In a skillet over medium heat, cook sausage with celery and onion until sausage is evenly browned, about 5 to 10 minutes. Drain excess grease, and set aside.
In another skillet over medium-low heat, combine tomatoes, tomato sauce, garlic powder, salt, and oregano. Simmer while preparing pasta.
Bring a large pot of lightly salted water to a boil. Cook pasta for 8 to 10 minutes, or until al dente; drain.
Preheat oven to 350 degrees F (175 degrees C). In a 3 quart baking dish, layer ziti, mushrooms, sausage, mozzarella cheese, and sauce. Repeat layers, and top with grated Parmesan.
Bake for 45 minutes in the preheated oven, or until browned and bubbly.
