Hydroponic cucumber problem
 

I have been trying to grow cucumbers in a drip irrigation system in
rockwool. My first attempt was a dismal failure. I got some new seed
(spacemaster) which is supposed to be resistant to multiple diseases. I
am using general hydroponics nutrients at a pH of 6.0. Temperature is
80-90F and I have good ventillation. The problem last time (which
appears to be starting to happen with the new batch) is that the leaf
margins initially turn yellow-brown and this quickly spreads with the
leaves turning completely tan-brown and dying. Could this be a nutrient
problem or some kind of disease? I am using the recomended mixture of
GH flor nutrients at full strength. Is this too strong for seedlings. My
tomatoes and peppers seem to be doing fine in the same system. HELP! I
don't want it to happen again.