just a little sun scold, just trim it off. you good

Potassium deficiency, due you fertilize? Well here's the deal, I can't tell you how much to apply because it's in a container, and I don't know what fertilizers you use if any. I can tell what NOT to use and why the older leaves are effected.

Do NOT use 10-10-10 fertilizers or general purpose fertilizers, the type of potassium used in those fertilizers are Potassium Chloride (KCI) and cucumbers are very sensitive to Chloride reduces plant vigor, causing yellowing and scorching of leaf margins, and premature leaf fall. Right away people think the plant has a disease and it was chloride. Nope, Chloride isn't your problem.

potassium is mobile plants and the new growth is taking potassium from the older leaves to support growth, it's what plants do so you need potassium and the right kinds.

The right kind is Potassium Sulfate or potassium nitrate, not a banana peel unless you think the plant can wait a month before microbes make the potassium available for the plant to use. Potassium Sulfate is preferred because of the Sulfate (sulfur). Plants need sulfur or they get sick and the leaves start looking like a Iron deficiency, so people dump a boat anchor of iron on the plant when only a few milligrams are needed, but it doesn't help so they think disease (again).

So tell me what fertilizer you're using.

More fun facts on Cucumbers and potassium:

Cucumbers are one of the only crops that require more potassium then nitrogen, 60 to 100% more, but too much potassium will cause a nitrogen deficiency (it's complicated).

To much or to little and your plants will suffer and if to little your cucumbers will look like a water balloon, the stem end skinny and the blossom end fat .

Potassium will protect the plant from downey mildew, in fact a potassium deficient plant with downey mildew can be cured with potassium. Really! I'm not making this up.

https://www.siftdesk.org/article-details/The-impact-of-potassium-on-downy-mildew-of-cucumber-and-its-leafsoil-nutritional-status/437

Sorry but this is the kind of stuff I love reading.

@outpaientzero

First off, this is fantastic! I have been gardening for about 3 years now with moderate success but still learning about soil and fertilizers.

Even since this posting my cucumbers are still growing with even more fruit coming in but the leaves are definitely yellowing. It could be Sun Scold someone else said?

However, in terms of fertilizer below is a screen shot of what I've used this year. I also sprayed some Epsom salt spray but that is it.

So let me know if this stuff is good , bad or indifferent and if I should be doing anything else or my cukes are gonna be just fine.

Thank you again!

Honestly, it looks like fertilizer burn. Your leaves are very dark green (where they aren't damaged) which would indicate high nitrogen, among other things. And I suggest you do some reading on using epsom salt from a scientific source. It is rarely, if ever, necessary. Epsom salt is magnesium sulphate and as the name suggests, the only thing epsom salt supplies is magnesium and sulphur. Two micronutrients not needed in large amounts to begin with and they are two micronutrients that are already contained in the Miracle-Gro you have used. You just doubled up on those two nutrients.

Rodney

P.S. Here's an article on fertilizer burn: https://extension.umd.edu/hgic/topics/fertilizer-or-pesticide-burn-vegetables

The miracle grow has just about everything your plants need, but I said just about.

Potassium is potassium sulfate so you're good there, no chloride.

One small problem and I mean small, microscopic, It lacks boron but hold on,

Boron is TOXIC to cucumbers in excess so PLEASE read on.

Boron deficiency is the most widespread micronutrient deficiency around the world

and Boron easily leaches out of sandy soil (or containers) or the plant can't take it up because of high soil PH. But applied in excess boron will easily kill your plant.

Does this look like your leaves? Minor Boron deficiency in cucumbers.

extreme boron deficiency in cucumbers........... looks like dried up sun scald doesn't it.

I'm going to give the amount to apply safely and where to get it.

Borax is 11% boron and can be used and also recommended by the "food and fertilizer technology center" as a source for boron.

The amount is 0.01 grams per square foot of your container surface, this is the amount of boron, not borax. The amount of borax used would be 0.09 grams per square foot or almost a tenth of a gram and will do fine per sq. ft. This is the minimum required but 3 to 4 times the amount would be harmful and easy to do if you guess, so a good digital scale is needed.

Mix it in a little water and use a sprayer to apply it all over the top of the soil.

You can also mix up a 0.25% borax water solution (1/10th gram of borax to 99.75 grams of water? I think) and use it as a foliar spray every 10 days until problem goes away.

I apply 1 gram of boron (9 grams of borax) per 100 square feet of garden space I use during the year with a tank sprayer. I have about 16,000 sq. ft. of garden space but rotate and fallow about a third of it.

I hesitate telling people about Boron because all they hear is dump on boron blocking out the warning about excess so please be careful on the amount.