That is a leaf disease caused by either a fungus or a bacteria. Try to avoid wetting the leaves when you water to avoid conditions which favor disease development. Often just removing infected foliage and avoiding needless wetting of the foliage is enough to get a leaf spot problem under control. There are fungicides available at your local garden center that may also be helpful. Products with chlorothalonil are usually effective against most leaf spot diseases.
Thanks for the question. Please stop in again soon!
Q&A Library Searching Tips