associates, says, "While volunteers are working to meet this year's pledge by Oct. 1, 1982, proceeds from the sale of the cookbook will now be diverted to the rebuilding fund."

Sales of the 6-year-old cookbook, now in its fourth printing, usually contribute about $9,000 annually to the associates' total fund-raising efforts. In the past, extra money--not a huge sum, says Spear, since the pledge for the opera company increases each year--has gone to such projects as improving the gift shop where, according to Spear, "the return will be great enough to upgrade our support for the company."

More money will come from at least one Washington-area bookstore, which will contribute profits from Wolf Trap cookbook sales. Kathy Heavey, manager of Alexandria's Book Annex, says that the $2.80 made from each cookbook sold through its three stores, "for at least the next two weeks," will be donated to the Wolf Trap Foundation.

The "Wolf Trap Picnic Cookbook" not only inspires "the most simple picnics on top of ice chests" as well as the most elaborate affairs, says Spear, but also contributes to the welfare of the performers. Beverly Sills, Zubin Mehta and Elizabeth Taylor have dined on food donated, prepared and delivered by the associates' hospitality committee to the performers back stage. The food at many of the receptions and special dinners held at the park is derived from recipes in the cookbook.

The picnic attitude permeates all activities at Wolf Trap, says Spear, right down to the annual gala. Picnicking "has been the whole theme of this place," says Spear, "that's what people want and they'll see that it continues."

Anyone wanting to join the rebuilding effort by purchasing a cookbook may order directly from the associates by sending $7 per copy to Wolf Trap Associates, 1624 Wolf Trap Road, Vienna, Va. 22180.

Wolf Trap Associates contributed the recipes in the cookbook. Here are samples from them.

POTATO SALAD WITH BLUE CHEESE DRESSING (4 servings) 4 large potatoes, boiled, peeled and cut into cubes 1/2 cup diced celery 1/2 cup water chestnuts, drained and sliced (or substitute toasted, slivered almonds) 1/2 cup chopped scallions, including 2 inches of green tops 2 tablespoons minced fresh parsley 2 1/2 teaspoons salt 1/4 teaspoon pepper 1/2 teaspoon celery seed 1 cup sour cream 2 ounces blue cheese, crumbled 1/8 cup white wine vinegar

In a large serving bowl, combine cubed potatoes, celery, water chestnuts, scallions, parsley, salt, pepper and celery seed. In another bowl, mix the sour cream, blue cheese and vinegar. Pour over the potato mixture and toss lightly. Season to taste. Chill overnight. Serve on lettuce leaves and garnish with parsley and radishes.

GRAPE NUTS/OATMEAL BREAD (2 loaves) 1 cup Grape-Nuts cereal 1 cup rolled oats (quick or regular) 4 tablespoons honey 2 3/4 cups warm water 1 teaspoon sugar 1/4 teaspoon ground ginger 2 packages active dry yeast 6 cups all-purpose flour 3/4 cup dried skim milk 4 tablespoons shortening 2 teaspoons salt Melted butter

In a large bowl, combine the cereal, rolled oats, honey and 2 1/4 cups warm water. Let stand for about 30 minutes, until cereals are soaked and softened.

In a small bowl, combine 1/2 cup warm water, sugar, ginger and yeast. Let stand in a warm place until bubbling.

Combine the two mixtures and beat thoroughly. Stir in 2 cups flour and the dried skim milk. Add shortening, salt and 2 1/2 cups additional flour. Stir until dough clears the bowl. Spread 1 cup flour on a pastry board, turn out the dough and knead well, using a little more flour if necessary to make an elastic, non-sticky dough. Return to the bowl, grease the top of the dough, cover with a towel and let stand in a warm spot until double in bulk (35 to 45 minutes).

Turn out, knead the dough lightly, divide in half and shape each into a loaf. Place dough in 2 greased 9-by-5-by-3-inch loaf pans, brush the tops with butter and allow to rise until double in bulk (about an hour). Bake at 375 degrees for 45 minutes. Brush the tops of the loaves with butter again as soon as they are taken from the oven, and allow to cool for a few minutes before removing from the pans. Cool loaves on wire rack.

STRAWBERRIES WITH RASPBERRY SAUCE (5 to 6 servings) 1 quart fresh strawberries, washed, hulled and quartered 1/4 to 1/3 cup kirsch 10 ounces frozen raspberries, thawed, with liquid reserved 1 pint heavy cream

In a small serving bowl, toss the strawberries with the kirsch. In a blender, pure'e the raspberries and their juices. Pour over the strawberries and toss lightly to mix. Refrigerate 2 hours. Serve the cream separately, whipped or not, as desired.