cucumber collapse!

Planted up a few marketmore seedlings on the plot last week,protected with makeshift shelters.....they were fine and then one day one had just toppled over and died. Thinking it was just one of those things I planted another...only to find the same result next day!....the others are fine,it's just this station....it's like the stem has been almost severed at ground level......suggestions as to possible cause??.... before I sacrifice another seedling on the station of doom

Have similar problem with Marketmore, they seem to be rotting from the bottom ie near soil level they turn yellow for about half inch then fold over and die, this is my third batch so far this year that have done this.

Any ideas please

They can rot very easily - you shouldn't water anywhere near the stem - from the bottom is better or some people cut the bottom out of a plastic bottle and insert into the soil/compost upside down, away from the stem and water through that.  Its the same with courgettes and happens to me every year.