The brown spots may be fungal leaf blight or lily disease. This type of fungus spreads freely in wet, humid conditions. The best way of preventing it is to grow lilies in sunny, well ventilated positions where the foliage will dry out quickly following rainfall. Also, take care to water directly into the pot and not from overhead.
Cut off and burn any severely infected leaves and let them die back this year as normal. Destroy any remaining plant debris in autumn. The spores overwinter in the soils surface so I would repot them once they have died back to ensure that they are in fresh, clean soil for next year. You shouldn’t need to use ericaceous compost as most lilies are quite happy in a neutral soil.
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