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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.