We have a couple of plants in our tomato garden that have yellow leaves with...
We have a couple of plants in our tomato garden that have yellow leaves with tiny grey and black spots. some of the stems have dark places where it intersects with the leaves. These plants already have fruit about 2 inches in diameter that are not ripe, but look fine so far. There is no sign of insects on the foliage. Could overwatering be the problem? Our other tomato plants look fine. Should we remove the infected plants or use some kind of chemical?
Unfortunately, without at least a photo of the problem you're describing, there's no way I can even attempt to tell you what might be wrong. We have a whole loose-leaf binder filled with photos of the different diseases that can affect tomatoes, and many involve the symptoms you're describing. If we can see a photo of the leaves you're referring to, and take a look at the darkened tissue area, we might be able to help.
In addition, it's perfectly normal for the lower leaves on tomato plants to turn yellow as they age, and they will often develop spotting that may or may not be a sign of any disease.
Quite often, overwatering will cause the leaves to curl with the outside edges rolling downward; a few days of dry conditions will cause them to go back to normal.
Vegetables in general do NOT need more than an inch to an inch and a half of water weekly. You can measure this, if you're using a sprinkler, by putting an empty cat food or tuna can with the lid removed in the middle of your sprinkler pattern. Leave the sprinkler running for an hour, then measure how much water is in the can. If it's half an inch, then doing the math would lead you to know that you need to set that sprinkler out in that location and run it for 2 hours to get the necessary inch of water for the week. You should also have a rain gauge (available very inexpensively at most lawn and garden centers and big box stores) near your garden, so you can see how much weekly rainfall your garden is getting, and adjust your watering accordingly.
If you have a smartphone, you can take some photos and send them directly to my regular email if you'd like -- that's email@example.com. I'll be happy to work on a diagnosis if I can have something to look at.