When the lowest leaves turn yellow the first thing I think of is too much water. When it comes to zucchini and tomatoes in the month of May the first thing I think of when foliage does not look healthy is temperature. If we have cool nights below 50 degrees these heat loving veggies will throw a bit of a fit by dropping some of the oldest leaves. A chilly but not freezing night will also make lower leaves on tomatoes turn yellow and also a bit purple on the newer foliage. Move the potted plants close to the house on cold nights, grow them in the hottest spot of the garden and do not water until the top one inch of soil is dry to the touch. Young plants do not need as much water as mature plants. Make sure the pots never sit in drainage water as poor drainage also shows up as yellow leaves. The good news is that summer is on the way and warmer nights and drier days means your tomatoes and zucchini will be looking happy again soon. Keep growing, Marianne Binetti