Preheat oven to 350.
Dissolve yeast in warm water in a small bowl.
In a large bowl, combine squash, milk, margarine, egg, brown sugar and salt and mix them well.
Add yeast mixture to squash mixture along with 1 1/2 cups flour. Mix well and add enough flour to form a soft dough. Turn onto floured surface and knead until smooth and elastic, around 6-8 minutes.
Coat a large bowl with oil, add the dough and turn it to coat with oil. Cover bowl and let rise in a warm, draft-free place until dough doubles, around 1 hour.
Punch down dough. Divide dough into thirds and roll each third into an 18 inch rope. On a greased baking sheet, braid the three ropes together. Pinch ends, cover and let the dough rise until it is almost doubled, around 30 minutes.
Combine beaten egg and water to make a glaze. Brush the glaze over the dough. Bake 20 to 25 minutes until the bread is golden brown. Remove from the baking sheet and cool on a wire rack.