Baking soda works well as a safe and inexpensive fungicide and insecticide. It is actually registered with the EPA for use against certain plant fungi, powdery mildew. [See: Baking Soda Natural Fungicide] But there's actually an added benefit to using baking soda on your tomato plants, in most cases it will actually make tomatoes sweeter. Home grown tomatoes are already miles ahead of store bought ones so far as taste and quality is concerned but why stop there - better is always , well ... better.
Sprinkle a modest amount of baking soda around the base of your tomato plants, about 1/8th cup per plant. The baking soda aka sodium bicarbonate will seep into the soil and lower its acidity making your harvest less tart and hence sweeter.
Don't over do it as the baking soda does effect the soil pH, if the soil is already quite alkaline, you could excessively alter it. [Optimal soil ph for tomatoes is 6.0 to 6.8]