Oregano Leaf Spots

I have oregano that I bought from a reputable greenhouse. Since its transplanting it has developed random black spots on the older leaves, new growth does not appear to be affected. There are no apparent signs of insects or their damage. It is planted in a mix of a garden compost soil with BM6 mix to lighten it up. One of my clients also has the same black spots on her oregano which has been established for a few years, and it is in a sandy loam soil. Any ideas on what this might be?

Since this is the wet and cool spring and summer of 2008, people are having more problems with fungal or bacterial infections than usual. Make sure the plant is in an airy spot with good air movement. You might like to try the usual organic anti-microbial sprays such as a suspension of powdered sulphur ("flower" of sulphur), cinnamon powder, neem oil or tea tree oil.