Pumpkin rotted

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I had a white tail bumble bee nest under the tool shed so no problem with pollination this year.
Marrows have done well with two or even three fruits on each plant then a bit of a lull when mildew set in followed by a second flush of leaves and fruits.
Turban squashes have about one fruit for every 10 green leaves and no mildew. The area around the original roots has lost about three quarters of the leaves but the vines have replaced these with new green leaves on the vines. There are about half a dozen fruits ready but keeping fresh on the vines and a few more set and swollen to full size.
Turbans abort at about pool ball size if there are not enough leaves on the vines.

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I have had a fruit rot on a turban plant.
It had got to full size before the stalk split along its length and allowed flies to enter the fruit taking rot in with them.
If there was a split in the stalk on your fruit it might be the same problem I had.