if you go to this link on nutritional deficiencies of cucumbers. Page 18 of this publication has a picture very similar to yours showing severe calcium deficiency in a cucumber leaf with the white necrotic spots on the leaves, curled and necrotic edges. The picture also shows prominent veinal chlorosis.
I grow a lot of cucumbers and I grow them year round. They are very sensitive to nutritional issues. I tried growing them in an organic soil less mix in containers with organic fertilizers and had severe deficiencies of potassium which took me a while to figure out since I didn't have those problems growing them with conventional fertilizer ( same seed variety and same soil mi). I corrected the potassium by adding potassium sulfate and while that fixed the potassium deficiency, the extra potassium raised the pH and caused an imbalance that interfered with phosphorus, zinc, and magnesium causing those deficiencies to show up later. The problem was with the organic fertilizer I used not being complete and I could not figure out how to blend the different fertilizers available to me to get it to balance without causing other issues. The nutritional disorders weakened the plant so it that affected the yield, susceptibility to pests and disease requiring more pest control, and shorter life.
Master blend is a good hydroponic mix. I have only used that for greens. If you checked the link in the other publication in my previous post you will see the nutrient requirements for cucumbers are different that those for greens and tomatoes. Container and hydro plants cannot draw nutrients from the soil, everything needs to be supplied when they need it by you. That is why I would go with a recipe that someone has already figured out. Then the most important things would be to keep the pH and EC within range and make sure the system is clean. For larger plants like tomatoes and cucumbers, we supplemented the dutch buckets (using black cinder as media) with Sustane and bone meal, but we were doing aquaponics, not hydroponics. The fish tanks still had to be monitored to keep pH around 6.0 and iron was added to the fish water as a supplement for the lettuce.
There were aeroponic towers for lettuce and hyroponic rails for lettuce, chard, and pac choi. Because of the diversity of the crops, the A,B, C solutions were in 50 gallon drums run a by a controller that constantly measured and adjusted the solution based on the parameters set for the controller.
The aquaponic system was organic, the hydroponic one was not. They do make organic hydroponic solutions, but at that time they weren't very good. I don't know if that has changed.
https://extension.psu.edu/hydroponics-s ... and-excess