Cucumbers turning black & dying: Disease or Deficiency?

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My IBC aquaponics system has been up and running since May.  I have 30 tilapia and have actively growing basil, sweet peas, watermelon, and cucumbers.  The fish are happy (no deaths, or disease & they eat well).  The plants are growing well & flowering, but once fruit develops (baby watermelons & cucumbers), they turn black, die, and fall off the vine - so there's no fruit.  Yet the vine keeps growing & flowering & repeating the same thing.  The leaves are dark green at the growing end, but toward the roots, the leaves turn yellow, then black and die (see pics).  As I said the fruit is also dying.  Is this a deficiency or disease?

Using 3 different test kits I have, here's my latest parameters:

Using API's Freshwater Master Test Kit:

    pH:  6.1 (using electronic meter)
    Nitrates: 40 ppm
    Nitrites: 0 ppm
    Ammonia: 0 pmm

Using Nutrafin Test Kit:

    Phosphate: 0 (Low / Non-existent)
    Calcium:  < 20 ppm (Low)
    Free Iron (Non-Cheleated): .1 (High)
    Free Iron (Toxic): 1.0 (High)
    Carbonate Hardness (KH): 20 ppm (Low)
    General Hardness (GH): 100 ppm (Moderately Hard)

Using Rapidtest Soil Test Kit (so may not be accurate since I have no soil)

    Phosphorus: P3 (sufficient)
    Potassium/Potash: K2 (adequate)
    Nitrogen: N3 (sufficient)

Just yesterday, I added some crushed shells in a mesh bag to the water to help buffer the pH and keep it up (had dropped to 5.7 some days). It should also help with the calcium.  I also added some Maxicrop Liquid Seaweed which should help bring up the Potassium/Potash.  To address the high iron, I am working on replacing the tank water over time.

Any ideas what is going on & what else I should do to fix it?   Thanks!





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Hi Dave,

I'd bump up the pH (slowly) to pH 6.4

I don't know what is causing your issues there, but I have a couple of questions.

Have you found out what's causing your cucumbers to turn black?


Have you taken some samples and your pics to the local garden store?.....In my part of the world I do that when I have issues and they are very helpful.


Do you have aeration?

Let's know how you get on.


Edited by bigdaddy (see edit history)

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Thanks for the reply.  No I still don't know why the fruit on my plants (both cucumbers and watermelons) turn black and die.  It's still happening, but with new shoots. 

I don't have a dedicated aeration pump.  The water pouring back into the tank causes quite a disturbance with air bubbles forming deep into the water, so have not had any issues with oxygen.  No fish have ever died, so I assume there is plenty of oxygen in the water, though I've never tested it (and don't know how to).

My pH is currently 5.9 so going down.  I've recently added a bag of seashells, but it's still staying low.  How do you suggest I raise it? 

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Hi Dave,

I use Hydrated lime to raise the pH and HydroChloric acid to lower it.

To raise my pH I dissolve Hydrated lime or builders lime(same thing over here but check the packet) in a bucket of water and add gradually to to fish tank...Generally I bump it up 1 or max 2 pH points per day until it gets to my desired pH level. I aim for 6.4 pH but bump it up to 6.8 and let it drop to 6.2 or 6.0 at the outside.

If I need to lower the pH I use gloves and goggles Mix some Hydro Chloric acid in a watering can and gradually add to fish tanks. again I try to lower it as slowly as I can and I am very careful not to spill any on me or my clothes.

It seems to me you have some issues with your media which is keeping the pH down. I'd persist and keep adding lime and hopefully sometime it will even out.

I don't think this is causing the problem but I certainly think it will help to get the pH at around 6.4.



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Did you get your problem fixed?   It sounds very similar to blossom end rot that tomatoes experience if the calcium level is low.  Adding the lime should help both the pH as well as the calcium level.  Just a word of caution: be careful not to add powered lime straight to the fish tank as it can kill your fish.

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