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Grey-brown spots made up of concentric markings appear on the leaves and can join together to create large dead patches. A small pinhead fruiting fungal body can sometimes be seen in the middle of the spots.
A variety of different fungi.
Odd leaves may die but, as a rule, the spots are not very harmful. However, their appearance suggests that the plant is suffering from some other disease or is stressed.
Organic - remove all infected leaves. Try to determine (and remedy) if the plant is stressed in some way, such as if it is in an position it doesn't like, or is over- or under-watered. Plants are often stressed by the soil drying out, so it is well worth mulching around the base of plants with well rotted farmyard manure. This helps condition the soil and enables it to retain more moisture.
Chemical - Spray with a fungicide at the first signs of an attack.