Cucumber disease or nutrient deficiency?

[rochie1211]

I have three cucumber plants that each are about 10 inches tall with five or six leaves on them. The leaves are about the size of my palm. The newest leaves appear healthy and dark green. One of the first leaves that emerged on two of the plants, however, is getting a bit of black at the edges, some yellow/brown spot in the middle. I'm hoping it's a nutrient deficiency: I have an aquaponic garden (closed-loop system where fish waste water is pumped up to growbeds then recirculated) which is relatively new, so many of the trace nutrients probably haven't built up yet. I recently added some Maxicrop with iron, a liquid seaweed product, to introduce some trace nutrients. But, I'm worried that it could be a disease that could spread.

I've read in a couple of forums that excessive nitrogen can cause bacterial spot, which perhaps this is. As an aquaponic system, the number one nutrient is nitrate (a form of nitrogen - ammonia in fish waste is converted to nitrite then nitrate by beneficial bacteria). Could this be the issue? I've read that the treatment is often a copper-based solution, but because the system is closed loop, I can't treat with anything that could harm the fish.

All thoughts appreciated.

Sick Leaf 1:
[img]https://images2.snapfish.com/232323232%7Ffp539%3A5%3Enu%3D747%3B%3E66%3B%3E258%3EWSNRCG%3D33%3B4%3B76495349nu0mrj[/img]

Sick 2:
[img]https://images2.snapfish.com/232323232%7Ffp53982%3Enu%3D747%3B%3E66%3B%3E258%3EWSNRCG%3D33%3B4%3B7649%3B349nu0mrj[/img]

Thanks!

[Francis Barnswallow]

I've got the same thing on my cuc leaves (actually much worse) but I'm still getting large amounts of cucs.......although some of the cucs are very odd shaped I.e. really fat on one end of the cuc itself and really thin on the other end where the blossom was.

[DoubleDogFarm]

Sick Leaf 1:
https://images2.snapfish.com/232323232%7Ffp539%3A5%3Enu%3D747%3B%3E66%3B%3E258%3EWSNRCG%3D33%3B4%3B76495349nu0mrj
Sick 2:
https://images2.snapfish.com/232323232%7Ffp53982%3Enu%3D747%3B%3E66%3B%3E258%3EWSNRCG%3D33%3B4%3B7649%3B349nu0mrj