sounds like powdery mildew, the solution is feeding them, a strong well fed, plant can deal with it themselves. Stuff like Tomorite will help, I tend to go for more natural feeding. I have a water butt that I suspend a bucket of horse manure in, besides that I add manure, fish blood and bone and a little wood ash to the bottom of the hole when I plant, and use the same mixture to spread around the plants before watering. I don't know if it is possible to over feed a tomato, but I have tried and not succeeded yet
If you do go the commercial fertiliser route you might do well to include a seaweed extract, they bring in nutrients and trace elements that the others might not have.
People who ride horses often have to pay to get the manure taken away, I went on my local facebook asking if anyone had some and got three replies before the day was out.