What are your water tests and pH saying?
Is this happening on all the leaves or just the old ones?
Pictures of deficiency of plants and what kind of deficiency
Thanks for the info, Johann there isn't cucumbers listed on that link but it's great info for future use. My AP system is outdoors getting full sun, so it's not the lights. My tests are all coming back "normal" I'll test again this morning just to be sure...and it's just on the older larger leaves, not all of them.
I'm glad to hear it doesn't seem to be a fungus, that was my main concern.
You say your tests came back normal. What is "normal?" What is your system pH? Have you ever used any maxicrop? How old is the system? What is the nitrate level? I know of many aquaponics systems with vastly different water test readings and they are likely to have different issues based on what their "normal" is.
I have a system with a normally high pH and I regularly have trouble with iron deficiency. In a system with now well buffered water and a very low pH calcium or phosphorus might be more of a problem. If you have never dosed with maxicrop you might be seeing potassium deficiency or other trace deficiencies.
I have a small outdoor system where the cucumbers did well, untill a "catastrophic event" both times, which I equate to nutriet loss due to heavy planting. Unless you have tests for all the micro-nutients at hand, it could be one of many that has run out. Mine literally hit a brick wall both times - going from healthy and fruiting to misearable and dying in days. It was not Nitroge, but with the plant load I had, I concluded that when the micro nutrients run low, the cucumbers are the first to show signs of distress.
The picture you added looks a lot like what I saw. Did the leaves turn brown on the spots that are gone in the photo?
Just seeing this post, but thought I'd add my two cents. I grew cucumbers last year and as a few people have said, they did really well in the beginning, but then seemed to 'hit a wall'. I got a pretty similar results on the leaves. I was doing some research and think root rot may have been the answer.
When planting in soil, cucumbers (and other cucurbits) are supposed to be planted on mounds of dirt. The extra elevation help keeps their roots a bit drier and hence avoid root rot. In my system, due to a design flaw, there was perhaps 2 inches of standing water at the bottom of the 12 inch deep growbed all the time. I think the plant grew well until their roots got to a point where they hit that standing water, then root rot destroyed them.
Guy this looks like Corynespora blight to me. I get this quite a bit on my cukes and cucurbits some seasons...I think in the US you call it "target spot" or "target leaf spot" or something...I'm trying preventative foliar potassium bicarbonate this year...hope it works. Copper sulfate and lime seemed to work in the garden, but that is totally inappropriate for an AP system, so I'm currently trying out the KHCO3.
I have/had the same issue. I added seaweed extract and all the new growth is coming out normal for 2 weeks now. I'll keep the other tips in mind throughout the summer.
Just a thought, but an air stone under the plant might fix the problems described above.