What causes dark brown patches on sage leaves?

I have pineapple sage and purple sage in pots outside on the patio (South UK). I keep them moist and they get a fair amount of sun. They are potted in compost. Brown patches have started appearing on the leaves and also a yellowing of the lower leaves of the plant . Almost as if they are wilting. Can anyone advise if this is a fungal or viral disease and if so, any treatment? Thank you.