Absolutely, it's purely a preventative measure, in making the pH on the leaf surfaces unreceptive to the fungal spores. So the sooner, and more often you spray, the better it will work. Since baking soda is soluble, it will rinse off if it rains, or the leaves are sprinkled.
As I said above, one of the best ways to keep these fungal things at bay is to keep the leaves of your plants dry as much as possible, so soaker watering systems that don't wet the leaves are a big help.
Air circulation around the plants is also very important in keeping the leaves dry. So be sure to space your plants far enough apart to allow the wind to blow . . .
