Marrows

It may be rot of the blossom end of the fruit. Blossom-end rot is caused by a lack of calcium in the developing fruit. Either the plant is not absorbing enough calcium from the soil or the soil doesn’t have enough calcium in it to start with. The solution is to get a PH soil test kit and test your soil (follow kit instructions). If it is lacking calcium, the solution would be to add calcium to your soil.

If the test shows the soil is okay, then you can increase nutrient uptake to the roots of the plant by mulching and by watering well.

Wouldnt be becasue of to much water as they drink loads and it hasnt rained much! Hope this works...

Thanks Nicky it is rotting from the blossom end, will get a test kit, Peter