Cucumber fruit turning yellow and withering from the blossom end can be caused by draughts, or the use of fresh manure but is usually due to poor drainage or overwatering.
The plants should be grown without a check to their development, taking care not to over water the plants. To correct the situation remove all the withered fruit and spray the plants with a foliar feed. Ventilate the greenhouse to the maximum to reduce the temperature and do not water the plants for a week but continue to damp down the pathway.
After this period continue to grow the plants in the normal way, they will then start to crop successfully.