Southern California coastal areas are a prime target for Powdery Mildew, which is a fungus that affects multiple plant species.
It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powdery spots on the leaves and stems. The lower leaves are the most affected, but the mildew can appear on any above ground part of the plant. As the disease progresses, the spots get larger and denser as large numbers of asexual spores are formed, and the mildew may spread up and down the length of the plant.