Pumpkin leaf disease
Mature leaf starts out with yellow freckles then spots turn larger and brown, finally whole leaf yellows
Harford County Maryland
We can't say exactly what the problem is with your pumpkin. If you can send us a digital photo, perhaps we could give a more authoritative diagnosis.
On the other hand, regardless of the exact problem, it is too late to correct it. Assuming that it is a fungal problem, it would help to know which fungal disease it is, so that we could recommend or help you find a more resistant variety.
Typically, fungal diseases can be prevented but not cured. Prevention involves engaging in a fungicidal spray regimen, beginning before any symptoms appear and spraying the plants every 7-10 days throughout the growing season.
One of the worst fungal diseases is downy mildew, which has appeared in Maryland this year. The following page provides more information about this disease: