Is there a powdery white substance on those leaves? If so your pumpkin has powdery mildew a fungal infection encouraged by stressed plants suffering drought, crowding, over feeding and in areas of high humidity. This usually occurs after a heat wave or from overhead watering. Good air circulation and not using too much high nitrogen plant food helps prevent this fungus among us. Powdery mildew grows best on fast growing foliage often on plants that have been overfertilized. A slow release plant food like Osmocote helps to prevent fast growth.  At this point I would water the roots but try to keep the foliage dry. You can also cut off the worst of the infected foliage and an application of Neem Oil could help. Even if the foliage dies the pumpkins that have formed may still ripen on the vine.  Next year don't let the plants dry out, give them room, use slow release plant food  and use a mulch on top of the soil to ensure even moisture. Keep growing, Marianne Binetti