Melt butter in a large saucepan over medium heat. Add onion, garlic, curry, and cumin; saute, stirring often, until onion is soft and fragrant.
Stir in apples, pumpkin, broth, water, and sugar. Bring to a boil, stirring often. Cover, and reduce heat to low. Simmer for 25 minutes, stirring occasionally.
Puree soup in a food processor or a blender.
Return soup to saucepan; reheat, covered, over low heat.
