Lettuce help needed

[Steve Robinson]

Can anyone advise me on this issue that's just come up in my DWC bed?  I've searched this forum as well as the internet and can't identify it.  Older leaves begin to wilt at the edges progressing until the entire leave turns white and dies.  New leaves seem to still be coming on and growing well.  System is 150 gallon FT with two 50 gallon grow beds alternately flooding for 15 minutes and draining 45 minutes.  GB's drain into a sump which pumps into two small (5 gallons each) DWC beds and drain back into the FT.  Each DWC bed has aeration.  System is in my garage which is currently about 68 degrees.  Don't see any obvious insects on the plant and it seems to be confined to the more tender varieties of lettuce.  Everything in the GB's look great and growing fast!  Today's readings are Ph 7.0, Ammonia .25 ppm, nitrites 0, nitrates 80+.  I also used a soil test kit and got a reading on the Phosphorus and potassium which showed medium to high on each.  Maxicrop was added a couple of months ago when the system was started. Only water and fish food have been added since.  Thanks for your help!

[David R. Russell]

I have seen this before in hydroponic lettuces with bibb/boston/butter and with romaine before.  Known as tip burn.  Causes are warm/hot temperatures with poor air circulation resulting transpiration not being adequate.  Air circulation and transpiration are needed for cooling and for allowing nutrient uptake for growth.  The lack of adequate transpiration can manifest itself similar to a nutrient deficiency.  Low calcium can also be associated with tip burn in lettuces.  Can also be caused by electroconductivity being too high on warmer days (the plants shut down transpiraton due to too much nutrient uptake with the high rate of transpiration that occurs with warm temps).  This is why hydroponic producers will lower Electroconductivity or nutrient strength in summer months.  I doubt that nutrient strength is an issue.  I assume problem is from too much heat and poor air circulation and possibly humidity related (higher transpiration with dryer air. less with very humid air. 

[Cory Cramer]

I'm having the exact same issue with a butter crunch variety, and briefly with a Romain, but that seems to have taken care of itself. I don't think heat was an issue with mine. They are in a cool room and I'm only using T12s. I do have knats too, but I'm thinking humidity and/or air circulation could be the main issue. Thanks for posting.