Cucumber plants - yellow splodges on leaves. Can anyone please help identify?

[Russel Sprout]

Morning all

I've only grown cucumbers once or twice in the past and that was many years ago, so they're all a bit new to me. I was wondering if anyone could be so kind as to help identify what might be wrong please? I've tried searching for myself but there seem so many possibilities.

Photos below. And here's some relevant info and possibly a big clue too!

- I don't know the variety. Not even sure if they are climbers or danglers, or whether that even matters in terms of how I let them grow? I did get two from one person and two from another, so not likely to all be the same.

- They are growing in large pots in a polytunnel.

- They were doing really well until a few days ago when I noticed two leaves had what looked liked powdery mildew.

- Having read up on powdery mildew last year, I convinced myself that prevention is better than cure. I just hadn't got round to it yet! So spurred on by this, I removed the two leaves and gave the rest of them a spray of Potassium Bicarbonate (with a little washing up liquid).

- I did so in the morning as it was forecast to be cloudy for the day. However, it turned out to be sunny for quite a few hours after I gave them the spray.

So could it be leaf scorch from the wet leaves/sun? Too much Potassium Bicarbonate in the mix? The washing up liquid? Or is that all a red herring and could it be something else entirely?

Many thanks,
Max

[Jungle Jane]

It is just leaf scorch don’t worry,it does seem cloudy & then the sun makes a surprise appearance. There’s a lot of cloud today,raining again now…Cucumber leaves can go white with sun burn when they’re young,if very hot in a greenhouse/outside a shade net will help.

[Russel Sprout]

Hi, and many thanks for the reply. Apologies too for my delayed response.

Someone told me elsewhere that the addition of washing up liquid may have caused problems. I use Bio-D which says it is "100% naturally derived", and that it has "environmentally responsible ingredients". The ingredients are:

Above 30% Anionic Surfactant. Below 5%: Amphoteric Surfactant, Sodium Chloride, Citric Acid. pH 11.5.

I was told the cucumber leaves wouldn't have liked the sodium chloride.

Maybe it was both things combined...

Anyhow, I was planning to spray them periodically over the growing season (and try to pick cloudier days!) as I've read that prevention is better than cure when it comes to powdery mildew. So next time I spray, should I use just Potassium Bicarbonate or add a little shower gel? We use Faith In Nature - Grapefruit and Orange in case that makes a difference.

And one last thing, should it be a very light spray or get the leaves nice and wet?

Thanks again

[Jungle Jane]

Hi,try & keep the leaves dry in the sun. The wash up liquid would help the bicarb coat the leaf & stick better,I don’t know about the shower gel & fruit oils in them? I would spray them in the evening at about 7-8pm so the droplets on the leaves won’t cause the burn in the sun. I’ve used milk on powdery mildew in the past but there’s a few recipes online for homemade solutions. I think lack of airflow can be a cause of powdery mildew,make sure the plants have space. I would do a medium spray of between the two wetnesses described,spray both sides but don’t go too mad

[Penellype]

Hi, and many thanks for the reply. Apologies too for my delayed response.

Someone told me elsewhere that the addition of washing up liquid may have caused problems. I use Bio-D which says it is "100% naturally derived", and that it has "environmentally responsible ingredients". The ingredients are:

Above 30% Anionic Surfactant. Below 5%: Amphoteric Surfactant, Sodium Chloride, Citric Acid. pH 11.5.

I was told the cucumber leaves wouldn't have liked the sodium chloride.

Maybe it was both things combined...

Anyhow, I was planning to spray them periodically over the growing season (and try to pick cloudier days!) as I've read that prevention is better than cure when it comes to powdery mildew. So next time I spray, should I use just Potassium Bicarbonate or add a little shower gel? We use Faith In Nature - Grapefruit and Orange in case that makes a difference.

And one last thing, should it be a very light spray or get the leaves nice and wet?

Thanks again

Reading this the first thing that struck me was pH 11.5. That is borderline strong alkali and I wouldn't want to be putting it near my plants!