Scraig1 from Ireland
The best way to keep it from spreading is to remove any infected leaves when the plant is dry. Also make sure to clean up any fallen leaf debris under the plant and avoid watering from overhead to keep leaves dry. Leaf spot can also be spread by dirty tools, so keep your garden tools clean to avoid spreading the spores. Treat existing leaf spot with a commercial fungicide containing fixed copper. Follow label directions carefully.
Ellen
Do you mist your plants? If you are spraying the leaves (with water) then letting it sit in the sun, the water droplets work like little magnifying glasses and will burn the leaves.
Many thanks for the message. Unfortunately its not a water & sun problem, I'm pretty careful about that. I think it may be a fungus so against all my instincts I'm going to spray... :(