The little round spheres might be leftover fertilizer capsules, Dawn. Pick one up with your glove on and squish it between your fingers. If it's empty, that's what it is. If there's goo inside, put some whole ones in a little jar and see what hatches before trying to treat for an unknown beastie. It might be something beneficial!
As to the zucchini leaves, is it only the lowest leaves on the plant that are turning yellow? You've got some good fat stems in those pictures and if the new leaves are still healthy, I think I'd just cut off those yellow leaves and 'wait and see' if any more go. Plants do shed old leaves as they make new ones, and squashes grow so very fast, they can do this quite quickly too.
Bear in mind that growing a heavy feeding plant like squash in a bucket, you need to feed them and water them steadily. They can't reach their roots out into surrounding soil to find more food or water like they could if they were in the ground. As the weather gets hotter, they will need more and more water. Don't fret if they wilt a little in the heat of the day, but I'd be watering thoroughly both morning and night if they're doing that. So, more watering means you are dissolving the nutrients in the soil faster too, so fertilizer more often may be needed too.
