Marrows

Question

My marrows are turning yellow at end of young fruit.

Answer 1

It may be improper pollination or blossom end rot. These articles will help you:
https://www.gardeningknowhow.com/edible/vegetables/squash/squash-fruit-falling-off-the-plant.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/squash-blossom-end-rot-causes-and-treatment.htm