Answer: You might be on to something here. Individual plants need adequate growing room with plenty of air circulation all around to help keep them disease free. When plants are growing in a crowded space, air circulation is poor but also, the roots have to compete for enough moisture and nutrients. If possible, try to redirect the cucumber vines so they're not choking out the rest of your plants. You might even consider erecting a temporary trellis so you can lift the cucumber vines up off the ground and away from the rest of your plants.
As for the tomato plant with yellowing leaves, if they are the oldest (lower) leaves, it's perfectly normal. The oldest leaves will be dropped and new leaves will develop higher up on the plant. It isn't a good idea to plant tomatoes in the same garden spot year after year. If insects or diseases are present that are specific to tomato plants, your plants could pick these problems up year after year. Try to rotate your tomato, pepper and eggplant to different areas of the garden each year.
Best wishes with your veggie garden!
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