Yellow foliage at the top of the plants could be a sign of cold weather or root damage. Too much water usually presents as lower leaves turning yellow, and too much fertilizer presents as brown tips on the leaves. You could have the dreaded blight disease brought on by cool moist weather. Snip out the infected parts and wait and see. Perhaps a change in the weather will solve this problem for you. If not, send a photo so I will have a better idea of what is going on. Keep growing, Marianne Binetti