my lemon cucumbers sprouts are dying

Hi All,
I am trying to grow lemon cucumbers this year. I got a bunch of beautiful sprouts pop up.
But, the two large leaves on the sprouts have started to get light brown patches, like circles. Eventually the whole leaf turns brown and falls off leaving me a stem. Eventually the stem dies too.
Too much water? Too little water? Too cold? I am baffled.
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