A little hard to tell but looks suspiciously like powdery mildew in the later stages of the leaf's infection. Use a diluted milk solution. High moisture levels could be contributing. Also what zone are you in? It's a bit late in the season for northern zones to successfully plant pumpkins.

Often, this is caused by lack of pollination. When a female flower is open, rub the pollen from a male flower into it.

Hell-o Tropictype and Rugosalover, thanks for your help. In case you are interested with what I am doing with my pumpkins; paper bags and newspaper will melt in the Mt. View rain which falls almost daily this time of year. I am using worn out nylon stockings to cover my fruit. The only way the fruit fly can introduce their larvae to the pumpkin is through the open female flower. Since bees carry pollen from the male flower to the female, keeping the pumpkins fruit fly free also keeps the bees from pollinating. So we must be aware of when the female flowers are opening for the first time. Early in the morning I find the newly open flower, hand pollinate it, fold the flower closed, slide the nylon "sock" over the fruit and close the bottom with a twisty tie.

I thought I would let you all know I was in SouthEast Idaho trying to grow this pumpkin.