Which deficiency is this?

[Ryan Garlington]

A couple of older, lower zucchini leaves look like this. My Bright Agrotech deficiency guide points to iron deficiency for this, but today's iron test is 3.04. Is iron ppm between 2 and 3 always right? Could it be something else? 

[Vlad Jovanovic]

Iron (Fe) is NOT mobile withing the plant, so inter-venal chlorosis will occur on the YOUNGEST of leaves. Magnesium (Mg) IS mobile within the plant and looks just like an iron deficiency (inter-venal chlorosis) only it manifests itself on OLDER leaves. When an element is mobile within a plant, and that element is not present in sufficient quantities within it's environment, the plant will usually move those resources from the older growth to the newer growth (much the same way that many animals, and us Humans do :)

Zucchini (and most cucurbits) need quite a bit of Mg (and K). So the fact that it is happening on older leaves and not the youngest leaves points to a Mg deficiency.

[Slinky]

Could well be a low nitrate reading, I have melons that had this problem and boosted the nitrate with a tablespoon of fish blood and bone meal. It takes a while to kick in as it needs to break down in the system and you have to keep a close eye on the ammonia levels and do a small water change if it goes too high but it certainly raises the nitrate.

If you put the powder in a gauze bag, it keeps the solids out of the system. I sometimes also add some dried seaweed and some epsom salts to the bag while I am at it which gives a bit of magnesium and other good stuff. Change the bag every two weeks or so as it can go anaerobic otherwise.

[Ryan Garlington]

Thanks- I put 260g of epsom salt in the 525L system this morning. We'll see what happens with that.

[Vlad Jovanovic]

That works out to about 50ppm Mg (as I'm sure you know). That should be fine for now. You might not expect the already affected leaves to get much better...sometimes Mg deficiencies are difficult to correct in that way, but that dose should go a ways in preventing any new manifestations from rearing their head for a while.

Those guys use up a lot of K as well, so if you buffer your water it would be wise to use a K containing substance,(Potassium bicarbonate, potassium carbonate, or potassium hydroxide) if you're not already doing so. I don't really see any yellowing along the leaf margins that would indicate the beginnings of a K deficiency...just saying 

[JoAnne Bell]

That's not a deficiency issue--it's a bug issue. Your zucchini is infested with spidermites. 

Look at the underside of the leaves for tiny brown bugs the size of a pinhead and webbing. If you can't see them, put a white paper towel under the leaves and shake them. Look for the bugs on the towel.

You have 3 options:

1) Toss the plants

2) Release predatory mites (benficial bugs that attack spidermites). You can ind them at Orcon.com

3) Spray weekly with organic pesticides, like Capt. Jacks or Insecticidal Soap. My preference is to alternate between the two. You can find both of these sprays at TheAquaponicSource.com. Make sure you spray the underside of the leaves.

[Alex Veidel]

I would say that's a magnesium deficiency. I see your point however. The leaf on the top left of the picture does have that sort of flecked look to it that spider mites give off. I thought maybe it was just the camera angle giving it that look, but it could be both. There's definitely a bit of magnesium deficiency in there though.

When checking for spider mites, I find it easier to see the droppings sometimes. Check for little turds on your plant. Or, if you've got a simple microscope, put a piece of leaf under there. Takes all the guesswork out of it.

JoAnne Bell said:

That's not a deficiency issue--it's a bug issue. Your zucchini is infested with spidermites. 

Look at the underside of the leaves for tiny brown bugs the size of a pinhead and webbing. If you can't see them, put a white paper towel under the leaves and shake them. Look for the bugs on the towel.

You have 3 options:

1) Toss the plants

2) Release predatory mites (benficial bugs that attack spidermites). You can ind them at Orcon.com

3) Spray weekly with organic pesticides, like Capt. Jacks or Insecticidal Soap. My preference is to alternate between the two. You can find both of these sprays at TheAquaponicSource.com. Make sure you spray the underside of the leaves.

[Ryan Garlington]

I have potassium bicarbonate, but my municipal water source adds some kind of bicarbonate to the water already. In the hot weather, I'm having to add about 20 gallons to the 140 gallon system every week. At that rate the bicarbonates test at about 11 dKh consistently. Ill take a close look for mites. I really should get some kind of fish-safe pesticide, regardless.

[JoAnne Bell]

Guys, I respectfully disagree. Every leaf in that picture has spidermite damage. Ryan, I think you'll find that if you control the bug population, your "deficiency" will go away. In the Grow Lab at the Aquaponic Source, we've treated plants for various deficiencies only to discover later what we really had was a bug issue (be it spidermites, root aphids, etc.). I've been working with plants for 12 years (both hydroponics and aquaponics) and I can tell you with absolute certainty, Ryan, those zucchini plant leaves are infested with spidermites. Treating a mag deficiency right now will be a total waste of your money.

[Ryan Garlington]

Joanne- which pest treatment should I buy? Liquid ladybug?

[Glenn]

Use of tissue paper soaked in alcohol or plain water to wipe them off the plant, is a good but temporary fix. They can reproduce very fast so total annihilation is difficult requiring a continued if not weekly wiping.

Here's a similar photo directly linked to spider mite damage.