Could it be a. powdery mildew, usually due to underwatering or b. red spider mite infestation, if its in a greenhouse, but I expect someone more knowledgeable than me will know the answer.

looks like mildew possibly --dry atmosphere/ dry roots-- hows the rest of the plant and the cropping-- also poor air circulation can cause it

The plants are outside the others in the other patch have done really well they have been watered regularly when we have had no rain I can see no red spider mite or anything else like that

I have an acanthus in a shady but dry part of the garden and it gets mildew every year without fail - I have given up and just pull the affected leaves off.

I find in the greenhouse some leaves go yellow so I remove them so its worth a try, do you feed them?

Powdery mildew needs dry warm days, and cool nights without dew to thrive. One of the treatments is to get up before dawn and spray the leaves with plain water, to imitate dew. That kills the spores just when they are trying to sprout. Spraying the leaves any other time of day just seems to make it worse.