Thanks for writing. It sounds like your plants are both infected with some common fungal diseases. This is fairly typical on these types of plants as both are very susceptible to disease.
Your squash sounds like it has powdery mildew. There's no cure for powdery mildew, but you may be able to stop it from spreading to other leaves. Remove the most affected leaves and throw them in the trash. If you need to water your squash, do it from the bottom so the plant's leaves don't get wet. And prune or train the vine to ensure there's good airflow around it. If the powdery mildew continues to spread, look for a fungicide labeled for use on edible plants. (There are organic and traditional sprays.)
The same pretty much goes for your tomato. Again, remove the infected leaves --- and pull off any leaves you see develop spots. Keep the foliage as dry as you can. If you need to water, do so only in the morning or early afternoon.
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