Unusual Curling of Cucumber Leaves
Cucumbers grown in container. Flowers and fruit appear healthy, along with new growth, but older leaves curl and darken as in photos. Owner says they did the same things last year, and he continued to pinch off the affected leaves during the season. He has grown the same variety of "climbing" cucumbers both years. After consultation, I placed one of the leaves in a plastic bag with a barely damp paper towel for a few days, but observed nothing that indicated fungal growth on the leaves.
Without seeing the entire cucumber plant it can be challenging to diagnose. Nutrient deficiency is a possibility. You mention these are grown in a container. New bagged potting soil should be used each year. Nutrients will leach out with watering and depending on the size of the container will need to be replaced with fertilizer throughout the season. Below is a link that references calcium deficiency. Again, without seeing the entire plant, it is difficult to know if this is the challenge. https://plantix.net/plant-disease/en/700036/calcium-deficiency-in-cucurbits