Lettuce has brown spots

[Gary Wong]

Hi all,

My lettuce has developed brown spots, wondering how I might fix it.  New system, 2-3 mos old, first crop.  Added liquid seaweed and seeds, then fish.  Was fine for a bit, but recently the lettuce has started to look like the pic below.  Any recommendations appreciated!  

[Rob Nash]

Ive seen this happen when there were tomatoes in the greenhouse with the lettuce. 

apparently the ethylene gas can cause spots like this..

[Gary Wong]

Hmmm, this is outdoors, no tomatoes in growbed only lettuce.  In Hydroton, new IBC system.  Should I try iron or more liquid seaweed?  I did lettuce in my 55 gallon barrel drum system and no brown spots on those.

[Jon Parr]

Is it possible that it is frost damage?

I've had similar conditions on slow growing winter lettuce, too many cold nights. There is chlorosis on the lettuce in the background, so low iron or hi pH may he culprit. Fortunately, it is just lettuce, and the new growth looks okay, so I'd make sure the pH is 6-7, and it will probably grow out fine.

They are pretty crowded, too. Crowding leads to poor air circulation and mold/blight.

[Gary Wong]

Thanks for the help.  65 degrees at night, 80 during day, but yes, there were a couple of cold nights in the past month where it might have gotten to 60 or so.  I just tossed seed in there, so maybe I'll try to thin the group.

I'll hold tight for now and see how the new leaves turn out, I think the coldest nights are past me.

Thanks again.

[Roger]

Are there any signs of pests?  I didn't see any in the picture, but this looks similar to thrip damage I've experienced in the past.  And yes, you definitely can't go wrong by adding a little bit of iron.  I use Maxicrop plus iron.  I have a 100 gallon FT and add about 60 mills to the FT twice a month.  It won't hurt the fish.

[Gary Wong]

No signs of pests.  I added a little iron and am in wait and see. Some of the older leaves are brown, some seem brown and wet, I just peeled them off.  Water is about an inch below the top of the media, constant flood.