Bacterial leaf spot is favored by warm, wet conditions in late spring onwards. Almost all hydrangea species are susceptible to the disease. The disease symptoms can start on planting stocks and are accelerated by high moist conditions.

Leaf spots have reddish-purple in color and may have angular edges when bordered with leaf veins. The size of the lesions can vary on different hydrangea species.

Lesions can also be seen on the lower side of the leaves. Yellow halo can be seen around the spots. The causal bacterium is spread easily through water splash. The bacterium is known to enter the plant through the natural openings (stomates and hydathodes)

The spots can have a shot-hole appearance due to the collapse of the central tissue of the spot lesions. Over time spots coalesce to form large blighted areas on the leaves.

An easy method to test for the presence of the bacterium is to cut an infected section and place on a slide with a water and observe under a stereo-microscope. This bacterial ooze is a quick test for the disease.