I'm a beginner also, so may not know what I am talking about. On some of my plants, the older leaves closest to the mound started yellowing and dying a couple weeks ago. I cut them off, including those that were just starting to change shade, and sprayed with a fungicide. The plants are looking better now. But I'm still not certain it was a fungus / mildew... my neighbor who plants cucumbers every year said it is normal for this to happen about this time of year.
Now, some of my plants have mature leaves going brown, similar to yours. I'm cutting them off. I figure, any leaf that is "going bad" can't be doing much good, and is probably draining energy from the plant.

I think it is ENVIRONMENTAL meaning that it is not caused by a disease.

1- possibly , it is sun scald.
2- possibly , due to fertilizer burns. Do you spray them , foliary ?

Anyway, If it is just isolated cases, I would just snipe those leaves off. IF the it recurs , I would look into disease identification and treat them accordingly.