Answer: Pumpkins are subject to a number of foliar problems and diseases. Unfortunately, based on your description I am unable to identify specifically which one it is. When growing pumpkins (and related plants such as cucumbers, melons and squash) it is very important to clean up all plant debris each fall, rotate your crops, and select disease resistant varieties. It is also important to control pests such as cucumber beetles which can help spread infections. You might try removing the affected leaves and avoid working on the plants when they are wet, in case it is just a fungal problem. Apart from that, there isn't much you can do once they are infected.
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