Answer: Without seeing the leaves, it's hard to positively diagnose. It could be something as simple as sun scald or as complicated as a fungal disease called powdery mildew. If the areas do not have a talcum powder like coating, I'd chalk it up to sunscald. This is something that the plant will overcome on its own. If there's a white coating on the leaves, it's probably powdery mildew.
There are some things you can do to minimize its effects. Powdery mildew is unique among plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather. The first line of defense is to grow resistant varieties. Next, remember that while the disease is unsightly, it doesn't cause any real harm to many of its victims (though it will damage some plants.) Here are some general rules for helping control fungal diseases. Start by making sure that your plants are getting enough direct sunlight. (Eight to ten hours a day is generally the minimum for plants that flower or bear fruit.) You'll also want to make sure that there's enough room between plants for air to circulate freely. Overcrowding not only makes plants more susceptible to diseases, if leaves touch other plants, those diseases can easily be spread. The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, you can actually inhibit infection with periodic strong sprays of water (not so strong as to damage the plant.) After the new growth of affected plants reaches about six inches long and UNTIL THE TEMPERATURE REACHES 90 degrees F, you may apply dusting sulfur to foliage every 14 days. If necessary apply fungicide. Fungicide works much better as a preventative measure, before the powdery mildew becomes a problem.
Hope this helps you diagnose the problem with your tomato plants.
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